Oilfield wellhead gas circulation injection system and technology

Abstract

The invention discloses an oil field well head gas circulation injection system and technology. The system comprises a purification unit, a pre-pressurization unit, a cooling water removal unit, a pressurization injection unit and a control system. Wherein the purification unit comprises a desulfurization tank, a centrifugal water removal tank and a filter which are communicated with one another, and the pre-pressurization unit comprises a diffusion pipeline and a gas inlet main pipeline which are arranged in parallel; the cooling and dewatering unit comprises a centrifugal dehydrator, a freezing dryer and a micro-heat regeneration adsorption type dryer; the pressurizing injection unit comprises a filtering dehydrator, a variable frequency motor and a four-stage piston compressor; the control system comprises a standard electrical control unit and a remote monitoring unit. Oilfield wellhead gas is recycled, purified and pressurized and then circularly injected into a well, separation, recycling, purification and supercritical pressurization of carbon dioxide in mixed gas are achieved, gaseous carbon dioxide can be restrained from being condensed into liquid based on the four-stage piston compressor, and therefore zero loss of carbon dioxide gas in the mixed gas is guaranteed, and the energy consumption is reduced. 100% recycling of carbon dioxide is achieved, zero emission of carbon dioxide in the external environment is achieved, and the problems that in the prior art, wellhead gas is directly exhausted into air, consequently, environment pollution is caused, and energy waste is avoided are solved.

Description

Technical Field

[0001] The invention relates to the field of oilfield wellhead gas circulation utilization, and in particular to an oilfield wellhead gas circulation injection system and process. Background Art

[0002] After the implementation of CCUS-EOR (carbon dioxide cycle injection) technology, the excess carbon dioxide gas injected into the oil layer will be discharged to the oil wellhead along with water vapor to form wellhead gas. Therefore, the wellhead gas contains a small amount of natural gas, a large amount of CO2 gas and water vapor. If the wellhead gas is directly discharged into the air, it will cause the emission of greenhouse gases, leading to environmental pollution and energy waste. Therefore, the CO2 in the wellhead gas must be recovered and reused.

[0003] In the prior art, the natural gas processing system only contains separation and drying units, lacks a boosting unit, and cannot achieve separation and recovery of carbon dioxide and water vapor; it is also impossible to achieve supercritical boosting of carbon dioxide and wellhead gas injection. The multi-stage piston compressor unit in the prior art cannot handle the mixed gas produced by CCUS-EOR technology. This is mainly because: the cooling machine of the multi-stage piston compressor unit in the prior art will cause the liquefaction of gaseous CO2 in the wellhead gas, and the liquefied CO2 will be discharged through the gas-liquid separation system of the multi-stage piston compressor unit in the prior art, thereby failing to achieve "zero" carbon dioxide emissions in oil field development and causing environmental pollution; in addition, the control method of the unit is single, and once the control fails, it will also cause serious damage to the piston compressor, resulting in the unit being unable to separate, recover and purify the mixed gas produced by CCUS-EOR technology.

[0004] Therefore, it is urgent to develop an injection system suitable for the separation, recovery and purification of carbon dioxide in the mixed gas after CCUS-EOR technology. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a wellhead gas circulation injection system and process for oil fields, which can recycle, purify and pressurize the wellhead gas generated after the implementation of CCUS-EOR technology and then inject it into the well, thereby achieving "zero" carbon dioxide emissions in oilfield development, and the displacement and pressure can be automatically adjusted according to the on-site working conditions.

[0006] The technical solution adopted by the present invention to solve the technical problem is: The oilfield wellhead gas circulation injection system includes a purification unit, a pre-pressurization unit, a cooling and water removal unit, a pressurization injection unit and a control system; The purification unit includes a desulfurization tank, a centrifugal water removal tank, and a filter that are interconnected; The pre-boosting unit comprises a venting pipeline and an air intake main pipeline arranged in parallel, and the air intake main pipeline is provided with a gas buffer tank, a variable frequency motor, a screw compression cylinder, and an aftercooler; The cooling and water removal unit includes a centrifugal water remover, a cold dryer, and a micro-heat regeneration adsorption dryer; The boost injection unit includes a filter dewatering device, a variable frequency motor, and a four-stage piston compressor; The control system includes a standard electrical control unit and a remote monitoring unit.

[0007] Furthermore, in the purification unit: The output gas enters from the gas inlet of the desulfurization tank and is used to remove impurities such as hydrogen sulfide in the output gas. The gas outlet of the desulfurization tank is connected to the inlet of the centrifugal dehydration tank, and the gas outlet of the centrifugal dehydration tank is connected to the gas inlet of the filter; the gas outlet of the filter is connected to the gas inlet of the gas buffer tank of the pre-boosting unit.

[0008] Furthermore, the desulfurization tank adopts the "sieve plate + alkali + filler" treatment process: The desulfurization tank is provided with a gas inlet at the lower end, an alkali solution inlet and an alkali solution spray nozzle at the top, and a gas outlet and a filler injection port is provided on the right side of the alkali solution inlet; a pressure meter and a temperature meter are installed at the upper end of the desulfurization tank, and a liquid level detection meter is installed at the lower end. The pressure meter, temperature meter and liquid level detection meter are connected to the control system through PIC; A sieve plate with through holes is installed at the lower end of the gas inlet on the inner wall of the desulfurization tank, and the sieve plate is evenly filled with fillers. A retention plate is also installed in the inner wall of the desulfurization tank to support the fillers and slow down the gas flow rate, form gas turbulence, promote gas-liquid mass transfer, and achieve desulfurization of the output gas.

[0009] Furthermore, the desulfurization principle of the desulfurization tank is as follows: the alkali liquid sprayed down from the top is in full contact with the hydrogen sulfide-containing gas flowing up from the bottom of the tower. Under the synergistic effect of the alkali liquid and the filler, the hydrogen sulfide in the gas is oxidized and reduced to sulfide salts and settles to the waste liquid area at the bottom of the tower with the alkali liquid for storage. When the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system.

[0010] Furthermore, the filler is glassy iron oxyhydroxide, which is used to remove hydrogen sulfide from the produced gas; The alkaline solution is preferably sodium hydroxide or potassium hydroxide, which is used to absorb hydrogen sulfide in coordination with glassy iron oxyhydroxide.

[0011] Furthermore, a gas inlet is provided in the middle of the centrifugal dewatering tank, a gas outlet is provided at the top, a pressure meter and a temperature meter are installed at the upper end, and a liquid level detection meter is installed at the lower end. The pressure meter, temperature meter and liquid level detection meter are connected to the control system through PIC.

[0012] Furthermore, a funnel-shaped sieve plate is installed on the inner wall of the centrifugal dewatering tank, and through holes are evenly distributed on the sieve plate, and gas can enter the through holes; when the output gas enters from the gas inlet, under the centrifugal rotation of the funnel-shaped sieve plate, the gas enters the through holes along the cross-sectional tangent of the funnel-shaped sieve plate, and performs a spiral upward motion in the sieve plate, and the gas and liquid are separated according to the cyclone separation effect, and then the waste liquid is condensed and gathered along the wall of the funnel-shaped sieve plate by relying on the gravity of the liquid, and flows into the bottom of the funnel-shaped sieve plate for collection; A conical intercepting plate is installed at the upper end of the inner wall of the centrifugal dewatering tank. Through holes are evenly distributed on the conical intercepting plate to intercept the rising gas and slow down the gas flow rate so that the residual liquid in the gas condenses and flows along the tank wall and the funnel-type sieve plate into the bottom arc-shaped wastewater collection area of ​​the centrifugal dewatering tank for storage. When the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system.

[0013] Furthermore, the filter adopts the "pressurization + active composite inorganic adsorption" treatment process, selects "activated carbon + inorganic silicon" fillers, takes advantage of the pore structure and specific surface area, utilizes capillary action and surface tension to form a physical-chemical adsorption structure, removes heavy hydrocarbons, and prevents the injected medium from being enriched and blocked after the purification unit.

[0014] Furthermore, a removable two-layer perforated support plate is installed at the lower end of the filter, and the middle of the perforated support plate is filled with an activated carbon adsorption layer for preliminarily removing C4 and above heavy hydrocarbon components in the output gas; the pores of the perforated support plate are used to discharge the waste liquid condensed in the filler into the arc space at the lower end of the filter for storage, and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system.

[0015] Furthermore, the perforated support plate is detachable, and when the activated carbon adsorption layer is saturated with adsorption, the window on the side wall of the filter can be opened to disassemble the perforated support plate, remove the saturated activated carbon adsorption layer, and add filler again.

[0016] Furthermore, the upper area of ​​the activated carbon adsorption layer is filled with an inorganic silicon adsorption layer, and retention plates are symmetrically and staggeredly installed on the inner wall of the filter where the inorganic silicon adsorption layer is located, which are used to cause gas turbulence and block the direct flow of the gas, so that the gas can fully contact the inorganic silicon adsorption layer to adsorb and remove impurities such as heavy hydrocarbons in the gas, thereby avoiding enrichment and blockage of the output gas during subsequent treatment.

[0017] Furthermore, the height ratio of the inorganic silicon adsorption layer to the activated carbon adsorption layer is 2-3:1. Under this ratio, the effect of adsorbing and removing impurities such as heavy hydrocarbons is best.

[0018] Furthermore, the air intake main line of the pre-boosting unit is connected to a release line through a stop valve, and can be released with one button in an emergency; The main air intake line is connected to the branch line I through the stop valve BV-1, and is connected to the branch line II through the stop valves BV-2 and BV-3, so as to realize two-stage pipeline diversion. In case of emergency, the branch line BV-1 is opened.

