Oilfield wellhead gas circulation injection system and process

Abstract

The present invention discloses an oilfield wellhead gas circulation injection system and process, comprising a purification unit, a pre-pressurization unit, a cooling and water removal unit, a pressurization injection unit, and a control system. The purification unit comprises a connected desulfurization tank, a centrifugal water removal tank, and a filter; the pre-pressurization unit comprises a venting pipeline and an air intake main pipeline arranged in parallel; the cooling and water removal unit comprises a centrifugal water remover, a cold dryer, and a micro-heat regeneration adsorption dryer; the pressurization injection unit comprises a filter water remover, a variable frequency motor, and a four-stage piston compressor; and the control system comprises a standard electrical control unit and a remote monitoring unit. The present invention recycles, purifies, and pressurizes oilfield wellhead gas before recirculating it back into the well, achieving separation, recovery, purification, and supercritical pressurization of carbon dioxide in the mixed gas. The four-stage piston compressor suppresses the condensation of gaseous carbon dioxide into liquid, thereby ensuring zero loss of carbon dioxide in the mixed gas, achieving 100% carbon dioxide recovery and utilization, and achieving zero carbon dioxide emissions to the external environment. This solves the problem of prior art where wellhead gas is directly discharged into the air, causing environmental pollution and avoiding energy waste.

Description

Technical Field

[0001] The present 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 implementing CCUS-EOR (carbon dioxide cycle injection) technology, excess carbon dioxide injected into the oil reservoir is discharged along with water vapor to the wellhead, forming wellhead gas. This wellhead gas contains a small amount of natural gas, but a large amount of CO2 and water vapor. Directly discharging this wellhead gas into the atmosphere would result in the release of greenhouse gases, environmental pollution, and energy waste. Therefore, the CO2 in the wellhead gas must be recovered and reused.

[0003] In existing technologies, natural gas processing systems only contain separation and drying units, lacking a pressurization unit. This makes it impossible to separate and recover carbon dioxide from water vapor, nor to achieve supercritical pressurization and wellhead gas injection of carbon dioxide. Existing multi-stage piston compressor units are unable to process the mixed gas produced by CCUS-EOR technology. This is primarily because the cooling mechanism of existing multi-stage piston compressor units causes the liquefaction of gaseous CO2 in the wellhead gas. This liquefied CO2 is then discharged through the gas-liquid separation system of the existing multi-stage piston compressor units, making it impossible to achieve "zero" carbon dioxide emissions in oilfield development and causing environmental pollution. Furthermore, the unit's single control method means that any control failure can cause severe damage to the piston compressor, rendering the unit 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] To solve the above technical problems, the present invention provides a wellhead gas circulation injection system and process for oil fields, which can recover, purify, and pressurize the wellhead gas generated after the implementation of CCUS-EOR technology and then recirculate 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 its technical problems is:

[0007] 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;

[0008] The purification unit includes a desulfurization tank, a centrifugal water removal tank, and a filter that are connected;

[0009] The pre-boost unit includes 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;

[0010] The cooling and water removal unit includes a centrifugal water remover, a cold dryer, and a micro-heat regeneration adsorption dryer;

[0011] The boost injection unit includes a filter and water remover, a variable frequency motor, and a four-stage piston compressor;

[0012] The control system includes a standard electrical control unit and a remote monitoring unit.

[0013] Furthermore, in the purification unit:

[0014] 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.

[0015] Furthermore, the desulfurization tank adopts the "sieve plate + alkali + filler" treatment process:

[0016] 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 end, and a gas outlet and 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 the PIC;

[0017] 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 filler. A retention plate is also installed in the inner wall of the desulfurization tank to support the filler and slow down the gas flow rate, form gas turbulence, promote gas-liquid mass transfer, and achieve desulfurization of the output gas.

[0018] 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 action of the alkali liquid and the filler, the hydrogen sulfide in the gas is oxidized and reduced to sulfide salt 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.

[0019] Furthermore, the filler is glassy iron oxyhydroxide, which is used to remove hydrogen sulfide from the produced gas;

[0020] The alkali solution is preferably sodium hydroxide or potassium hydroxide, which is used to absorb hydrogen sulfide in coordination with glassy iron oxyhydroxide.

[0021] 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.

[0022] 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, through which gas can enter. 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. According to the cyclonic separation effect, the gas and liquid are separated, 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.

[0023] 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, 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-shaped sieve plate into the arc-shaped wastewater collection area at the bottom 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.

[0024] Furthermore, the filter adopts the "pressurization + active composite inorganic adsorption" treatment process, selects "activated carbon + inorganic silicon" filler, gives full play to the advantages 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 injection medium from being enriched and blocked after the purification unit.

[0025] 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-shaped 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.

[0026] Furthermore, the perforated support plate is detachable. When the activated carbon adsorption layer is saturated with adsorption, the window on the side wall of the filter can be opened to remove the perforated support plate, remove the saturated activated carbon adsorption layer, and re-add fillers.

[0027] 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 straight-through rise of the gas, so that the gas can fully contact with 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.

[0028] 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.

[0029] Furthermore, the air intake main line of the pre-boost unit is connected to a relief line through a stop valve, which can be released with one button in an emergency;

[0030] 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, realizing two-stage pipeline diversion. In case of emergency, the BV-1 branch line is opened.

