Pulse differential pressure-based gas-water alternating drive extraction process and simulation device thereof

Abstract

The present application belongs to the technical field of unconventional oil and gas development, and particularly relates to a gas-water alternating displacement extraction process based on pulse differential pressure and a simulation device thereof. The extraction process comprises target core pretreatment, saturation of crude oil, baseline water displacement, pulse differential pressure setting, injection of CO2 slug, injection of formation water slug, cyclic injection of gas and water slug, and data processing. The gas-water alternating displacement extraction process based on pulse differential pressure innovatively proposes design differential pressure response characteristics, fully utilizes periodic pulsation of differential pressure to improve displacement effect. Through pulse differential pressure disturbance, the blocked pore throat is dredged, sweep volume is improved, single dominant channel is inhibited, and channeling time is delayed, thereby improving the ultimate recovery of reservoir crude oil.

Description

Technical Field

[0001] This invention belongs to the technical field of unconventional oil and gas development, specifically relating to a gas-water alternating drive extraction process based on pulsed differential pressure and its simulation device. Background Technology

[0002] Tight oil reservoirs are an important component of unconventional oil and gas. Conventional waterflooding for the development of tight oil reservoirs suffers from slow results, "injection failure, and extraction failure," resulting in low oil recovery rates. CO2 has strong diffusion capabilities, small molecules, and low viscosity, making it easy to utilize crude oil in micropores. It can be used to displace crude oil through mechanisms such as expansion, viscosity reduction, lowering interfacial tension, and dissolved gas flooding, thereby improving oil recovery. However, due to the low viscosity of CO2 and its high oil-gas ratio, viscous fingering and gas channeling are highly likely to occur during the displacement process. Therefore, water-gas alternating flooding (WAG) has been proposed to improve the effectiveness of CO2 oil displacement.

[0003] The alternating gas-water displacement method combines the advantages of CO2 injection and water injection, sequentially injecting CO2 slugs and water slugs. The injected CO2 slugs can expand the swept volume and improve the oil displacement efficiency, while the injected water slugs can effectively suppress gas channeling and improve the mobility ratio. Through alternating gas-water injection, the dual goals of improving oil displacement efficiency and overall mobility control are achieved.

[0004] Currently, to save development costs and improve economic efficiency, oilfield sites generally first adopt single water drive and then switch to gas-water alternating drive. Therefore, gas-water alternating drive usually involves injecting CO2 into the sluice gate first, followed by water into the sluice gate, and then cyclically injecting in sequence. Injection is usually divided into two methods: one is constant pressure injection at a certain pressure; the other is constant speed injection at a certain rate.

[0005] The constant pressure injection method has a long-term unidirectional effect of displacement pressure difference, which makes it difficult to clear blocked pores. Once the dominant channel is formed, it is difficult to change, which exacerbates the risk of crossflow. Under the mobility difference, the velocity is out of control and cannot be actively controlled to affect the flow. It has limited ability to utilize the micropore throats of tight reservoirs.

[0006] In the constant-rate injection method, the pressure difference is passively responded to, and the pressure change is not actively used to improve the oil displacement effect. Continuous constant-rate displacement is difficult to avoid the problem of solidification of the dominant channel. Long-term constant rate may lead to continuous pressure rise, which poses a risk of rupture but lacks an active pressure relief mechanism. It cannot effectively utilize the additional oil displacement mechanism brought about by the pressure difference "pressurization-depressurization" cycle.

[0007] The two conventional gas-water alternating displacement methods essentially require the establishment of a relatively stable displacement state before continuous operation. They suffer from problems such as solidification of dominant channels, difficulty in expanding the affected volume, and insufficient utilization of microscopic pore throats.

[0008] Currently, in the existing technologies in related fields, some pulsed water injection / pulse displacement methods mainly change the time rhythm of fluid injection by periodically opening and closing wells and intermittently injecting fluid. That is, according to the opening and closing frequency of injecting for a period of time and stopping for a period of time, it is attempted to improve the sweep effect through such intermittent disturbances.

[0009] However, these methods essentially focus on the pulse changes in the injection frequency, without designing or precisely controlling the actual displacement forces established within the formation during each injection process, such as how the pressure difference itself changes, to what extent it changes, and at what rate it is established and released.

[0010] In addition, some existing technologies improve displacement uniformity by periodically changing the slug volume or injection rate of the injected medium, such as adjusting the injection volume of the medium slug in one round and changing it in the next round, utilizing the mobility differences of different medium slugs. However, the core variable for optimization in these methods is the periodic change in the injection volume. Summary of the Invention

[0011] The purpose of this invention is to provide a gas-water alternating displacement extraction process based on pulsed differential pressure and its simulation device. The aforementioned pulsed differential pressure alternating displacement extraction process innovatively proposes a design differential pressure response characteristic, fully utilizing the periodic pulsation of the differential pressure to improve the displacement effect. Through pulsed differential pressure disturbance, it clears blocked pore throats, improves swept volume, suppresses single dominant channels, and delays crossflow time, thereby increasing the ultimate recovery rate of reservoir crude oil.