[0019] Furthermore, a total pressure detection gauge, a total intake temperature detection gauge and a total intake solenoid valve are installed on the intake main pipe, and the total intake solenoid valve is connected to a gas buffer tank through a pipeline; The upper end of the gas buffer tank is equipped with a pressure detection gauge and a safety valve, and the lower end is equipped with a liquid level detection gauge. The safety valve is connected to the release pipeline. The lower end of the gas buffer tank is provided with a drainage pipeline, and a stop valve is provided on the drainage pipeline. When the waste liquid reaches the liquid level monitoring point, the stop valve can be controlled by the control system to automatically discharge the waste liquid through the drainage pipeline to achieve the external discharge of the waste liquid; The gas outlet of the gas buffer tank is connected with an air filter tank, and a screw front air intake solenoid valve is arranged at the front end of the pipeline connected with the air filter tank. A branch pipeline III is arranged at the front end pipeline of the screw front air intake solenoid valve. The branch pipeline III is connected with the air filter tank gas outlet pipeline through a stop valve BV-4 and is collected into a main pipeline. Here, the main pipeline is connected with a variable frequency motor, and the gas outlet of the variable frequency motor is connected with a screw compression cylinder through a pipeline. The gas outlet of the screw compression cylinder is connected with a minimum pressure valve through a pipeline, and the outlet of the minimum pressure valve is connected with an aftercooler through a pipeline. The aftercooler realizes gas cooling through a fan, and the gas outlet of the aftercooler is connected with a centrifugal water remover through a pipeline. A safety valve is provided at the upper end of the screw compression cylinder, and the safety valve is connected to the venting pipeline; a discharge pipeline is provided at the lower end of the screw compression cylinder, and a stop valve is provided on the discharge pipeline. When the waste liquid reaches the liquid level monitoring point, the stop valve can be controlled by the control system to automatically discharge the waste liquid through the discharge pipeline, thereby realizing the external discharge of the waste liquid; The oil outlet of the screw compression cylinder is connected to an oil filter through an oil pipeline, the oil filter is connected to an oil cooler through a pipeline, and the oil cooler is connected to the oil inlet of the variable frequency motor through a pipeline to achieve oil cooling of the screw compression cylinder and the variable frequency motor.

[0020] Furthermore, the gas outlet of the aftercooler of the pre-boosting unit is connected to the gas inlet of the centrifugal water separator through a pipeline, the gas outlet of the centrifugal water separator is connected to the gas inlet of the cold dryer, a temperature detection meter is arranged on the pipeline between the centrifugal water separator and the cold dryer, the gas outlet of the cold dryer is connected to tank A and tank B of the micro-heat regeneration adsorption dryer through a pipeline, a first valve is arranged on the pipeline between tank A and the cold dryer, a second valve is arranged on the pipeline between tank B and the cold dryer, and the main pipeline before the air intake pipeline of tank A, tank B and the gas buffer tank is connected through a third valve.

[0021] The main pipeline of the micro-heat regeneration adsorption dryer and the cold dryer is provided with a branch pipeline, and a stop valve is installed on the branch pipeline. The branch pipeline is connected with the gas outlet pipelines of tank A and tank B and then merges into the main pipeline. A total pressure flow detection instrument is installed on the main pipeline. If the gas medium after cooling by the cold dryer reaches the gas dew point required by the process, the micro-heat regeneration adsorption dryer is not needed for further drying and dehydration. At this time, close the first valve and the second valve to allow the gas medium to be injected into the next device through the branch pipeline.

[0022] Further, the four-stage piston compressor comprises: First-stage air intake steam-water separator, first-stage air intake buffer tank, first-stage piston compression cylinder, first-stage exhaust buffer tank, first-stage air cooler, second-stage air intake steam-water separator, second-stage air intake buffer tank, second-stage piston compression cylinder, second-stage exhaust buffer tank, second-stage air cooler, third-stage air intake steam-water separator, third-stage air intake buffer tank, third-stage piston compression cylinder, third-stage exhaust buffer tank, third-stage air cooler, fourth-stage air intake steam-water separator, fourth-stage air intake buffer tank, fourth-stage piston compression cylinder, fourth-stage exhaust buffer tank, fourth-stage air cooler; The gas outlet of the four-stage air cooler is connected to an exhaust pipeline; The lower ends of the secondary air intake steam-water separator, the tertiary air intake steam-water separator and the quaternary air intake steam-water separator are provided with electric heating reboilers.

[0023] In the four-stage piston compressor of the present invention, the electric heating reboiler is mainly arranged after the first-stage piston compression cylinder. This is mainly because: after filtering and dehydrating by the filter dehydrator and further dehydrating by the first-stage air intake steam-water separator, and no air cooler is arranged in front of the first-stage air intake steam-water separator, liquefaction of carbon dioxide will not occur in the first-stage air intake steam-water separator, and it can be ensured that the gas entering the first-stage piston compression cylinder is all gas medium. Therefore, there is no need to arrange an electric heating reboiler in the first-stage air intake steam-water separator.

[0024] Since the waste liquid in the first-stage air intake steam-water separator does not contain liquid CO2, the waste liquid stored in the waste liquid area of ​​the first-stage air intake steam-water separator does not need to be strictly controlled to have a temperature above 35°C. When the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system.

[0025] The air cooler is mainly set after the first-stage piston compression cylinder. This is mainly because: the first-stage piston compression cylinder belongs to primary compression, the pressure (3-3.5MPa) is relatively small, and the impact on subsequent equipment is relatively small, so there is no need to set an air cooler in front of the first-stage piston compression cylinder. The subsequent second-stage piston compression cylinder has a pressure of 7.5-8.5MPa and a temperature of 140℃±10℃, the third-stage piston compression cylinder has a pressure of 14-16MPa and a temperature of 140℃±10℃, and the fourth-stage piston compression cylinder has a pressure of 28-30MPa and a temperature of 140℃±10℃. The high-temperature and high-pressure gas after the second-stage, third-stage, and fourth-stage piston compression will cause the aging of the seals of the subsequent equipment. In order to reduce the impact of high temperature and high pressure on subsequent equipment, air coolers are introduced before the second-stage piston compression cylinder, the third-stage piston compression cylinder, and the fourth-stage piston compression cylinder.

[0026] However, the air cooler cannot accurately control the temperature of the gas medium to a constant value (35-65°C), and under the influence of environmental factors, there may be a problem of over-cooling, resulting in the liquefaction of gaseous carbon dioxide. Therefore, a heat tracing reboiler must be installed in the air inlet steam-water separator behind the air cooler to gasify the liquefied carbon dioxide.

[0027] Furthermore, the gas inlet of the filter dehydrator is connected to the air intake pipe, and the gas outlet is connected to the inlet of the first-stage air intake steam-water separator through a pipeline. A first-stage suction pressure gauge is provided on the left side of the upper end of the first-stage air intake steam-water separator, and a safety valve SV-2 is provided on the upper end of the first-stage air intake steam-water separator, and the safety valve SV-2 is connected to a release pipeline; The first-stage exhaust buffer tank is provided with a first-stage exhaust pressure gauge and a first-stage exhaust temperature gauge, and the air intake pipeline of the first-stage air cooler is provided with a safety valve SV-3, which is connected to the vent pipeline; The secondary exhaust buffer tank is provided with a secondary exhaust pressure gauge and a secondary exhaust temperature gauge, and the air intake pipeline of the secondary air cooler is provided with a safety valve SV-4, which is connected to the vent pipeline; The three-stage exhaust buffer tank is provided with a three-stage exhaust pressure gauge and a three-stage exhaust temperature gauge, and the air intake pipeline of the three-stage air cooler is provided with a safety valve SV-5, which is connected to the vent pipeline; The four-stage exhaust buffer tank is provided with a four-stage exhaust pressure gauge and a four-stage exhaust temperature gauge, and the air intake pipeline of the four-stage air cooler is provided with a safety valve SV-6, which is connected to the vent pipeline.

[0028] Furthermore, a main gas production valve is installed on the exhaust pipeline, and the exhaust pipeline located on the left side of the main gas production valve is also connected to the release pipeline through a main release valve; The safety valves on each component are controlled by the main relief valve, so that the boost injection unit (four-stage piston compressor) can be released with one button in emergency situations (such as compressor shutdown).