[0031] Furthermore, a total pressure detection gauge, a total intake air temperature detection gauge and a total intake air solenoid valve are installed on the main intake air line, and the total intake air solenoid valve is connected to a gas buffer tank through a pipeline;

[0032] 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 waste liquid discharge;

[0033] The gas outlet of the gas buffer tank is connected to an air filter tank, and a screw front air intake solenoid valve is provided at the front end of the pipeline connected to the air filter tank. A branch pipeline III is provided at the front end of the pipeline of the screw front air intake solenoid valve. The branch pipeline III is connected to the air filter tank gas outlet pipeline through a stop valve BV-4 and is collected into the main pipeline. The main pipeline is connected to a frequency conversion motor. The gas outlet of the frequency conversion motor is connected to a screw compression cylinder through a pipeline. The gas outlet of the screw compression cylinder is connected to a minimum pressure valve through a pipeline. The outlet of the minimum pressure valve is connected to an aftercooler through a pipeline. The aftercooler cools the gas by a fan. The gas outlet of the aftercooler is connected to a centrifugal water remover through a pipeline.

[0034] 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 discharge of the waste liquid;

[0035] 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 frequency conversion motor through a pipeline to achieve oil cooling of the screw compression cylinder and the frequency conversion motor.

[0036] Furthermore, the gas outlet of the aftercooler of the pre-boost 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 tank A and tank B of the micro-heat regeneration adsorption dryer through a pipeline, a first valve is provided on the pipeline between tank A and the cold dryer, a second valve is provided on the pipeline between tank B and the cold dryer, and the main pipeline before the air inlet pipeline of tank A, tank B and the gas buffer tank is connected through a third valve.

[0037] The main pipelines of the micro-heat regenerative adsorption dryer and the cold dryer are equipped with branch pipelines, each equipped with a shutoff valve. This branch pipeline connects to the gas outlet pipelines of tanks A and B before merging into the main pipeline. A total pressure and flow meter is installed on this main pipeline. If the gas medium cooled by the cold dryer reaches the required gas dew point, further drying and water removal in the micro-heat regenerative adsorption dryer is no longer required. At this point, the first and second valves are closed, allowing the gas medium to be injected into the next device through the branch pipeline.

[0038] Furthermore, the four-stage piston compressor includes:

[0039] 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;

[0040] The gas outlet of the four-stage air cooler is connected to an exhaust pipeline;

[0041] 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.

[0042] 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 dehydration by the filter dehydrator and further dehydration 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, carbon dioxide liquefaction 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 set an electric heating reboiler in the first-stage air intake steam-water separator.

[0043] 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.

[0044] Air coolers are primarily installed after the first-stage piston compression cylinder. This is primarily because the first-stage piston compression cylinder represents primary compression, with a relatively low pressure (3-3.5 MPa), which minimizes the impact on subsequent equipment. Therefore, an air cooler is not required before the first-stage piston compression cylinder. The subsequent second-stage piston compression cylinder has a pressure of 7.5-8.5 MPa and a temperature of 140°C ± 10°C; the third-stage piston compression cylinder has a pressure of 14-16 MPa and a temperature of 140°C ± 10°C; and the fourth-stage piston compression cylinder has a pressure of 28-30 MPa and a temperature of 140°C ± 10°C. The high temperature and high pressure of the gas after compression in the second, third, and fourth stages can cause aging of the seals in subsequent equipment. To mitigate the impact of high temperature and high pressure on subsequent equipment, air coolers are introduced before the second, third, and fourth-stage piston compression cylinders, respectively.

[0045] However, air coolers cannot precisely control the temperature of the gaseous medium to a constant value (35-65°C), and due to environmental factors, there may be a problem of overcooling, causing the gaseous carbon dioxide to liquefy. Therefore, a heated reboiler must be installed in the air inlet steam-water separator after the air cooler to vaporize the liquefied carbon dioxide.

[0046] Furthermore, the gas inlet of the filter dehumidifier 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. 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 discharge pipeline.

[0047] 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 inlet pipe of the first-stage air cooler is provided with a safety valve SV-3, which is connected to the discharge pipe;

[0048] The secondary exhaust buffer tank is provided with a secondary exhaust pressure gauge and a secondary exhaust temperature gauge, and the air inlet pipe of the secondary air cooler is provided with a safety valve SV-4, which is connected to the discharge pipe;

[0049] The three-stage exhaust buffer tank is provided with a three-stage exhaust pressure gauge and a three-stage exhaust temperature gauge. The air inlet pipe of the three-stage air cooler is provided with a safety valve SV-5, which is connected to the discharge pipe.

[0050] 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 inlet pipe of the four-stage air cooler is provided with a safety valve SV-6, which is connected to the discharge pipe.

[0051] Furthermore, a main gas production valve is installed on the exhaust pipeline, and the exhaust pipeline on the left side of the main gas production valve is also connected to the release pipeline through a main release valve;

[0052] 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 relieved with one button in an emergency (such as compressor shutdown).

[0053] Furthermore, the control process of the boost injection unit is:

[0054] 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 removing water, the dew point at normal pressure reaches -45℃±5℃.

[0055] 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, it can be automatically discharged through the control system;

[0056] After gas-liquid separation, the gas enters the first-stage air inlet buffer tank through the pipeline to reduce the impact of the gas on the piston compression cylinder. The pressure difference is 0-0.1MPa;

[0057] Then it enters the first-stage piston compression cylinder, and after pressurization, the pressure is 3-3.5MPa and the temperature is 140℃±10℃;

[0058] 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;

[0059] 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℃.

[0060] The gas cooled by the first-stage air cooler enters the second-stage air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the second-stage air intake steam-water separator is monitored in real time in the control system:

[0061] If the control system detects that the gas temperature is less than 35°C, this indicates that carbon dioxide will be liquefied in the secondary air inlet separator, causing the liquefied carbon dioxide to enter the liquid. 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 vaporize the carbon dioxide condensed in the liquid.

[0062] Otherwise, the control system does not perform the corresponding control action;

[0063] The liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the secondary air intake gas-liquid 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;

[0064] The gas then enters the secondary air intake buffer tank through the pipeline for buffering, with a pressure difference of 0-0.1MPa to reduce the impact on the secondary piston compression cylinder;

[0065] 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℃;

[0066] 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;

[0067] 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℃.