[0012] The technical solution of this invention is as follows: A gas-water alternating displacement and extraction process based on pulsed differential pressure includes the following steps: (1) Pretreatment of target core: S1. Wash and dry the target core. You can refer to the standard GB / T 29172-2012 "Core Analysis Methods" to wash and dry the target core.

[0013] S2. Obtain the physical properties of the target core, including pore volume. V pore Porosity φ, core permeability K, and core matrix T2 spectrum; pore volume V can be obtained using a helium porosimeter. pore And confirm the porosity φ. The core permeability K can be measured using a permeability testing device.

[0014] The matrix T2 spectrum can be obtained by scanning the initial T2 relaxation spectrum using nuclear magnetic resonance (NMR). Transverse relaxation is a parameter characterizing the transverse relaxation time distribution; the crude oil distribution is confirmed based on the signal intensity at different relaxation times.

[0015] Online nuclear magnetic resonance (NMR) devices can continuously scan relaxation time (T2) spectra and perform online imaging to characterize the fluid occurrence state in pores of different sizes within the core in real time, as well as the amount and extent of crude oil mobilization and its impact during the displacement process.

[0016] The transverse relaxation time (T2) spectrum of nuclear magnetic resonance (NMR) directly reflects the scale of hydrogen-containing fluid in pores through NMR signals. The relaxation parameters obtained through attenuation signal inversion are fundamental to the study of rock physical properties. The integral area of ​​the T2 spectrum is proportional to the hydrogen content of the fluid in the core pores. By calibrating and obtaining the correlation between the NMR signal and porosity, the NMR porosity of the rock sample can be obtained. Different pore sizes (small pores - bound fluid, large pores - mobile fluid) correspond to different T2 relaxation times. Therefore, the T2 spectrum essentially reflects the pore structure and fluid mobility distribution. By analyzing the changes in the integral area of ​​the T2 spectrum, oil saturation and oil recovery can be quantitatively calculated.

[0017] Online nuclear magnetic resonance (NMR) imaging: The main steps for acquiring NMR signals are as follows: During the core displacement process, NMR software is used to acquire two-dimensional images simultaneously to monitor the displacement process; a 90° radio frequency pulse is added to the imaging pulse sequence, followed by a 180° pulse after time τ, and a spin echo appears after another time τ, at which point signal data is acquired; the data is post-processed to form images. Real-time visualization of the spatial distribution of fluids (including hydrogen signals) inside the core during the displacement process is achieved. Online imaging testing (visualizing fluid distribution, intuitively showing how the spatial distribution of oil, gas, and water inside the core evolves with the displacement process) is also performed.

[0018] S3. Evacuate the displacement system to a pressure <50Pa and continue for 12 hours; pressurize and saturate the formation water until saturation is complete after 48 hours.

[0019] (2) Saturated crude oil: S1. Set the temperature of the displacement system to the formation temperature, and apply confining pressure to the core placed in the displacement system after pretreatment in step (1) according to the formation pressure; the confining pressure is generally 2-3 MPa higher than the displacement pressure.

[0020] S2. The pressure at the production end of the displacement system is the formation pressure; the production end pressure is the back pressure of the displacement system.

[0021] S3. Using the displacement saturation method, simulated formation crude oil is injected into the displacement system at a constant rate or constant pressure until oil is continuously produced at the production end of the displacement system and the water cut is <2%.

[0022] S4. T2 spectra were scanned using nuclear magnetic resonance (NMR) and online core imaging tests were performed to obtain the bound water saturation of the target core. S wi Initial oil saturation S oiInitial crude oil volume V oil,initial .

[0023] (3) Baseline water displacement: S1. Water drive is performed on the target core after it has been saturated with crude oil in step (2). Formation water is injected at constant pressure or constant speed and the water drive is stopped when the water cut at the production end of the displacement system is ≥98%. The water drive is then complete.

[0024] S2. T2 spectrum was scanned using nuclear magnetic resonance instrument, and online core imaging test was performed.

[0025] S3. Reset and stabilize the pressure difference between the injection end and the production end of the displacement system at 0 MPa as the starting point of the pulsed pressure difference cycle; prepare for subsequent pulsed pressure difference gas-water alternating displacement.

[0026] (4) Pulse differential pressure setting: The maximum pressure difference between the injection and production ends in the displacement system is set to ΔP. max Maximum pressure difference ΔP max That is, the pulse amplitude of the pulsed differential pressure.

[0027] Let the size of a slug in a single fluid medium be nPV, where nPV = n times the core pore volume. V pore The sum of all pore spaces inside the target core corresponds to 1 PV.