[0029] Furthermore, the control process of the boost injection unit is: The gas (medium) has a pressure of 1.0-2.5MPa and a temperature of 40℃±5℃ (the temperature and pressure parameters are obtained through real-time monitoring). It enters the filter dehumidifier through the air inlet pipe. After filtering and dehydration, the dew point at normal pressure reaches -45℃±5℃. The gas enters the first-stage air inlet steam-water separator for gas-liquid separation, and the waste liquid is stored in the waste liquid area of ​​the first-stage air inlet steam-water separator. When the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system; After gas-liquid separation, the gas enters the first-stage air intake buffer tank through the pipeline to reduce the impact of the gas on the piston compression cylinder, with a pressure difference of 0-0.1MPa; Then it enters the first-stage piston compression cylinder, and after pressurization, the pressure is 3-3.5MPa and the temperature is 140℃±10℃; After being compressed by the first-stage piston, the gas enters the first-stage exhaust buffer tank for gas buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; The gas then enters the first-stage air cooler, which is used to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the first-stage air cooler is 3-3.5MPa and the temperature is 35-55℃; The gas cooled by the primary air cooler enters the secondary air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the secondary air intake steam-water separator is monitored in real time in the control system: If the control system detects that the gas temperature is less than 35°C, it indicates that carbon dioxide will be liquefied in the secondary air inlet separator, causing the liquefied carbon dioxide to enter the liquid. In order to avoid the loss of carbon dioxide, the control system automatically shuts down the primary air cooler through the programmable controller and automatically starts the electric heating reboiler of the secondary air inlet separator to raise the temperature to 35-55°C to gasify the carbon dioxide condensed in the liquid. Otherwise, the control system does not perform the corresponding control action; The liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the secondary air intake gas-water separator. When the gas temperature in the secondary air intake gas-liquid separator is above 35°C and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system; After the gas enters the secondary air intake buffer tank through the pipeline for buffering, the pressure difference is 0-0.1MPa to reduce the impact on the secondary piston compression cylinder; After being buffered, the gas enters the secondary piston compression cylinder for compression, and the pressure after pressurization is 7.5-8.5MPa and the temperature is 140℃±10℃; The gas then enters the secondary exhaust buffer tank for buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; The gas then enters the secondary air cooler, which is used to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the secondary air cooler is 7.5-8.5MPa and the temperature is 40-60℃; The gas cooled by the secondary air cooler enters the third-stage air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the third-stage air intake steam-water separator is monitored in real time in the control system: If the control system detects that the gas temperature is less than 35°C, it indicates that carbon dioxide will be liquefied in the three-stage air inlet steam-water separator, causing the liquefied carbon dioxide to enter the liquid. In order to avoid the loss of carbon dioxide, the control system automatically shuts down the secondary air cooler through the programmable controller and automatically starts the electric heating reboiler of the three-stage air inlet steam-water separator to heat it to 40-60°C to gasify the carbon dioxide condensed in the liquid; Otherwise, the control system does not perform the corresponding control action; The liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the three-stage air intake gas-water separator. When the gas temperature in the three-stage air intake gas-liquid separator is above 35°C and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system; After the gas enters the three-stage air intake buffer tank for buffering, the pressure difference is 0-0.1MPa to reduce the impact on the three-stage piston compression cylinder; The gas after buffering enters the three-stage piston compression cylinder for compression. After pressurization, the pressure is 14-16MPa and the temperature is 140℃±10℃; The gas then enters the third-level exhaust buffer tank for buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; Then it enters the third-stage air cooler to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the air cooler is 14-16MPa and the temperature is 45-65℃. The gas cooled by the three-stage air cooler enters the four-stage air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the four-stage air intake steam-water separator is monitored in real time in the control system: If the control system detects that the gas temperature is less than 35°C, it indicates that carbon dioxide will be liquefied in the four-stage air inlet steam-water separator, causing the liquefied carbon dioxide to enter the liquid. In order to avoid the loss of carbon dioxide, the control system automatically shuts down the three-stage air cooler through the programmable controller, and automatically starts the electric heating reboiler of the four-stage air inlet steam-water separator to heat it to 45-65°C to gasify the carbon dioxide condensed in the liquid; Otherwise, the control system does not perform the corresponding control action; Finally, the liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the four-stage air intake gas-water separator. When the gas temperature in the four-stage air intake gas-liquid separator is above 35°C and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system; The gas enters the four-stage air intake buffer tank for buffering, with a pressure difference of 0-0.1MPa to avoid excessive impact on the piston compression cylinder; The gas after buffering enters the four-stage piston compression cylinder for compression, and the pressure after pressurization is 28-30MPa and the temperature is 140℃±10℃; The gas then enters the fourth-stage exhaust buffer tank for buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; Then it enters the four-stage air cooler to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the four-stage air cooler is 28-30MPa and the temperature is 60℃±10℃. After cooling, the gas is injected into the lower-level equipment through the exhaust pipeline. At this time, the pressure of the carbon dioxide gas medium is 28-30MPa and the temperature is 60℃±10℃, reaching the supercritical carbon dioxide state (the outlet temperature can be adjusted according to production conditions, and the minimum outlet temperature is the ambient temperature + 10℃). The gas is injected into the lower-level equipment through the total gas production valve on the exhaust pipeline to achieve supercritical injection of carbon dioxide.

[0030] Furthermore, the standard electrical control unit includes a programmable controller and a control panel, wherein the control panel is provided with a liquid crystal display window for displaying or querying setting values, fault alarms and operating status; The standard electrical control unit monitors and controls the operation process, emergency faults and equipment start and stop of the entire circulating injection system through a programmable controller (PLC); and automatically monitors the abnormal state of the system, such as compressor abnormality, residual gas in the circulating injection system, etc., automatically displays an alarm signal and shuts down, thereby realizing fault diagnosis, recording and self-elimination functions. When the fault is eliminated, the compressor automatically resumes work.

[0031] Furthermore, the remote control unit is connected to a remote computer through an RTD input card, a communication card, and an expansion slot reserved on a standard electrical control unit to achieve remote data transmission and control.

[0032] Furthermore, a reporting unit is installed on the remote computer, which has the function of recording operating parameters, with a recording time period greater than 9999 hours, and real-time display of the pulsation curves of the import and export pipelines of the circulation injection system, the inside of the booster unit and the entire process pipeline.

[0033] The present invention is to solve the technical problem and also provides a wellhead gas circulation injection process for oil fields.

[0034] The cyclic injection process is: The wellhead gas first enters the desulfurization tank to remove hydrogen sulfide in the gas, and then enters the centrifugal dewatering tank along the cutting groove for preliminary gas-liquid phase separation. The gas enters the filter to filter and remove C4 and above heavy hydrocarbon components in the gas. The gas is then sent to the screw compression cylinder for initial pressure increase to 0.5-1.0MPa, and then sent to the freeze dryer to make the atmospheric pressure dew point of the gas reach -20±5℃, and then deep dehydration is carried out by the micro-heat regeneration adsorption dryer to reduce the dew point temperature of the gas medium to -40±5℃ (this dew point temperature can generally suit the winter temperature in the north and is suitable for the national promotion and use of the system). The gas is then sent to the filter dehydrator for further dehydration, and then sent to the four-stage piston compressor to increase the pressure to 28-30MPa, and then the gas (carbon dioxide concentration 100%) is injected into the formation to achieve carbon dioxide recycling and zero emission.

[0035] Furthermore, when the sewage in the desulfurization tank, centrifugal water removal tank, filter, screw compressor, freeze dryer, micro-heat regeneration adsorption dryer, filter dehydrator and four-stage compression piston machine reaches the liquid level monitoring point, the control system can control the stop valve to automatically discharge the waste liquid into the sewage tank through the sewage pipeline for unified treatment.

[0036] The beneficial effects brought by the present invention are: The wellhead gas circulation injection system of the present invention can be used for processing the mixed gas generated by CCUS-EOR technology. It can recycle, purify and pressurize the wellhead gas generated after the implementation of CCUS-EOR technology and then recycle it into the well, not only realizing 100% recycling of carbon dioxide in the mixed gas, but also realizing supercritical pressurization of carbon dioxide (28-30MPa, 60℃±10℃ for carbon dioxide to reach supercritical state), and recycle it into the wellhead, finally realizing "zero" emission of carbon dioxide in oilfield development, solving the problem of environmental pollution caused by direct discharge of wellhead gas into the air in the prior art, and avoiding the waste of energy.

[0037] The four-stage piston compressor independently developed by the present invention can prevent the condensation of gaseous carbon dioxide into liquid, thereby preventing the discharge of liquid carbon dioxide from the gas-liquid separator, thereby ensuring zero loss of CO2 gas in the mixed gas, realizing 100% recycling of CO2, and achieving zero emission of CO2 in the external environment, which is in line with the concept of environmental protection.

[0038] At the same time, the wellhead gas circulation injection system of the present invention has various control modes, which solves the problem that the traditional unit has a single control mode, is prone to control failure accidents, and causes serious damage to the piston compressor. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0040] Figure 1A schematic diagram of the framework of the circulation injection system; Figure 2 It is a structural diagram of the circulating injection system; Figure 3 It is a structural schematic diagram of the desulfurization tank; Figure 4 It is a structural schematic diagram of a centrifugal water removal tank; Figure 5 It is a schematic diagram of the funnel-type sieve plate structure; Figure 6 It is a schematic diagram of the filter structure; Figure 7 It is the process flow chart of the pre-pressurization unit and the cooling and water removal unit; Figure 8 This is the process flow chart of the boost injection unit; Fig. 9 This is the process flow chart of the lubricating oil circuit of the four-stage piston compressor.

[0041] In the figure: Desulfurization tank: 11-gas inlet, 12-alkali solution inlet, 13-gas outlet, 14-pressure instrument, 15-temperature instrument, 16-sieve plate, 17-filler, 18-retention plate I, 19-alkali solution spray nozzle; Centrifugal dewatering tank: 21-gas inlet, 22-gas outlet, 23-funnel-type sieve plate, 24-retention plate II, 25-liquid level detection meter; Filter: 31-gas inlet, 32-gas outlet, 33-filler injection port, 34-perforated support plate, 35-activated carbon adsorption layer, 36-inorganic silicon adsorption layer, 37-retention plate III. DETAILED DESCRIPTION

[0042] The following will be combined with the embodiments of the present invention and the accompanying drawings to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] Example 1 Reference Figure 1 , Figure 2 The oilfield wellhead gas circulation injection system of this embodiment includes a purification unit, a pre-pressurization unit, a cooling and water removal unit, a pressurization injection unit and a control system; wherein The purification unit includes a desulfurization tank, a centrifugal water removal tank, and a filter that are interconnected; The pre-boosting unit comprises a venting pipeline and an air intake main pipeline arranged in parallel, and the air intake main pipeline is provided with a gas buffer tank, a variable frequency motor, a screw compression cylinder, and an aftercooler; The cooling and water removal unit includes a centrifugal water remover, a cold dryer, and a micro-heat regeneration adsorption dryer; The boost injection unit includes a filter dewatering device, a variable frequency motor, and a four-stage piston compressor; The control system includes a standard electrical control unit and a remote monitoring unit.

[0044] As a preferred embodiment, the output gas enters from the gas inlet 11 of the desulfurization tank of the purification unit, which is used to remove impurities such as hydrogen sulfide in the output gas. The gas outlet 13 of the desulfurization tank is connected to the gas inlet 21 of the centrifugal dehydration tank, and the gas outlet 22 of the centrifugal dehydration tank is connected to the gas inlet 31 of the filter; the gas outlet 32 ​​of the filter is connected to the gas inlet of the gas buffer tank of the pre-boosting unit.

[0045] Reference Figure 3 The desulfurization tank adopts the "sieve plate + alkali + filler" treatment process to remove the hydrogen sulfide component in the wellhead gas. The equipment is simple, easy to operate, high purification degree, large sulfur capacity, low desulfurization cost, desulfurization efficiency ≥ 90%, and no secondary pollution.