[0068] The gas cooled by the secondary air cooler enters the tertiary air inlet steam-water separator for gas-liquid separation, and the temperature of the gas in the tertiary air inlet steam-water separator is monitored in real time in the control system:

[0069] If the control system detects that the gas temperature is less than 35°C, this indicates that carbon dioxide will be liquefied in the three-stage air inlet separator, causing the liquefied carbon dioxide to enter the liquid. 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 separator to raise the temperature to 40-60°C to gasify the carbon dioxide condensed in the liquid.

[0070] Otherwise, the control system does not perform the corresponding control action;

[0071] The liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the three-stage air inlet gas-liquid separator. When the gas temperature in the three-stage air inlet 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;

[0072] 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;

[0073] 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℃;

[0074] 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;

[0075] 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℃.

[0076] The gas cooled by the three-stage air cooler enters the four-stage air inlet steam-water separator for gas-liquid separation, and the temperature of the gas in the four-stage air inlet steam-water separator is monitored in real time in the control system:

[0077] 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 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 separator to raise the temperature to 45-65°C to gasify the carbon dioxide condensed in the liquid.

[0078] Otherwise, the control system does not perform the corresponding control action;

[0079] Finally, the liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the four-stage air inlet gas-liquid separator. When the gas temperature in the four-stage air inlet 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;

[0080] 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;

[0081] 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℃;

[0082] 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;

[0083] The gas then enters a four-stage air cooler, which cools the gas and mitigates the impact of high temperature and high pressure on subsequent equipment. The pressure in the four-stage air cooler is 28-30MPa and the temperature is 60°C ± 10°C. After cooling, the gas is injected into the downstream equipment through the exhaust pipeline. At this point, the carbon dioxide gas medium has a pressure of 28-30MPa and a temperature of 60°C ± 10°C, reaching a supercritical state (the outlet temperature can be adjusted according to production conditions, with the minimum outlet temperature being ambient temperature + 10°C). The main gas production valve on the exhaust pipeline controls the gas injection into the downstream equipment, achieving supercritical CO2 injection.

[0084] 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;

[0085] The standard electrical control unit monitors and controls the operation process, emergency faults and equipment start-up and shutdown of the entire circulating injection system through a programmable controller (PLC); and automatically monitors abnormal conditions of the system, such as compressor abnormalities, residual gas in the circulating injection system, etc., automatically displays an alarm signal and shuts down the system, thereby realizing fault diagnosis, recording and self-elimination functions. When the fault is eliminated, the compressor automatically resumes operation.

[0086] Furthermore, the remote control unit is connected to a remote computer through the RTD input card, communication card, and expansion slot reserved on the standard electrical control unit to achieve remote data transmission and control.

[0087] 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 inlet and outlet pipelines of the circulating injection system, the inside of the booster unit and the entire process pipeline.

[0088] The present invention aims to solve the technical problem and provides a wellhead gas circulation injection process for oil fields.

[0089] The cyclic injection process is:

[0090] 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℃. It is then deeply dehydrated by the micro-heat regeneration adsorption dryer to reduce the dew point temperature of the gas medium to -40±5℃ (the dew point temperature is generally suitable for 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 water removal, and then sent to the four-stage piston compressor to increase the pressure to 28-30MPa. The gas (carbon dioxide concentration 100%) is injected into the formation to achieve carbon dioxide recycling and zero emission.

[0091] 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.

[0092] The beneficial effects brought by the present invention are:

[0093] The wellhead gas circulation injection system of the present invention can be used to treat the mixed gas generated by the CCUS-EOR technology. It can recover, purify, and pressurize the wellhead gas generated after the implementation of the CCUS-EOR technology and then recirculate it into the well. This not only achieves 100% recovery and utilization of the carbon dioxide in the mixed gas, but also achieves supercritical pressurization of the carbon dioxide (28-30MPa, 60°C±10°C for carbon dioxide to reach a supercritical state) and recirculates it into the wellhead, ultimately achieving "zero" carbon dioxide emissions in oilfield development. This solves the problem of environmental pollution caused by direct discharge of wellhead gas into the air in the prior art and avoids energy waste.

[0094] 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 ring, which is in line with the concept of environmental protection.

[0095] 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 and is prone to control failure accidents, causing serious damage to the piston compressor. BRIEF DESCRIPTION OF THE DRAWINGS

[0096] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0097] Figure 1 The schematic diagram of the loop injection system.

[0098] Figure 2 This is a structural diagram of the circulating injection system;

[0099] Figure 3 It is a structural diagram of the desulfurization tank;

[0100] Figure 4 It is a structural diagram of a centrifugal water removal tank;

[0101] Figure 5 It is a schematic diagram of the funnel-type sieve plate structure;

[0102] Figure 6 It is a schematic diagram of the filter structure;

[0103] Figure 7 This is the process flow chart of the pre-pressurization unit and the cooling and water removal unit;

[0104] Figure 8 This is the process flow chart of the boost injection unit;

[0105] Figure 9 This is the process flow chart of the lubricating oil circuit of the four-stage piston compressor.

[0106] In the picture:

[0107] 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;

[0108] Centrifugal dewatering tank: 21-gas inlet, 22-gas outlet, 23-funnel-type sieve plate, 24-retention plate II, 25-liquid level detection gauge;

[0109] 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

[0110] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention and the accompanying drawings. Obviously, the embodiments described 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 making creative efforts are within the scope of protection of the present invention.

[0111] Example 1

[0112] 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

[0113] The purification unit includes a desulfurization tank, a centrifugal water removal tank, and a filter that are connected;

[0114] The pre-boost unit includes 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;

[0115] The cooling and water removal unit includes a centrifugal water remover, a cold dryer, and a micro-heat regeneration adsorption dryer;

[0116] The boost injection unit includes a filter and water remover, a variable frequency motor, and a four-stage piston compressor;

[0117] The control system includes a standard electrical control unit and a remote monitoring unit.

[0118] As a preferred embodiment, the output gas enters from the gas inlet 11 of the desulfurization tank of the purification unit, and 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.

[0119] Reference Figure 3 The desulfurization tank uses a "sieve plate + alkali + filler" treatment process to remove hydrogen sulfide from wellhead gas. The equipment is simple, easy to operate, has a high purification level, a large sulfur capacity, low desulfurization cost, a desulfurization efficiency of ≥90%, and no secondary pollution.

[0120] The desulfurization tank is provided with a gas inlet 11 at the lower end, an alkali solution inlet 12 and an alkali solution spray nozzle 19 at the top, and a gas outlet 13 serving as a filler injection port is provided on the right side of the alkali solution inlet 12; a pressure gauge 14 and a temperature gauge 15 are installed at the upper end of the desulfurization tank, and a liquid level detection gauge is installed at the lower end. The pressure gauge 14, temperature gauge 15, and liquid level detection gauge are connected to the control system via a PIC;

[0121] 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 filler 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.

[0122] The desulfurization principle of the desulfurization tank is as follows: the alkali liquid sprayed from the top is fully in contact with the hydrogen sulfide 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 with the alkali liquid to the waste liquid area at the bottom of the tower for storage. When the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system.

[0123] Preferably, the filler 17 is glassy iron oxyhydroxide, which is used to remove hydrogen sulfide from the produced gas;

[0124] The alkali solution is preferably sodium hydroxide or potassium hydroxide, which is used to absorb hydrogen sulfide in coordination with glassy iron oxyhydroxide.

[0125] Reference Figure 4 A gas inlet 21 is provided in the middle of the centrifugal water removal 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.

[0126] The inner wall of the centrifugal dewatering tank is provided with a funnel-type sieve plate 23. Figure 5 As shown, the funnel-shaped sieve plate 23 is evenly distributed with through holes, through which gas can enter. When the output gas enters from the gas inlet 21, the gas enters the through holes along the cross-sectional tangent of the funnel-shaped sieve plate 23 under the centrifugal rotation of the funnel-shaped sieve plate 23, and performs a spiral upward motion inside the sieve plate. According to the cyclonic separation effect, the gas and liquid are separated. Then, relying on the gravity of the liquid, the waste liquid condenses and gathers along the wall of the funnel-shaped sieve plate 23, and flows into the bottom of the funnel-shaped sieve plate 23 for collection.

[0127] A conical intercepting plate II24 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 II24, 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-shaped sieve plate 23 into the arc-shaped wastewater collection area at the bottom 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.

[0128] Reference Figure 6 The filter adopts the "pressurization + active composite inorganic adsorption" treatment process, selects "activated carbon + inorganic silicon" filler, gives full play to the advantages of pore structure and specific surface area, uses capillary action and surface tension to form a physical-chemical adsorption structure to remove heavy hydrocarbons, so as to prevent the injection medium from enrichment and clogging after the purification unit.

[0129] A gas inlet 31 is provided in the middle of the filter, a gas outlet 32 ​​and a filler injection port 33 are 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 PLC.

[0130] A detachable two-layer perforated support plate 34 is installed at the lower end of the filter. 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 produced gas; the pores of the perforated support plate 34 are used to discharge the waste liquid condensed in the filler into the arc-shaped 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.

[0131] 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 prevent the gas from rising in a straight-through manner, 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 during subsequent treatment.

[0132] Preferably, the perforated support plate 34 is detachable. When the activated carbon adsorption layer 35 is saturated with adsorption, the window on the side wall of the filter can be opened to remove the perforated support plate 34, remove the saturated activated carbon adsorption layer 35, and add filler again.

[0133] 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.

[0134] Reference Figure 7 The air intake main line of the pre-boost unit is connected to a relief line through a stop valve, which can be released with one button in an emergency;

[0135] 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, realizing two-stage pipeline diversion. In case of emergency, the BV-1 branch line is opened.

[0136] Preferably, a total pressure detection gauge, a total intake air temperature detection gauge and a total intake air solenoid valve are installed on the main intake air line, and the total intake air solenoid valve is connected to a gas buffer tank through a pipeline;

[0137] 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.

[0138] The gas outlet of the gas buffer tank is connected to an air filter tank, and a screw front air intake solenoid valve is provided at the front end of the pipeline connected to the air filter tank. A branch pipeline III is provided at the front end of the pipeline of the screw front air intake solenoid valve. The branch pipeline III is connected to the air filter tank gas outlet pipeline through a stop valve BV-4 and is collected into the main pipeline. The main pipeline is connected to a frequency conversion motor. The gas outlet of the frequency conversion motor is connected to a screw compression cylinder through a pipeline. The gas outlet of the screw compression cylinder is connected to a minimum pressure valve through a pipeline. The outlet of the minimum pressure valve is connected to an aftercooler through a pipeline. The aftercooler cools the gas by a fan. The gas outlet of the aftercooler is connected to a centrifugal water remover through a pipeline.

[0139] The upper end of the screw compression cylinder is provided with a safety valve, which is connected to the venting 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 to realize the discharge of waste liquid;

[0140] 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 frequency conversion motor through a pipeline to achieve oil cooling of the screw compression cylinder and the frequency conversion motor.