[0046] The lower end of the desulfurization tank is provided with a gas inlet 11, the top end is provided with an alkali liquid inlet 12 and an alkali liquid spray nozzle 19, and the right side of the alkali liquid inlet 12 is provided with a gas outlet 13 which also serves as a filler injection port; the upper end of the desulfurization tank is provided with a pressure meter 14 and a temperature meter 15, and the lower end is provided with a liquid level detection meter, and the pressure meter 14, the temperature meter 15, and the liquid level detection meter are connected to the control system through the PIC; A sieve plate 16 with through holes is installed at the lower end of the gas inlet 11 on the inner wall of the desulfurization tank, and the sieve plate 16 is evenly filled with fillers 17. A retention plate 118 is also installed in the inner wall of the desulfurization tank to support the filler 17 and slow down the gas flow rate, form gas turbulence, promote gas-liquid mass transfer, and achieve desulfurization of the output gas.

[0047] The desulfurization principle of the desulfurization tank is as follows: the alkali liquid sprayed down from the top is fully in contact with the hydrogen sulfide-containing gas flowing up from the bottom of the tower. Under the synergistic effect of the alkali liquid and the filler 17, the hydrogen sulfide in the gas is oxidized and reduced to sulfide salts and settles to the waste liquid area at the bottom of the tower with the alkali liquid for storage. When the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system.

[0048] Preferably, the filler 17 is glassy iron oxyhydroxide, which is used to remove hydrogen sulfide from the produced gas; The alkaline solution is preferably sodium hydroxide or potassium hydroxide, which is used to absorb hydrogen sulfide in coordination with glassy iron oxyhydroxide.

[0049] Reference Figure 4 A gas inlet 21 is provided in the middle of the centrifugal dewatering tank, a gas outlet 22 is provided at the top, a pressure meter and a temperature meter are installed at the upper end, and a liquid level detection meter 25 is installed at the lower end. The pressure meter, temperature meter and liquid level detection meter 25 are connected to the control system through PLC.

[0050] The inner wall of the centrifugal dewatering tank is provided with a funnel-type sieve plate 23, such as Figure 5 As shown, the funnel-shaped sieve plate 23 is evenly provided with through holes, through which gas can enter; when the output gas enters from the gas inlet 21, under the centrifugal rotation of the funnel-shaped sieve plate 23, the gas enters the through holes along the tangent of the cross section of the funnel-shaped sieve plate 23, and performs a spiral upward motion in the sieve plate, and the gas and liquid are separated according to the cyclone separation effect, and then the waste liquid is condensed and gathered along the wall of the funnel-shaped sieve plate 23 by relying on the gravity of the liquid, and flows into the bottom of the funnel-shaped sieve plate 23 for collection; A conical intercepting plate II24 is installed at the upper end of the inner wall of the centrifugal dewatering tank. The conical intercepting plate II24 has through holes evenly distributed on it, which is used to intercept the rising gas and slow down the gas flow rate, so that the residual liquid in the gas condenses and flows along the tank wall and the funnel-type sieve plate 23 into the bottom arc-shaped wastewater collection area of ​​the centrifugal dewatering tank for storage. When the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system.

[0051] Reference Figure 6 The filter adopts the "pressurization + active composite inorganic adsorption" treatment process, selects "activated carbon + inorganic silicon" filler, takes advantage of pore structure and specific surface area, utilizes capillary action and surface tension to form a physical-chemical adsorption structure, removes heavy hydrocarbons, and prevents the injected medium from being enriched and blocked after the purification unit.

[0052] The filter is provided with a gas inlet 31 in the middle, a gas outlet 32 ​​and a filler injection port 33 at the top, a pressure meter and a temperature meter are installed at the upper end, and a liquid level detection meter is installed at the lower end. The pressure meter, temperature meter and liquid level detection meter are connected to the control system through PLC.

[0053] A removable two-layer perforated support plate 34 is installed at the lower end of the filter, and the middle of the perforated support plate 34 is filled with an activated carbon adsorption layer 35, which is used to preliminarily remove C4 and above heavy hydrocarbon components in the output gas; the pores of the perforated support plate 34 are used to discharge the waste liquid condensed in the filler into the arc space at the lower end of the filter for storage. When the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system.

[0054] The upper area of ​​the activated carbon adsorption layer 35 is filled with an inorganic silicon adsorption layer 36, and retention plates III37 are symmetrically and staggeredly installed on the inner wall of the filter where the inorganic silicon adsorption layer 36 is located, which are used to cause gas turbulence and block the straight-through rise of the gas, so that the gas can fully contact with the inorganic silicon adsorption layer 36 to adsorb and remove impurities such as heavy hydrocarbons in the gas, thereby avoiding enrichment and blockage of the output gas in the subsequent treatment process.

[0055] Preferably, the perforated support plate 34 is detachable, and when the activated carbon adsorption layer 35 is saturated with adsorption, the window on the side wall of the filter can be opened to disassemble the perforated support plate 34, remove the saturated activated carbon adsorption layer 35, and add filler again.

[0056] Preferably, the height ratio of the inorganic silicon adsorption layer 36 to the activated carbon adsorption layer 35 is 2-3:1. Under this ratio, the effect of adsorbing and removing impurities such as heavy hydrocarbons is best.

[0057] Reference Figure 7 The air intake main line of the pre-boost unit is connected to a release line through a stop valve, which can be released with one button in an emergency; The main air intake line is connected to the branch line I through the stop valve BV-1, and is connected to the branch line II through the stop valves BV-2 and BV-3, so as to realize two-stage pipeline diversion. In case of emergency, the branch line BV-1 is opened.

[0058] Preferably, a total pressure detection gauge, a total intake temperature detection gauge and a total intake solenoid valve are installed on the main intake pipe, and the total intake solenoid valve is connected to a gas buffer tank through a pipeline; A pressure detection gauge and a safety valve are installed at the upper end of the gas buffer tank, and a liquid level detection gauge is installed at the lower end. The safety valve is connected to the release pipeline. A drainage pipeline is provided at the lower end of the gas buffer tank, and a stop valve is provided on the drainage pipeline to realize the discharge of waste liquid.

[0059] The gas outlet of the gas buffer tank is connected with an air filter tank, and a screw front air intake solenoid valve is arranged at the front end of the pipeline connected with the air filter tank. A branch pipeline III is arranged at the front end pipeline of the screw front air intake solenoid valve. The branch pipeline III is connected with the air filter tank gas outlet pipeline through a stop valve BV-4 and is collected into a main pipeline. Here, the main pipeline is connected with a variable frequency motor, and the gas outlet of the variable frequency motor is connected with a screw compression cylinder through a pipeline. The gas outlet of the screw compression cylinder is connected with a minimum pressure valve through a pipeline, and the outlet of the minimum pressure valve is connected with an aftercooler through a pipeline. The aftercooler realizes gas cooling through a fan, and the gas outlet of the aftercooler is connected with a centrifugal water remover through a pipeline. A safety valve is provided at the upper end of the screw compression cylinder, and the safety valve is connected to the release pipeline; a discharge pipeline is provided at the lower end of the screw compression cylinder, and a stop valve is provided on the discharge pipeline to realize the discharge of waste liquid; The oil outlet of the screw compression cylinder is connected to an oil filter through an oil pipeline, the oil filter is connected to an oil cooler through a pipeline, and the oil cooler is connected to the oil inlet of the variable frequency motor through a pipeline to achieve oil cooling of the screw compression cylinder and the variable frequency motor.

[0060] In this embodiment, the screw compression cylinder is mainly used for the preliminary pressurization of the wellhead produced gas to provide power for subsequent processing. The screw compression cylinder has a wide range of application for air intake, ranging from 0-0.7MPa, and the state of the gas-liquid components does not affect the operation of the booster; the exhaust pressure is high, and the maximum pressure can reach 2.4MPa, which can reduce the workload of the back-end unit. The screw compression cylinder adopts adaptive frequency conversion motor adjustment to save energy and meet the requirements of different gas volumes. The screw compression cylinder uses a twin-screw compression cylinder with the characteristics of oil lubrication, integrated design, and simple maintenance. The booster head is composed of an air filter, an intake control valve, a rotor, an oil filter, a temperature control valve, and a minimum pressure valve. It is equipped with a variable frequency three-phase asynchronous motor to provide power for the screw compression cylinder, and the variable frequency speed regulation is 15-65 Hz. At the same time, the screw compression cylinder is also equipped with an air cooler. The cooling system of the air cooler is designed according to the ambient temperature of -20℃~+40℃, which can ensure that the screw compressor can work effectively under complex climatic conditions; the radiator temperature of the screw machine and piston machine is controlled by the variable frequency motor fan, so that the temperature of the exhaust wellhead gas is 15℃ higher than the ambient temperature. The cylinder cooling design and efficient air-gas interstage cooling system are used to reduce the wear of the piston ring. The cooling system adopts a vertical structure as a whole, which is placed at one end of the skid body, saving the height of the skid body, facilitating transportation, and having extremely strong maneuverability.

[0061] Reference Figure 7 , the gas outlet of the aftercooler of the pre-boosting unit is connected to the gas inlet of the centrifugal dehumidifier through a pipeline, the gas outlet of the centrifugal dehumidifier is connected to the gas inlet of the cold dryer, a temperature detection meter is provided on the pipeline between the centrifugal dehumidifier and the cold dryer, the gas outlet of the cold dryer is connected to the A tank and the B tank of the micro-heat regeneration adsorption dryer through a pipeline, the pipeline between the A tank and the cold dryer is provided with a first valve, the pipeline between the B tank and the cold dryer is provided with a second valve, and the main pipeline before the air inlet pipeline of the A tank, the B tank and the gas buffer tank is connected through a third valve; The main pipelines of the micro-heat regeneration adsorption dryer and the cold dryer are provided with branch pipelines, on which stop valves are installed. The branch pipelines are connected with the gas outlet pipelines of tanks A and B and then merged into the main pipeline, where a total pressure flow detection instrument is installed.