[0141] In this embodiment, the screw compression cylinder is mainly used to preliminarily pressurize the gas produced at the wellhead to provide power for subsequent processing. The screw compression cylinder has a wide range of application for air intake, and can be used from 0-0.7MPa. 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 variable frequency 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 consists 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. The screw compressor cylinders are also equipped with air coolers. The cooling system is designed for ambient temperatures ranging from -20°C to +40°C, ensuring the screw compressors can operate effectively in complex climates. A variable-frequency motor fan controls the radiator temperature of the screw and piston compressors, keeping the exhaust gas temperature 15°C above the ambient temperature. The cylinder cooling design and efficient air-gas interstage cooling system reduce piston ring wear. The cooling system is a vertical structure, positioned at one end of the skid, saving height, facilitating transportation, and ensuring exceptional maneuverability.

[0142] 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 tank A and tank B of the micro-heat regeneration adsorption dryer through a pipeline, a first valve is provided on the pipeline between tank A and the cold dryer, a second valve is provided on the pipeline between tank B and the cold dryer, and the main pipeline before the air inlet pipeline of tank A and tank B and the gas buffer tank is connected through a third valve;

[0143] The main pipeline of the micro-heat regeneration adsorption dryer and the cold dryer is provided with a branch pipeline, and a shut-off 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.

[0144] Preferably, the freeze dryer is primarily used for deep water removal from wellhead gas. The freeze dryer rapidly freezes the gas being dried. Under a suitable vacuum environment, the frozen water molecules sublime and escape as water vapor. After heat exchange with air, the water vapor enters the waste liquid discharge system. The dew point of the wellhead gas treated by the freeze dryer reaches -20°C at atmospheric pressure.

[0145] Preferably, the micro-heat regenerative adsorption dryer is primarily used to further dehydrate and dry wellhead gas. This micro-heat regenerative adsorption dryer performs deep adsorption dehydration, leaving the gas dry enough to meet the operating conditions of the subsequent main compressor. The atmospheric pressure dew point of the wellhead gas treated by the micro-heat regenerative adsorption dryer reaches -40 to -50°C.

[0146] Reference Figure 8 、 Figure 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:

[0147] 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;

[0148] The gas outlet of the four-stage air cooler is connected to an exhaust pipeline;

[0149] 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.

[0150] 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 through a pipeline. 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.

[0151] Furthermore, the gas inlet of the filter dehumidifier 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. 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 discharge pipeline.

[0152] 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 inlet pipe of the first-stage air cooler is provided with a safety valve SV-3, which is connected to the discharge pipe;

[0153] The secondary exhaust buffer tank is provided with a secondary exhaust pressure gauge and a secondary exhaust temperature gauge, and the air inlet pipe of the secondary air cooler is provided with a safety valve SV-4, which is connected to the discharge pipe;

[0154] The three-stage exhaust buffer tank is provided with a three-stage exhaust pressure gauge and a three-stage exhaust temperature gauge. The air inlet pipe of the three-stage air cooler is provided with a safety valve SV-5, which is connected to the discharge pipe.

[0155] 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 inlet pipe of the four-stage air cooler is provided with a safety valve SV-6, which is connected to the discharge pipe.

[0156] Furthermore, a main gas production valve is installed on the exhaust pipeline, and the exhaust pipeline on the left side of the main gas production valve is also connected to the release pipeline through a main release valve;

[0157] 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 click in an emergency (compressor shutdown). The residual gas in the four-stage piston compressor will be released to a safe area outside the skid through the relief pipe, ensuring the no-load start-up of the four-stage piston compressor and reducing equipment power consumption and maintenance rate.

[0158] In this embodiment, a four-stage piston compressor uses four-stage compression and step-by-step pressure boosting. Gas buffer tanks are installed between each of the four compressor stages to achieve gas pulsation buffering. Through the cooperation of air cooling and the electric heating reboiler of the steam-water separator, the gas temperature in the gas-liquid separator reaches dynamic equilibrium, so that the gas temperature is maintained within the range of 35-65°C. This temperature range can ensure the complete gasification of the liquefied CO2 in the gas-liquid separator without gasifying the water in the liquid, achieving the separation of liquefied CO2 and water, thereby preventing the discharge of liquefied CO2 from the gas-liquid separator, thereby ensuring zero loss of the CO2 gas medium entering the oilfield wellhead gas circulation injection system, and achieving 100% recovery and zero emission of CO2. The liquid that does not contain 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°C and when the waste liquid reaches the liquid level monitoring point, the waste liquid can be automatically discharged through the control system.

[0159] Each component of the four-stage piston compressor is equipped with a safety valve, which is controlled by a master relief valve, enabling one-touch relief of the four-stage piston compressor in an emergency. The motor's variable frequency control saves energy and meets varying gas volume requirements. The piston compressor utilizes 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, and crosshead. The piston compressor is powered by a variable frequency three-phase asynchronous motor with variable frequency speed regulation of 15-65 Hz. An elastic coupling connects the compressor and motor to achieve optimal alignment accuracy, automatically compensate for axial displacement, and eliminate potential hazards to explosion-proof equipment caused by frictional static electricity.

[0160] Furthermore, the secondary, tertiary, and quaternary steam-water separators are equipped with electric heating reboilers, which can vaporize the carbon dioxide condensed in the liquid in the gas-liquid separator, preventing the discharge of liquefied CO2 from the gas-liquid separator, thereby ensuring zero loss of the CO2 gas medium entering the oilfield wellhead gas circulation injection system, and achieving 100% recovery and zero emissions of CO2. This is because: carbon dioxide-rich wellhead gas will undergo a phase change at temperatures below 31°C, producing 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, making it impossible to achieve 100% recovery and utilization of the carbon dioxide gas.