[0062] Preferably, the freeze dryer is mainly used for deep water removal of wellhead gas. The freeze dryer quickly freezes the gas to be dried, and in a proper vacuum environment, the frozen water molecules are directly sublimated into water vapor and escape. After the water vapor exchanges heat with the air, it enters the waste liquid discharge system. The atmospheric pressure dew point of the wellhead gas treated by the freeze dryer reaches -20°C.

[0063] Preferably, the micro-heat regeneration adsorption dryer is mainly used to further dehydrate and dry the wellhead gas. The micro-heat regeneration adsorption dryer performs deep dehydration by adsorption, and the gas source after adsorption dehydration is in a dry state, which meets the subsequent main compressor operating conditions. The atmospheric pressure dew point of the wellhead gas treated by the micro-heat regeneration adsorption dryer reaches -40 to -50°C.

[0064] Reference Figure 8 , Fig. 9 The four-stage piston compressor is mainly used to pressurize the wellhead gas to 28-30MPa and then inject it into the formation, including the following connected in sequence: First-stage air intake steam-water separator, first-stage air intake buffer tank, first-stage piston compression cylinder, first-stage exhaust buffer tank, first-stage air cooler, second-stage air intake steam-water separator, second-stage air intake buffer tank, second-stage piston compression cylinder, second-stage exhaust buffer tank, second-stage air cooler, third-stage air intake steam-water separator, third-stage air intake buffer tank, third-stage piston compression cylinder, third-stage exhaust buffer tank, third-stage air cooler, fourth-stage air intake steam-water separator, fourth-stage air intake buffer tank, fourth-stage piston compression cylinder, fourth-stage exhaust buffer tank, fourth-stage air cooler; The gas outlet of the four-stage air cooler is connected to an exhaust pipeline; The lower ends of the secondary air intake steam-water separator, the tertiary air intake steam-water separator and the quaternary air intake steam-water separator are provided with electric heating reboilers.

[0065] Specifically, the right side gas outlet of the first-stage air intake steam-water separator is connected to the gas inlet of the first-stage air intake buffer tank through a pipeline, the gas outlet of the first-stage air intake buffer tank is connected to the gas inlet of the first-stage piston compression cylinder through a pipeline, the gas outlet of the first-stage piston compression cylinder is connected to the gas inlet of the first-stage exhaust buffer tank through a pipeline, the gas outlet of the first-stage exhaust buffer tank is connected to the gas inlet of the first-stage air cooler through a pipeline, the gas outlet of the first-stage air cooler is connected to the gas inlet of the second-stage air intake steam-water separator through a pipeline, the gas outlet of the second-stage air intake steam-water separator is connected to the gas inlet of the second-stage air intake buffer tank through a pipeline, the gas outlet of the second-stage air intake buffer tank is connected to the gas inlet of the second-stage piston compression cylinder through a pipeline, the gas outlet of the second-stage piston compression cylinder is connected to the gas inlet of the second-stage exhaust buffer tank through a pipeline, the gas outlet of the second-stage exhaust buffer tank is connected to the gas inlet of the second-stage air cooler through a pipeline, and the gas outlet of the second-stage air cooler is connected to the third-stage air intake The gas inlet of the steam-water separator and the gas outlet of the three-stage air intake steam-water separator are connected to the gas inlet of the three-stage air intake buffer tank through a pipeline, the gas outlet of the three-stage air intake buffer tank is connected to the gas inlet of the three-stage piston compression cylinder through a pipeline, the gas outlet of the three-stage piston compression cylinder is connected to the gas inlet of the three-stage exhaust buffer tank through a pipeline, the gas outlet of the three-stage exhaust buffer tank is connected to the gas inlet of the three-stage air cooler through a pipeline, the gas outlet of the three-stage air cooler is connected to the gas inlet of the four-stage air intake steam-water separator through a pipeline, the gas outlet of the four-stage air intake steam-water separator is connected to the gas inlet of the four-stage air intake buffer tank through a pipeline, the gas outlet of the four-stage air intake buffer tank is connected to the gas inlet of the four-stage piston compression cylinder through a pipeline, the gas outlet of the four-stage piston compression cylinder is connected to the gas inlet of the four-stage exhaust buffer tank through a pipeline, the gas outlet of the four-stage exhaust buffer tank is connected to the gas inlet of the four-stage air cooler through a pipeline, and the gas outlet of the four-stage air cooler is connected to the exhaust pipeline.

[0066] Furthermore, the gas inlet of the filter dehydrator is connected to the air intake pipe, and the gas outlet is connected to the inlet of the first-stage air intake steam-water separator through a pipeline. A first-stage suction pressure gauge is provided on the left side of the upper end of the first-stage air intake steam-water separator, and a safety valve SV-2 is provided on the upper end of the first-stage air intake steam-water separator, and the safety valve SV-2 is connected to a release pipeline; The first-stage exhaust buffer tank is provided with a first-stage exhaust pressure gauge and a first-stage exhaust temperature gauge, and the air intake pipeline of the first-stage air cooler is provided with a safety valve SV-3, which is connected to the vent pipeline; The secondary exhaust buffer tank is provided with a secondary exhaust pressure gauge and a secondary exhaust temperature gauge, and the air intake pipeline of the secondary air cooler is provided with a safety valve SV-4, which is connected to the vent pipeline; The three-stage exhaust buffer tank is provided with a three-stage exhaust pressure gauge and a three-stage exhaust temperature gauge, and the air intake pipeline of the three-stage air cooler is provided with a safety valve SV-5, which is connected to the vent pipeline; The four-stage exhaust buffer tank is provided with a four-stage exhaust pressure gauge and a four-stage exhaust temperature gauge, and the air intake pipeline of the four-stage air cooler is provided with a safety valve SV-6, which is connected to the vent pipeline.

[0067] Furthermore, a main gas production valve is installed on the exhaust pipeline, and the exhaust pipeline located on the left side of the main gas production valve is also connected to the release pipeline through a main release valve; By controlling the safety valves on each component through the main relief valve, the boost injection unit (four-stage piston compressor) can be released with one button in an emergency (compressor shutdown), so that the residual gas in the four-stage piston compressor will be released to the safe area outside the skid through the relief pipe, ensuring the no-load start-up of the four-stage piston compressor and reducing the power consumption and maintenance rate of the equipment.

[0068] In this embodiment, the four-stage piston compressor is compressed in four stages and pressurized step by step. A gas buffer tank is set between each stage of the four stages of the compressor to achieve gas pulsation buffering. Through the cooling of the air cooler and the cooperation of the electric heating reboiler of the steam-water separator, the gas temperature in the gas-liquid separator reaches a dynamic balance, so that the gas temperature is maintained in the range of 35~65℃. This temperature range can ensure the complete gasification of the liquefied CO2 in the gas-liquid separator without gasifying the water in the liquid, so as to achieve the separation of liquefied CO2 and water, thereby avoiding the discharge of liquefied CO2 from the gas-liquid separator, and then ensuring the zero loss of CO2 gas medium entering the oil field wellhead gas circulation injection system, and achieving 100% recycling and zero emission of CO2. The liquid without liquid carbon dioxide is stored in the waste liquid area of ​​the steam-water separator. When the gas temperature in the gas-liquid separator is above 35℃ and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system.

[0069] Safety valves are installed on each component of the four-stage piston compressor. The safety valves on each component are controlled by the total relief valve to achieve one-button relief of the four-stage piston compressor in an emergency. The frequency conversion adjustment of the motor can save energy and meet the requirements of different gas volumes. The piston compressor adopts a horizontally symmetrical balanced reciprocating piston compressor. The cylinder adopts an oil-free structure, a low-oil lubrication design, and forced lubrication of the crankshaft, connecting rod, crosshead, etc. A variable frequency three-phase asynchronous motor is equipped to provide power for the piston compressor, and the frequency conversion speed regulation is 15-65 Hz. The compressor and the motor are connected by an elastic coupling to achieve the best alignment accuracy, automatically compensate for axial displacement, and eliminate the potential hidden dangers of friction static electricity to explosion-proof equipment.

[0070] Furthermore, the second, third and fourth stage steam-water separators are equipped with electric heating reboilers, which can gasify the carbon dioxide condensed in the liquid of the gas-liquid separator and avoid the discharge of liquefied CO2 from the gas-liquid separator, thereby ensuring zero loss of the CO2 gas medium entering the oil field wellhead gas circulation injection system, and achieving 100% recovery and zero emission of CO2. This is because: the wellhead gas rich in carbon dioxide will undergo a phase change at a temperature below 31°C to produce liquid carbon dioxide. If the liquefied CO2 is not heated and gasified, it will be discharged with the waste liquid in the gas-liquid separator, thus failing to achieve 100% recovery and utilization of carbon dioxide gas.

[0071] Therefore, an electric heating reboiler is installed between the second, third and fourth stage steam-water separators of the piston compression cylinder. The electric heating reboiler of the second, third and fourth stage steam-water separators is automatically started by the control system, and the second, third and fourth stage air coolers are closed, so that the electric heating reboiler and the air cooler work together, so that the temperature in the gas-liquid separator is kept in the dynamic equilibrium range of 35~65℃. In this temperature range, the liquefied CO2 liquefied in the gas-liquid separator can be completely gasified without gasifying the water in the liquid, so as to achieve the separation of liquefied CO2 and water, and avoid the discharge of liquefied CO2 from the gas-liquid separator, thereby ensuring zero loss of CO2 gas medium entering the oilfield wellhead gas circulation injection system, and achieving 100% recovery and utilization of CO2 and zero emission.

[0072] The purpose of keeping the working temperature of the second-stage electric heating reboiler of this embodiment consistent with the outlet air temperature of the first-stage air cooler, the working temperature of the third-stage electric heating reboiler consistent with the outlet air temperature of the second-stage air cooler, and the working temperature of the fourth-stage electric heating reboiler consistent with the outlet air temperature of the third-stage air cooler is to perform thermal compensation for the air cooler through the electric heating reboiler so that the temperature is fully maintained in the dynamic equilibrium range of 35~65℃.

[0073] As a preferred embodiment, the control process of the boost injection unit is: The gas (medium) pressure is 1.0-2.5MPa and the temperature is 40℃±5℃ (obtained through real-time monitoring). It enters the filter dehumidifier through the air inlet pipe. After filtering and dehydration, the dew point at normal pressure reaches -45℃±5℃. The dew point parameter design basis is: In order to adapt to the winter environment (-35℃~-30℃) of the field application, the dew point is set to 45℃±5℃ according to the parameter redundancy design principle; The gas enters the first-stage air inlet steam-water separator for gas-liquid separation, and the waste liquid is stored in the waste liquid area of ​​the first-stage air inlet steam-water separator. When the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system; After gas-liquid separation, the gas enters the first-stage air intake buffer tank through the pipeline to reduce the impact of the gas on the piston compression cylinder, with a pressure difference of 0-0.1MPa; Then it enters the first-stage piston compression cylinder, and after pressurization, the pressure is 3-3.5MPa and the temperature is 140℃±10℃ (the actual measured temperature after the compression cylinder works); After being compressed by the first-stage piston, the gas enters the first-stage exhaust buffer tank for gas buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; The gas then enters the first-stage air cooler, which is used to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the first-stage air cooler is 3-3.5MPa and the temperature is 35-55℃; The gas cooled by the primary air cooler enters the secondary air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the secondary air intake steam-water separator is monitored in real time in the control system: If the control system detects that the temperature of the gas in the secondary air intake separator is less than 35°C, this indicates that carbon dioxide may be liquefied in the secondary air intake separator, causing liquefied carbon dioxide to enter the liquid. In order to avoid the loss of carbon dioxide, the control system automatically shuts down the primary air cooler through the programmable controller, and automatically starts the electric heating reboiler of the secondary air intake separator to raise the temperature to 35-55°C, so as to gasify the carbon dioxide condensed in the liquid, so that 100% concentration carbon dioxide gas can enter the next stage of equipment; Otherwise, the control system does not perform the corresponding control action; The liquid that does not contain liquid carbon dioxide in the secondary air intake steam-water separator is stored in its waste liquid area. When the gas temperature in the gas-liquid separator is above 35°C and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system; After the gas enters the secondary air intake buffer tank through the pipeline for buffering, the pressure difference is 0-0.1MPa to reduce the impact on the secondary piston compression cylinder; After being buffered, the gas enters the secondary piston compression cylinder for compression, and the pressure after pressurization is 7.5-8.5MPa and the temperature is 140℃±10℃; The buffered gas enters the secondary exhaust buffer tank for buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; The gas then enters the secondary air cooler to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the secondary air cooler is 7.5-8.5MPa and the temperature is 40-60℃. The gas cooled by the secondary air cooler enters the third-stage air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the third-stage air intake steam-water separator is monitored in real time in the control system: If the control system detects that the gas temperature is less than 35°C, it indicates that carbon dioxide may be liquefied in the three-stage air inlet steam-water separator, causing liquefied carbon dioxide to enter the liquid. In order to avoid the loss of carbon dioxide, the control system automatically shuts down the secondary air cooler through the programmable controller, and automatically starts the electric heating reboiler of the three-stage air inlet steam-water separator to heat up to 40-60°C, so as to gasify the carbon dioxide condensed in the liquid and achieve 100% concentration of carbon dioxide gas entering the next stage of equipment; Otherwise, the control system does not perform the corresponding control action; The liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the three-stage air intake steam-water separator. When the gas temperature in the gas-liquid separator is above 35°C and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system; After the gas enters the three-stage air intake buffer tank for buffering, the pressure difference is 0-0.1MPa to reduce the impact on the three-stage piston compression cylinder; The gas after buffering enters the three-stage piston compression cylinder for compression. After pressurization, the pressure is 14-16MPa and the temperature is 140℃±10℃; The gas then enters the third-level exhaust buffer tank for buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; Then it enters the third-stage air cooler to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the air cooler is 14-16MPa and the temperature is 45-65℃. The gas cooled by the three-stage air cooler enters the four-stage air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the four-stage air intake steam-water separator is monitored in real time in the control system: If the control system detects that the gas temperature is less than 35°C, this indicates that carbon dioxide may be liquefied in the four-stage air inlet steam-water separator, causing liquefied carbon dioxide to enter the liquid. In order to avoid the loss of carbon dioxide, the control system automatically shuts down the three-stage air cooler through the programmable controller, and automatically starts the electric heating reboiler of the four-stage air inlet steam-water separator to heat it to 45-65°C, so as to gasify the carbon dioxide condensed in the liquid and achieve 100% concentration of carbon dioxide gas entering the next stage of equipment; If the control system detects that the temperature of the gas is within the range of 45°C-65°C, this indicates that no liquefaction of carbon dioxide will occur in the four-stage inlet gas-water separator, and the control system will not perform corresponding control actions, will not start the electric heating reboiler of the four-stage inlet gas-water separator through the programmable controller, and will not automatically shut down the three-stage air cooler; The liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the four-stage air intake steam-water separator. The gas temperature in the four-stage air intake steam-water separator is greater than 35°C, and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system; The gas enters the four-stage air intake buffer tank for buffering, with a pressure difference of 0-0.1MPa to avoid excessive impact on the piston compression cylinder; The gas after buffering enters the four-stage piston compression cylinder for compression, and the pressure after pressurization is 28-30MPa and the temperature is 140℃±10℃; The gas then enters the fourth-stage exhaust buffer tank for buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; Then it enters the four-stage air cooler to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the four-stage air cooler is 28-30MPa and the temperature is 60℃±10℃ (the outlet temperature can be adjusted according to production conditions, and the lowest outlet temperature is ambient temperature + 10℃). At this time, the pressure of the carbon dioxide gas medium is 28-30MPa and the temperature is 60℃±10℃, reaching a supercritical state. The gas is injected into the lower-level equipment through the total gas production valve on the exhaust pipeline to achieve supercritical injection of carbon dioxide.

[0074] It should be noted that, according to common knowledge, the CO2 phase diagram is in the gas phase when the temperature is higher than the critical temperature of 31°C. The optimal temperature range of the air cooler of the present invention under ideal conditions is set to 35~65°C. In theory, this temperature range can ensure that the CO2 in the gas medium always exists in a gaseous state, but in actual operation, the air cooler cannot accurately control the temperature of the gas medium to a constant value (35-65°C); and under the influence of environmental factors such as winter, there is heat loss between the pipelines and equipment between the air cooler and the steam-water separator, resulting in the actual temperature of the gas after cooling being lower than the preset temperature of the air cooler (35-65°C). For example, when the ambient temperature is too cold, the actual temperature of the gas after cooling is lower than the critical temperature of CO2, 31°C, resulting in the liquefaction of the gas CO2, so that the liquefied CO2 enters the waste liquid area of ​​the gas-liquid separator along with the condensed liquid. At this time, if the liquefied CO2 waste liquid is not reboiled, the liquefied CO2 will be discharged from the gas-liquid separator along with the waste liquid, thereby failing to achieve 100% recovery and utilization of carbon dioxide. In order to adapt to the changes in the external environment, the monitoring temperature is set to 35°C in this embodiment, which is higher than the critical temperature of 31°C, to ensure that the monitoring is free of deviation and the monitoring results are accurate and reliable. The monitoring temperature can be set to any value between 31°C and 35°C.

[0075] The specific principle of the air cooler and the electric heating reboiler of the present invention working together to control the CO2 gas medium to be always in the gaseous state is as follows: a temperature sensor is provided between each air inlet steam-water separator of the four-stage piston compressor, and the temperature sensor transmits the temperature data to the programmable controller (PLC); the programmable controller monitors the gas temperature in the steam-water separator, thereby automatically shutting down the air cooler in the four-stage piston compressor, and starting the reboiler to heat to a temperature of 35-65°C, so as to completely gasify the liquefied CO2 liquefied in the gas-liquid separator, and avoid the gasification of water, so as to achieve the CO2 gas medium to be always in the gaseous state. For example, when the control system monitors that the gas temperature in the steam-water separator is less than 31°C, that is, when the condition of CO2 liquefaction is reached, the air cooler is automatically shut down by the control system, and the heating reboiler is automatically started to vaporize the CO2 liquefied in the waste liquid, so that the CO2 always enters the next stage piston compression cylinder in the state of gas medium, thereby achieving 100% recovery and utilization of CO2 and zero emission.

[0076] For waste liquid discharge, the waste liquid collected in the secondary, tertiary and quaternary gas-water separators is stored in the waste liquid area. The discharge conditions of the waste liquid must meet the following conditions: the gas temperature in the gas-liquid separator is above 35°C, and the waste liquid reaches the liquid level monitoring point. Both are indispensable.

[0077] For example, when the temperature in the secondary, tertiary, and quaternary steam-water separators is greater than 35°C, it indicates that the waste liquid does not contain liquid carbon dioxide, and the waste liquid can only be automatically discharged through the control system when it reaches the liquid level monitoring point. When the temperature of the gas in the steam-water separator in the secondary, tertiary, and quaternary steam-water separators is less than 31°C, the control system will not automatically discharge the waste liquid even if the waste liquid reaches the liquid level monitoring point. By setting this condition, it is possible to avoid the waste liquid containing liquefied carbon dioxide from being discharged from the gas-liquid separator without reboil treatment, thereby failing to achieve "zero" emissions of carbon dioxide.

[0078] The waste liquid in the first-stage air intake steam-water separator is stored in the waste liquid area of ​​the first-stage air intake steam-water separator. It does not need to strictly control the temperature at 35°C. When the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system.

[0079] It should be noted that the wellhead gas circulation injection system for oil fields in this embodiment is equipped with an intake valve group to control the wellhead gas on and off, and balance the intake pressure before entering the circulation injection system. The intake valve group includes a relief valve, a pressure regulating and stabilizing valve, etc., to achieve emergency shutdown relief and overpressure relief functions, and adjust the inlet pressure at the same time to make the pressure run in a relatively stable state. When the compressor stops, the residual gas in the circulation injection system will be released to the safe area outside the skid through the relief pipe, ensuring that the compressor starts without load, greatly reducing the power consumption and maintenance rate of the equipment.