[0161] Therefore, an electric heating reboiler is installed between the second, third, and fourth-stage steam-water separators of the piston compression cylinder. The control system automatically starts the electric heating reboiler of the second, third, and fourth-stage steam-water separators and shuts down the second, third, and fourth-stage air coolers, so that the electric heating reboiler and the air cooler work together to keep the temperature in the gas-liquid separator within a dynamic equilibrium range of 35-65°C. Within this temperature range, the liquefied CO2 in the gas-liquid separator can be completely vaporized without vaporizing the water in the liquid, achieving the separation of liquefied CO2 and water, and preventing the liquefied CO2 from being discharged from the gas-liquid separator, thereby ensuring zero loss of the CO2 gas medium entering the oilfield wellhead gas circulation injection system, and achieving 100% recovery and utilization of CO2 and zero emissions.

[0162] In this embodiment, the operating temperature of the second-stage electric heating reboiler is consistent with the outlet air temperature of the first-stage air cooler, the operating temperature of the third-stage electric heating reboiler is consistent with the outlet air temperature of the second-stage air cooler, and the operating temperature of the fourth-stage electric heating reboiler is consistent with the outlet air temperature of the third-stage air cooler. The purpose is to thermally compensate the air cooler through the electric heating reboiler so that the temperature is fully maintained in the dynamic equilibrium range of 35~65℃.

[0163] As a preferred embodiment, the control process of the boost injection unit is:

[0164] The gas (medium) has a pressure of 1.0-2.5MPa and a temperature of 40℃±5℃ (obtained through real-time monitoring). It enters the filter dehumidifier through the air inlet pipe. After filtration and water removal, the dew point at atmospheric pressure reaches -45℃±5℃. The dew point parameter is designed based on the principle of parameter redundancy design to adapt to the winter environment (-35℃~-30℃) of the field application.

[0165] 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, it can be automatically discharged through the control system;

[0166] After gas-liquid separation, the gas enters the first-stage air inlet buffer tank through the pipeline to reduce the impact of the gas on the piston compression cylinder. The pressure difference is 0-0.1MPa;

[0167] 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 measured temperature after the compression cylinder works);

[0168] 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;

[0169] 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℃.

[0170] The gas cooled by the first-stage air cooler enters the second-stage air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the second-stage air intake steam-water separator is monitored in real time in the control system:

[0171] If the control system detects that the temperature of the gas in the secondary air inlet separator is less than 35°C, this indicates that carbon dioxide may be liquefied in the secondary air inlet separator, causing the liquefied carbon dioxide to enter the liquid. 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 vaporize the carbon dioxide condensed in the liquid, achieving 100% concentration of carbon dioxide gas entering the next stage of equipment;

[0172] Otherwise, the control system does not perform the corresponding control action;

[0173] The liquid in the secondary air inlet steam-water separator that does not contain liquid carbon dioxide 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.

[0174] The gas then enters the secondary air intake buffer tank through the pipeline for buffering, with a pressure difference of 0-0.1MPa to reduce the impact on the secondary piston compression cylinder;

[0175] 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℃;

[0176] 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;

[0177] 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℃;

[0178] The gas cooled by the secondary air cooler enters the tertiary air inlet steam-water separator for gas-liquid separation, and the temperature of the gas in the tertiary air inlet steam-water separator is monitored in real time in the control system:

[0179] If the control system detects that the gas temperature is less than 35°C, this indicates that carbon dioxide may be liquefied in the three-stage air inlet separator, causing the liquefied carbon dioxide to enter the liquid. 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 separator to raise the temperature 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;

[0180] Otherwise, the control system does not perform the corresponding control action;

[0181] The liquid that does not contain liquid carbon dioxide is stored in the waste liquid area of ​​the three-stage air inlet 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;

[0182] 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;

[0183] 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℃;

[0184] 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;

[0185] Then it enters the third-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 air cooler is 14-16MPa and the temperature is 45-65℃.

[0186] The gas cooled by the three-stage air cooler enters the four-stage air inlet steam-water separator for gas-liquid separation, and the temperature of the gas in the four-stage air inlet steam-water separator is monitored in real time in the control system:

[0187] If the control system detects that the gas temperature is less than 35°C, this indicates that carbon dioxide may be liquefied in the fourth-stage air inlet separator, causing the liquefied carbon dioxide to enter the liquid. To avoid the loss of carbon dioxide, the control system automatically shuts down the third-stage air cooler through the programmable controller and automatically starts the electric heating reboiler of the fourth-stage air inlet separator to raise the temperature 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;

[0188] If the control system detects that the gas temperature is within the range of 45°C to 65°C, this indicates that carbon dioxide liquefaction will not occur in the fourth-stage inlet steam-water separator. The control system will not perform corresponding control actions, will not start the electric heating reboiler of the fourth-stage inlet steam-water separator through the programmable controller, and will not automatically shut down the third-stage air cooler.

[0189] 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;

[0190] 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;

[0191] 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℃;

[0192] 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;

[0193] 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 the 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 downstream equipment through the main gas production valve on the exhaust pipeline to achieve supercritical injection of carbon dioxide.

[0194] It should be noted that, according to common knowledge, the CO2 phase diagram indicates that temperatures above the critical temperature of 31°C constitute the gas phase. The optimal temperature range for the air cooler of the present invention is set at 35-65°C under ideal conditions. This temperature range theoretically ensures that the CO2 in the gas medium remains in a gaseous state. However, in actual operation, the air cooler cannot precisely control the gas medium temperature to a constant value (35-65°C). Furthermore, under the influence of environmental factors such as winter, heat loss between the piping and equipment between the air cooler and the steam-water separator can cause the actual temperature of the cooled gas to fall below the air cooler's preset temperature (35-65°C). For example, in extreme conditions such as excessively cold ambient temperatures, the actual temperature of the cooled gas can fall below the critical temperature of 31°C for CO2, causing the gaseous CO2 to liquefy. The liquefied CO2 enters the waste liquid area of ​​the gas-liquid separator along with the condensed liquid. In this case, if the liquefied CO2 waste liquid is not reboiled, it will be discharged from the gas-liquid separator along with the waste liquid, preventing 100% carbon dioxide recovery. In order to adapt to changes in the external environment, this embodiment sets the monitoring temperature to 35°C, 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.