[0080] In this embodiment, the wellhead gas circulation injection system for oil fields is equipped with a control system, including a standard electrical control unit and a remote monitoring unit.

[0081] Furthermore, the standard electrical control unit includes a programmable controller (PLC) and a control panel, and the control panel is provided with a liquid crystal display window for displaying or querying setting values, fault alarms and operating status.

[0082] The standard electrical control unit is based on PLC for central control of the circulating injection system. Working instruments are attached to each equipment component. Instrument signals: 4-20mA, RTD. The PLC control cabinet is installed outside the skid. The cabinet has a front door. It has flexible control, high automation, reliable performance, fault self-diagnosis, display and other functions. It is highly integrated. The skid is equipped with an instrument local display panel to facilitate intuitive display of necessary operating parameters and control the local start and stop of the unit.

[0083] Based on this, the standard electrical control unit can monitor and control the operation process, emergency faults and equipment start-up and shutdown of the entire circulating injection system through a programmable controller; automatically monitor abnormal conditions (compressor abnormalities, residual gas in the circulating injection system), automatically display alarm signals and shut down; enable the gas injection system to have fault diagnosis, recording and self-elimination functions, and when the fault is eliminated, the compressor automatically resumes work.

[0084] Furthermore, the remote control unit is connected to a remote computer through an RTD input card, a communication card, and an expansion slot reserved on a standard electrical control unit to achieve remote data transmission and control.

[0085] Information is obtained from the PLC controller of the standard electrical control unit through a remote computer, and all operating parameters monitored by the PLC controller are displayed on the remote computer, thereby realizing real-time monitoring of the operating status of each device in the circulating injection system; the previous operating parameter records, fault diagnosis and troubleshooting parameter records of the circulating injection system can also be viewed on the remote computer, providing better technical support for the operator and reducing equipment downtime, maintenance time and personnel costs.

[0086] Preferably, the remote computer is equipped with a reporting unit, which has the function of recording operating parameters, with a recording time period greater than 9999 hours, and real-time display of the pulsation curves of the import and export pipelines of the circulation injection system, the inside of the booster unit and the entire process pipeline. The remote computer is also equipped with a "Gas Injection Technology Service Platform" to achieve paperless network reporting, and can generate analysis reports for the recorded data, so that the information is always available.

[0087] Preferably, the control system sends an ESD shutdown command to the PLC on each device and pipeline, so that the valve body ball valve rotates to the closed position, cuts off the gas source, and realizes the automatic control of the circulation injection system. A manual ESD button is provided on the control system. When the equipment is running, an emergency occurs in the surrounding environment or other situations require manual shutdown of the equipment, the equipment can be manually shut down to cut off the gas circuit and power supply.

[0088] In the preferred embodiment, the electrical and instrument wiring of the oilfield wellhead gas circulation injection system, all cables and electrical accessories are explosion-proof and isolated. Each box tray and source extraction component meets the explosion-proof and protection safety level requirements, all boxes have prefabricated holes, and are equipped with stainless steel explosion-proof sealing joints, stainless steel explosion-proof flexible pipes and stainless steel wire plugs for wiring.

[0089] In a preferred embodiment, carbon dioxide, hydrogen sulfide, and methane gas detectors, alarms, and ventilation systems are installed on the skid of the oilfield wellhead gas circulation injection system; when the gas concentration in the skid exceeds the standard set value, the alarm sounds and the compressor power and gas circuit are cut off.

[0090] The standard electrical control unit of the present invention monitors and controls the operation process, emergency failures and equipment start-up and shutdown of the entire circulation injection system through a programmable controller (PLC), and automatically monitors abnormal conditions. For example, when the programmable controller (PLC) monitors compressor abnormalities or residual gas in the circulation injection system, the programmable controller will automatically display an alarm signal, and the control system sends an ESD shutdown command to the PLC on each device and pipeline, so that the valve body ball valve rotates to the closed position, cuts off the gas source, realizes automatic control of the circulation injection system and shuts down, and realizes fault diagnosis, recording and self-elimination functions. When the fault is eliminated, the compressor automatically resumes work.

[0091] The control system is also equipped with a manual ESD button. When the equipment is running, if an emergency occurs in the surrounding environment or other situations require manual stopping of the equipment, the equipment can be manually stopped to cut off the gas and power supply.

[0092] Example 2 This embodiment provides a circulating injection process applied to the oilfield wellhead gas circulating injection system of embodiment 1, and the circulating injection process is: The wellhead gas first enters the desulfurization tank to remove hydrogen sulfide in the gas, and then enters the centrifugal dewatering tank along the cutting groove for preliminary gas-liquid phase separation. The gas enters the filter to filter and remove C4 and above heavy hydrocarbon components in the gas. The gas is then sent to the screw compression cylinder for initial pressure increase to 0.5-1.0MPa, and then sent to the freeze dryer to make the atmospheric pressure dew point of the gas reach -20±5℃, and then deep dehydration is carried out by the micro-heat regeneration adsorption dryer to reduce the dew point temperature of the gas medium to -40±5℃. The gas is then sent to the filter dehydrator for further dehydration, and then sent to the four-stage piston compressor to increase the pressure to 28-30MPa, and then the gas (the concentration of carbon dioxide is 100%) is injected into the formation to achieve the recycling of carbon dioxide.

[0093] Furthermore, the sewage in the desulfurization tank, centrifugal water removal tank, filter, screw compressor, freeze dryer, micro-heat regeneration adsorption dryer, filter dehydrator and four-stage compression piston machine is concentrated in the sewage tank through the sewage pipeline for unified treatment.

[0094] The technical specifications of the system are shown in Table 1: Table 1

[0095] It should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. Oilfield wellhead gas circulation injection system, characterized by: It includes a purification unit, a pre-pressurization unit, a cooling and water removal unit, a pressurization injection unit and a control system; The purification unit includes a desulfurization tank, a centrifugal water removal tank, and a filter that are interconnected; The pre-boosting unit comprises a venting pipeline and an air intake main pipeline arranged in parallel, and the air intake main pipeline is provided with a gas buffer tank, a variable frequency motor, a screw compression cylinder, and an aftercooler; The cooling and water removal unit includes a centrifugal water remover, a cold dryer, and a micro-heat regeneration adsorption dryer; The boost injection unit includes a filter dewatering device, a variable frequency motor, and a four-stage piston compressor; The control system includes a standard electrical control unit and a remote monitoring unit.

2. The oilfield wellhead gas circulation injection system according to claim 1, characterized in that: The desulfurization tank is provided with a gas inlet at the lower end, an alkali solution inlet and an alkali solution spray nozzle at the top, and a gas outlet and filler injection port is provided on the right side of the alkali solution inlet; A sieve plate with through holes is installed at the lower end of the gas inlet on the inner wall of the desulfurization tank, and the sieve plate is evenly filled with fillers. A retention plate is also installed in the inner wall of the desulfurization tank to support the fillers and slow down the gas flow rate, form gas turbulence, promote gas-liquid mass transfer, and achieve desulfurization of the output gas.

3. The oilfield wellhead gas circulation injection system according to claim 1, characterized in that: The centrifugal dewatering tank is provided with a gas inlet in the middle and a gas outlet at the top; The inner wall of the centrifugal dewatering tank is installed with a funnel-shaped sieve plate, and the sieve plate is evenly distributed with through holes, and gas can enter the through holes; when the output gas enters from the gas inlet, under the centrifugal rotation of the funnel-shaped sieve plate, the gas enters the through holes along the cross-sectional tangent of the funnel-shaped sieve plate, and performs a spiral upward movement in the sieve plate, and the gas and liquid are separated according to the cyclone separation effect, and then relying on the gravity of the liquid, the waste liquid condenses and gathers along the wall of the funnel-shaped sieve plate, and flows into the bottom of the funnel-shaped sieve plate for collection; A conical intercepting plate is installed at the upper end of the inner wall of the centrifugal dewatering tank. Through holes are evenly distributed on the conical intercepting plate to intercept the rising gas and slow down the gas flow rate so that the residual liquid in the gas condenses and flows along the tank wall and the funnel-type sieve plate into the bottom arc-shaped wastewater collection area of ​​the centrifugal dewatering tank for storage.

4. The oilfield wellhead gas circulation injection system according to claim 1, characterized in that: The lower end of the filter is provided with a detachable two-layer perforated support plate, and the middle of the perforated support plate is filled with an activated carbon adsorption layer; The upper area of ​​the activated carbon adsorption layer is filled with an inorganic silicon adsorption layer, and interception plates are symmetrically and staggeredly installed on the inner wall of the filter where the inorganic silicon adsorption layer is located, which are used to cause gas turbulence and block the direct flow of gas to make the gas fully contact with the inorganic silicon adsorption layer to adsorb and remove impurities in the gas.

5. The oilfield wellhead gas circulation injection system according to claim 1, characterized in that: The air intake main line of the pre-boost unit is connected to the release line through a stop valve, and can be released with one button in an emergency; The main air intake line is connected to the branch line I through a stop valve, and is connected to the branch line II through a stop valve to achieve two-stage pipeline diversion, and the BV-1 branch line is opened in an emergency; The main air intake pipe is equipped with a total pressure detection gauge, a total air intake temperature detection gauge and a total air intake solenoid valve, and the total air intake solenoid valve is connected to the gas buffer tank through a pipeline; The upper end of the gas buffer tank is equipped with a pressure detection gauge and a safety valve, and the lower end is equipped with a liquid level detection gauge. The safety valve is connected to the release pipeline. The lower end of the gas buffer tank is provided with a drainage pipeline, and a stop valve is provided on the drainage pipeline. The gas outlet of the gas buffer tank is connected with an air filter tank, and a screw front air intake solenoid valve is arranged at the front end of the pipeline connected with the air filter tank. A branch pipeline III is arranged at the front end pipeline of the screw front air intake solenoid valve. The branch pipeline III is connected with the air filter tank gas outlet pipeline through a stop valve and is collected into the main pipeline. The main pipeline is connected with a variable frequency motor. The gas outlet of the variable frequency motor is connected with a screw compression cylinder through a pipeline. The gas outlet of the screw compression cylinder is connected with a minimum pressure valve through a pipeline. The outlet of the minimum pressure valve is connected with an aftercooler through a pipeline. The aftercooler realizes gas cooling through a fan. The gas outlet of the aftercooler is connected with a centrifugal water remover through a pipeline. The upper end of the screw compression cylinder is provided with a safety valve, and the safety valve is connected to the release pipeline; the lower end of the screw compression cylinder is provided with a discharge pipeline, and a stop valve is provided on the discharge pipeline; The oil outlet of the screw compression cylinder is connected to an oil filter through an oil pipeline, the oil filter is connected to an oil cooler through a pipeline, and the oil cooler is connected to the oil inlet of the variable frequency motor through a pipeline to achieve oil cooling of the screw compression cylinder and the variable frequency motor.

6. The oilfield wellhead gas circulation injection system according to claim 1, characterized in that: The gas outlet of the aftercooler of the pre-boosting unit is connected to the gas inlet of the centrifugal dehumidifier through a pipeline, and the gas outlet of the centrifugal dehumidifier is connected to the gas inlet of the cold dryer. A temperature detection meter is provided on the pipeline between the centrifugal dehumidifier and the cold dryer. The gas outlet of the cold dryer is connected to the A tank and the B tank of the micro-heat regeneration adsorption dryer through a pipeline, and a first valve is provided on the pipeline between the A tank and the cold dryer, and a second valve is provided on the pipeline between the B tank and the cold dryer. The main pipeline before the air inlet pipeline of the A tank, the B tank and the gas buffer tank is connected through a third valve; The main pipelines of the heat regeneration adsorption dryer and the cold dryer are provided with branch pipelines, on which stop valves are installed. The branch pipelines are connected with the gas outlet pipelines of tanks A and B and then merged into the main pipeline.

7. The oilfield wellhead gas circulation injection system according to claim 1, characterized in that: The four-stage piston compressor comprises: First-stage air intake steam-water separator, first-stage air intake buffer tank, first-stage piston compression cylinder, first-stage exhaust buffer tank, first-stage air cooler, second-stage air intake steam-water separator, second-stage air intake buffer tank, second-stage piston compression cylinder, second-stage exhaust buffer tank, second-stage air cooler, third-stage air intake steam-water separator, third-stage air intake buffer tank, third-stage piston compression cylinder, third-stage exhaust buffer tank, third-stage air cooler, fourth-stage air intake steam-water separator, fourth-stage air intake buffer tank, fourth-stage piston compression cylinder, fourth-stage exhaust buffer tank, fourth-stage air cooler; The gas outlet of the four-stage air cooler is connected to an exhaust pipeline; A heating reboiler is arranged at the lower end of the secondary air intake steam-water separator, the tertiary air intake steam-water separator and the quaternary air intake steam-water separator.

8. The oilfield wellhead gas circulation injection system according to claim 7, characterized in that: The gas inlet of the filter dehydrator is connected to the air intake pipe, and the gas outlet is connected to the inlet of the first-stage air intake steam-water separator through a pipeline. A first-stage suction pressure gauge is arranged on the left side of the upper end of the first-stage air intake steam-water separator. A safety valve SV-2 is arranged on the upper end of the first-stage air intake steam-water separator, and the safety valve SV-2 is connected to a release pipeline. The first-stage exhaust buffer tank is provided with a first-stage exhaust pressure gauge and a first-stage exhaust temperature gauge, and the air intake pipeline of the first-stage air cooler is provided with a safety valve SV-3, which is connected to the vent pipeline; The secondary exhaust buffer tank is provided with a secondary exhaust pressure gauge and a secondary exhaust temperature gauge, and the air intake pipeline of the secondary air cooler is provided with a safety valve SV-4, which is connected to the vent pipeline; The three-stage exhaust buffer tank is provided with a three-stage exhaust pressure gauge and a three-stage exhaust temperature gauge, and the air intake pipeline of the three-stage air cooler is provided with a safety valve SV-5, which is connected to the vent pipeline; The four-stage exhaust buffer tank is provided with a four-stage exhaust pressure gauge and a four-stage exhaust temperature gauge, and the air intake pipeline of the four-stage air cooler is provided with a safety valve SV-6, which is connected to the vent pipeline. Furthermore, a main gas production valve is installed on the exhaust pipeline, and the exhaust pipeline located on the left side of the main gas production valve is also connected to the release pipeline through a main release valve.

9. The oilfield wellhead gas circulation injection system according to claim 7, characterized in that: The control process of the boost injection unit is: The gas pressure is 1.0-2.5MPa and the temperature is 40℃±5℃. It enters the filter dehumidifier through the intake pipe. After filtering and dehydrating, the dew point at normal pressure reaches -45℃±5℃. The gas enters the first-stage intake steam-water separator for gas-liquid separation. The waste liquid is stored in the waste liquid area of ​​the first-stage intake steam-water separator. When the waste liquid reaches the liquid level monitoring point, it can be automatically discharged through the control system. The gas after gas-liquid separation enters the first-stage air intake buffer tank through the pipeline to reduce the impact of the gas on the piston compression cylinder, with a pressure difference of 0-0.1MPa; Then it enters the first-stage piston compression cylinder, and after pressurization, the pressure is 3-3.5MPa and the temperature is 140℃±10℃; After being compressed by the first-stage piston, the gas enters the first-stage exhaust buffer tank for gas buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; The gas then enters the first-stage air cooler, which is used to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the first-stage air cooler is 3-3.5MPa and the temperature is 35-55℃; The gas cooled by the primary air cooler enters the secondary air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the secondary air intake steam-water separator is monitored in real time in the control system: If the control system detects that the temperature of the gas is less than 35°C, the control automatically shuts down the primary air cooler and automatically starts the heating reboiler of the secondary air inlet steam-water separator to raise the temperature to 35-55°C to gasify the carbon dioxide condensed in the liquid; otherwise, the control system does not perform the corresponding control action; The liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the secondary air intake gas-water separator. When the gas temperature in the secondary air intake gas-liquid separator is above 35°C and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system; After the gas enters the secondary air intake buffer tank through the pipeline for buffering, the pressure difference is 0-0.1MPa to reduce the impact on the secondary piston compression cylinder; The gas after buffering enters the secondary piston compression cylinder for compression. After pressurization, the pressure is 7.5-8.5MPa and the temperature is 140℃±10℃; The gas then enters the secondary exhaust buffer tank for buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; The gas then enters the secondary air cooler, which is used to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the secondary air cooler is 7.5-8.5MPa and the temperature is 40-60℃; The gas cooled by the secondary air cooler enters the third-stage air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the third-stage air intake steam-water separator is monitored in real time in the control system: If the control system detects that the temperature of the gas is less than 35°C, the control automatically shuts down the secondary air cooler and automatically starts the heating reboiler of the third-stage air inlet steam-water separator to raise the temperature to 40-60°C to gasify the carbon dioxide condensed in the liquid; otherwise, the control system does not perform the corresponding control action; The liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the three-stage air intake gas-water separator. When the gas temperature in the three-stage air intake gas-liquid separator is above 35°C and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system; After the gas enters the three-stage air intake buffer tank for buffering, the pressure difference is 0-0.1MPa to reduce the impact on the three-stage piston compression cylinder; The gas after buffering enters the three-stage piston compression cylinder for compression. After pressurization, the pressure is 14-16MPa and the temperature is 140℃±10℃; The gas then enters the third-level exhaust buffer tank for buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; Then it enters the third-stage air cooler to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the air cooler is 14-16MPa and the temperature is 45-65℃. The gas cooled by the three-stage air cooler enters the four-stage air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the four-stage air intake steam-water separator is monitored in real time in the control system: If the control system detects that the temperature of the gas is less than 35°C, the control automatically shuts down the third-stage air cooler and automatically starts the heating reboiler of the fourth-stage air inlet steam-water separator to raise the temperature to 45-65°C to gasify the carbon dioxide condensed in the liquid; otherwise, the control system does not perform the corresponding control action; The liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the four-stage air intake gas-water separator. When the gas temperature in the four-stage air intake gas-liquid separator is above 35°C and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system; The gas enters the four-stage air intake buffer tank for buffering, with a pressure difference of 0-0.1MPa to avoid excessive impact on the piston compression cylinder; The gas after buffering enters the four-stage piston compression cylinder for compression, and the pressure after pressurization is 28-30MPa and the temperature is 140℃±10℃; The gas then enters the fourth-stage exhaust buffer tank for buffering, with a pressure difference of 0-0.1MPa to reduce the impact on subsequent equipment; Then it enters the four-stage air cooler to cool the gas and reduce the impact of high temperature and high pressure on subsequent equipment. The pressure of the four-stage air cooler is 28-30MPa and the temperature is 60℃±10℃. After cooling, the gas is injected into the lower-level equipment through the exhaust pipeline.

10. Oilfield wellhead gas circulation injection process, characterized by: The process is applied to the oilfield wellhead gas circulation injection system according to any one of claims 1 to 9, comprising: The wellhead gas first enters the desulfurization tank to remove hydrogen sulfide in the gas, and then enters the centrifugal dewatering tank along the cutting groove for preliminary gas-liquid phase separation. The gas enters the filter to filter and remove C4 and above heavy hydrocarbon components in the gas. The gas is then sent to the screw compression cylinder for initial pressure increase to 0.5-1.0MPa, and then sent to the freeze dryer to make the atmospheric pressure dew point of the gas reach -20±5℃, and then deep dehydration is carried out by the micro-heat regeneration adsorption dryer to reduce the dew point temperature of the gas medium to -40±5℃. The gas is then sent to the filter dehydrator for further dehydration, and then sent to the four-stage piston compressor to increase the pressure to 28-30MPa, and then the gas is injected into the formation to achieve the recycling of carbon dioxide.

Images