[0195] The present invention utilizes an air cooler and an electrically heated reboiler to coordinate and maintain the CO2 gaseous medium in a constant gaseous state. Temperature sensors are installed between each intake separator of a four-stage piston compressor, transmitting temperature data to a programmable logic controller (PLC). The PLC monitors the gas temperature in the separators, automatically shutting down the air cooler in the four-stage piston compressor and activating the reboiler to heat the compressor to a temperature between 35°C and 65°C. This ensures that the liquefied CO2 in the separators is completely vaporized while preventing water vaporization, thereby maintaining the CO2 gaseous medium in a constant gaseous state. For example, when the control system detects that the gas temperature in the separators is below 31°C, meaning that the CO2 liquefaction conditions have been met, the air cooler is automatically shut down and the electrically heated reboiler is activated to vaporize the CO2 liquefied in the wastewater. This ensures that the CO2 enters the next-stage piston compression cylinder as a constant gaseous medium, thereby achieving 100% CO2 recovery and zero emissions.

[0196] 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.

[0197] For example, when the temperature in the secondary, tertiary, and quaternary separators is above 35°C, indicating that the waste liquid does not contain liquid carbon dioxide, and only when the waste liquid reaches the liquid level monitoring point can the waste liquid be automatically discharged through the control system. When the temperature of the gas in the secondary, tertiary, and quaternary separators is below 31°C, the control system will not automatically discharge the waste liquid even if the waste liquid reaches the liquid level monitoring point. This condition setting can prevent waste liquid containing liquefied carbon dioxide from being discharged from the gas-liquid separator without reboiled, thereby failing to achieve "zero" carbon dioxide emissions.

[0198] 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.

[0199] It should be noted that the oilfield wellhead gas circulation and injection system of this embodiment is equipped with an intake valve assembly to control the flow of wellhead gas and balance the intake pressure before it enters the circulation and injection system. The intake valve assembly includes a bleed valve and a pressure-regulating valve, which provide emergency shutdown and overpressure relief functions, while also regulating the inlet pressure to maintain a relatively stable pressure level. When the compressor shuts down, residual gas in the circulation and injection system is released through a bleed pipe to a safe area outside the skid, ensuring that the compressor can start without load, significantly reducing equipment power consumption and maintenance costs.

[0200] 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.

[0201] Furthermore, the standard electrical control unit includes a programmable controller (PLC) and a control panel. The control panel is provided with a liquid crystal display window for displaying or querying setting values, fault alarms and operating status.

[0202] The standard electrical control unit, based on a PLC, centrally controls the circulating injection system. Operating instruments are attached to each component, providing 4-20mA and RTD signals. The PLC control cabinet, mounted externally on a skid with a front-hung door, offers flexible control, a high degree of automation, reliable performance, and a highly integrated system with self-diagnostic and display functions. A local instrument display panel on the skid provides intuitive display of essential operating parameters and local start and stop control of the unit.

[0203] 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; and enable the gas injection system to have fault diagnosis, recording and self-elimination functions. When the fault is eliminated, the compressor automatically resumes work.

[0204] Furthermore, the remote control unit is connected to a remote computer through the RTD input card, communication card, and expansion slot reserved on the standard electrical control unit to achieve remote data transmission and control.

[0205] 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 circulation injection system; the previous operating parameter records, fault diagnosis and troubleshooting parameter records of the circulation 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.

[0206] Preferably, the remote computer is equipped with a reporting unit capable of recording operating parameters for a period greater than 9,999 hours, and displaying real-time pulsation curves for the inlet and outlet pipelines of the circulating injection system, the interior of the booster unit, and the entire process pipeline. The remote computer also includes a supporting "Gas Injection Technology Service Platform," enabling paperless online reporting and generating analytical reports based on recorded data, ensuring permanent documentation.

[0207] Preferably, the control system sends an ESD shutdown command to the PLCs on each device and pipeline, causing the ball valve on the valve body to rotate to the closed position, cutting off the gas supply and thus achieving automated control of the circulating injection system. The control system is equipped with a manual ESD button. When the equipment is running, an emergency occurs in the surrounding environment, or other circumstances require manual shutdown of the equipment, which can manually shut down the equipment and cut off the gas and power supply.

[0208] In a preferred embodiment, the electrical and instrumentation wiring for the oilfield wellhead gas circulation injection system, along with all cables and electrical accessories, is explosion-proof and isolated. Each tray and source component meets explosion-proof and safety-grade requirements. All boxes have prefabricated holes and are equipped with stainless steel explosion-proof sealing connectors, stainless steel explosion-proof flexible tubing, and stainless steel wire plugs for wiring.

[0209] In a preferred embodiment, the skid of the oilfield wellhead gas circulation injection system is equipped with carbon dioxide, hydrogen sulfide, and methane gas detectors, alarms, and ventilation systems; when the gas concentration in the skid exceeds the standard set value, the alarm sounds and the power supply and gas line of the compressor are cut off at the same time.

[0210] The standard electrical control unit of the present invention uses a programmable logic controller (PLC) to monitor and control the entire circulation injection system's operation, emergency failures, and equipment startup and shutdown. It automatically detects abnormal conditions. For example, if the PLC detects a compressor anomaly or residual gas in the circulation injection system, it automatically displays an alarm signal. The control system then sends an ESD shutdown command to the PLCs of each device and pipeline, rotating the ball valve to the closed position and cutting off the gas supply. This automatically controls and shuts down the circulation injection system, providing fault diagnosis, recording, and self-correction capabilities. Once the fault is resolved, the compressor automatically resumes operation.

[0211] 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 shutdown of the equipment, the equipment can be manually shut down to cut off the gas and power supply.

[0212] Example 2

[0213] This embodiment provides a circulating injection process applied to the oilfield wellhead gas circulating injection system of embodiment 1. The circulating injection process is as follows:

[0214] 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℃. It is then deeply dehydrated 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 water removal, and then sent to the four-stage piston compressor to increase the pressure to 28-30MPa. The gas (carbon dioxide concentration is 100%) is injected into the formation to achieve the recycling of carbon dioxide.

[0215] 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.

[0216] The technical specifications of the system are shown in Table 1:

[0217] Table 1

[0218]

[0219] 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, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or make equivalent replacements for some of the technical features therein. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection 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 connected; The pre-boost unit includes 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 and water remover, a variable frequency motor, and a four-stage piston compressor; The control system includes a standard electrical control unit and a remote monitoring unit; The four-stage piston compressor includes: 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 fourth stage air intake steam-water separator are provided with a heating reboiler.

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 filler. A retention plate is also installed in the inner wall of the desulfurization tank to support the filler 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, through which gas can enter. 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. According to the cyclonic separation effect, the gas and liquid are separated. 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 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, 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-shaped sieve plate into the arc-shaped wastewater collection area at the bottom 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 prevent the gas from rising in a straight line, so that the gas can 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, which 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 also connected to the branch line II through a stop valve to achieve two-stage pipeline diversion. In case of emergency, the BV-1 branch line is opened; The main air intake line 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 drain pipeline, and a stop valve is provided on the drain pipeline. The gas outlet of the gas buffer tank is connected to an air filter tank, and a screw front air intake solenoid valve is provided at the front end of the pipeline connected to the air filter tank. A branch pipeline III is provided at the front end of the pipeline of the screw front air intake solenoid valve. The branch pipeline III is connected to the air filter tank gas outlet pipeline through a stop valve and is gathered into the main pipeline. The main pipeline is connected to a frequency conversion motor. The gas outlet of the frequency conversion motor is connected to a screw compression cylinder through a pipeline. The gas outlet of the screw compression cylinder is connected to a minimum pressure valve through a pipeline. The outlet of the minimum pressure valve is connected to an aftercooler through a pipeline. The aftercooler cools the gas by a fan. The gas outlet of the aftercooler is connected to a centrifugal water remover through a pipeline. The upper end of the screw compression cylinder is provided with a safety valve, which is connected to the relief pipeline; the lower end of the screw compression cylinder is provided with a discharge pipeline, which is provided with a stop valve; 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 frequency conversion motor through a pipeline to achieve oil cooling of the screw compression cylinder and the frequency conversion 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, 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 tank A and tank B of the micro-heat regeneration adsorption dryer through a pipeline, a first valve is provided on the pipeline between tank A and the cold dryer, a second valve is provided on the pipeline between tank B and the cold dryer, and the main pipeline before the air inlet pipeline of tank A, tank B and the gas buffer tank is connected via 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 to the gas outlet pipelines of tanks A and B and then merge into the main pipeline.

7. The oilfield wellhead gas circulation injection system according to claim 1, characterized in that: The gas inlet of the filter dehumidifier 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. 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 discharge 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 inlet pipe of the first-stage air cooler is provided with a safety valve SV-3, which is connected to the discharge pipe; The secondary exhaust buffer tank is provided with a secondary exhaust pressure gauge and a secondary exhaust temperature gauge, and the air inlet pipe of the secondary air cooler is provided with a safety valve SV-4, which is connected to the discharge pipe; The three-stage exhaust buffer tank is provided with a three-stage exhaust pressure gauge and a three-stage exhaust temperature gauge. The air inlet pipe of the three-stage air cooler is provided with a safety valve SV-5, which is connected to the discharge pipe. 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 inlet pipe of the four-stage air cooler is provided with a safety valve SV-6, which is connected to the discharge pipe; 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 relief pipeline through a main relief valve.

8. The oilfield wellhead gas circulation injection system according to claim 1, characterized in that: The control process of the boost injection unit is as follows: The gas pressure is 1.0-2.5MPa and the temperature is 40℃±5℃. It enters the filter dehumidifier through the intake pipe. After filtration and water removal, the atmospheric pressure dew point 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. The pressure difference is 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 first-stage air cooler enters the second-stage air intake steam-water separator for gas-liquid separation, and the temperature of the gas in the second-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 automatically shuts down the primary air cooler and automatically starts the heated reboiler of the secondary inlet air 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-liquid 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; The gas then enters the secondary air intake buffer tank through the pipeline for buffering, with a pressure difference of 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 tertiary air inlet steam-water separator for gas-liquid separation, and the temperature of the gas in the tertiary air inlet 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 automatically shuts down the secondary air cooler and automatically starts the heated reboiler of the tertiary 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 inlet gas-liquid separator. When the gas temperature in the three-stage air inlet 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, which is used 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 inlet steam-water separator for gas-liquid separation, and the temperature of the gas in the four-stage air inlet 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 automatically shuts down the third-stage air cooler and automatically starts the heated reboiler of the fourth-stage inlet air separator to raise the temperature to 45-65°C to vaporize 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 inlet gas-liquid separator. When the gas temperature in the four-stage air inlet 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.

9. 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 8, 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℃. It is then deeply dehydrated 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 water removal, and then sent to the four-stage piston compressor to increase the pressure to 28-30MPa. The gas is then injected into the formation to realize the recycling of carbon dioxide.

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