[0028] Each pulsed differential pressure cycle T = the time taken to inject 2nPV of fluid; the pulsed differential pressure cycle is the frequency of differential pressure change.

[0029] In the displacement system, the total fluid injection volume for gas-water alternating displacement is mPV, where mPV = m times the core pore volume. V pore Where m is a positive integer multiple of 2n.

[0030] Total displacement time T of air-water alternating drive total =The time taken to inject fluid into mPV.

[0031] In the overall displacement process of gas-water alternating drive, the number of complete pulse differential pressure cycles = .

[0032] (5) Inject CO2 into the sluice gate: S1. CO2 is continuously injected into the displacement system from the injection end at a constant rate.

[0033] S2. Monitor the pressure at both ends of the displacement system (injection and extraction) in real time, and calculate the pressure difference between the two ends in real time.

[0034] S3. By dynamically adjusting the flow rate of CO2 medium in the displacement system in real time, and simultaneously adjusting the pressure at the extraction end of the displacement system, the pressure difference at both ends is controlled to rise steadily according to the pressure difference rise frequency shown in formula (1). (1); In the formula, F Rise-pressure differential rise frequency, MPa / PV; —The maximum pressure difference between the injection end and the extraction end, in MPa.

[0035] S4. When the cumulative injection volume of the CO2 slug reaches the set nPV, the pressure difference simultaneously reaches the set maximum value △P. max Stop injecting CO2 and switch to injecting formation water.

[0036] (6) Injecting formation water slugs: S1. Formation water is continuously injected into the displacement system from the injection end at a constant rate. The initial pressure difference between the injection end and the production end is ΔP. max .

[0037] S2. Monitor the pressure at both ends of the displacement system (injection and extraction) in real time, and calculate the pressure difference between the two ends in real time.

[0038] S3. By dynamically adjusting the flow rate of the formation water medium in the displacement system in real time, and simultaneously adjusting the pressure at the production end of the displacement system, the pressure difference at both ends is controlled to continuously and stably decrease according to the pressure difference decrease frequency shown in formula (2). (2); In the formula, F Decrease—Frequency of pressure differential decrease, MPa / PV; —The maximum pressure difference between the injection end and the extraction end, in MPa.

[0039] S4. When the cumulative injection volume of the formation water slug reaches the set nPV, the pressure difference simultaneously reaches 0MPa, and the injection of formation water is stopped.

[0040] (7) Circulating injection of gas and water sluice gate: Continue to inject CO2 slug and formation water slug in sequence according to steps (5) and (6) to carry out gas-water alternating drive; Injection continues until the total fluid injection volume reaches mPV, at which point injection is stopped.

[0041] Completed online imaging tests of nuclear magnetic resonance T2 and core samples, and ended the gas-water alternating drive of the displacement system.

[0042] (8) Data processing: The baseline water displacement recovery rate is calculated according to formula (3): (3); In the formula, RF water —Baseline water displacement recovery, % V oil，water —Baseline water displacement oil production, mL; V oil，initial —Initial saturated oil volume, mL; S water —The sum of signal intensities in the T2 NMR spectrum after baseline water displacement; S initial —The sum of signal intensities from the matrix NMR T2 spectrum; S oil —The sum of signal intensities in the T2 NMR spectrum after saturating crude oil.

[0043] The final recovery rate is calculated according to formula (4): (4); In the formula, RF finial —Final recovery rate, % V oil，finial —Final oil production, mL; V oil，initial —Initial saturated oil volume, mL; S finial —The sum of signal intensities in the T2 NMR spectrum after gas-water alternating drive; S initial —The sum of signal intensities from the matrix NMR T2 spectrum; S oil —The sum of signal intensities in the T2 NMR spectrum after saturating crude oil.

[0044] The oil yield increase of gas-water alternating drive is calculated according to formula (5): (5); In the formula, RF increase —Oil increase rate of gas-water alternating drive, % RF finial —Final recovery rate, % RFwater —Baseline water displacement recovery rate, %.

[0045] During the displacement process, relevant tests such as T2 and online imaging can be performed, and the production metering system measures the volume of the gas, water, and oil phases produced in real time. By organizing and calculating the oil production, water production, and gas production obtained after saturating crude oil, after benchmark water displacement, and after gas-water alternating displacement, the following can be obtained: benchmark water displacement recovery rate, gas-water alternating displacement oil enhancement rate, and ultimate recovery rate.

[0046] Based on the analysis of T2 spectral data, the crude oil mobilization pattern in pores of different sizes is generally defined as follows: pores with relaxation times of 0.001ms-1ms are micropores; pores with relaxation times of 1ms-10ms are small pores; pores with relaxation times of 10ms-100ms are medium pores; and pores with relaxation times of more than 100ms are large pores.

[0047] The changes in signal intensity within different relaxation time ranges can clarify the crude oil mobilization in pores of different sizes.

[0048] According to the online imaging test images, the darker the color at the core location, the crude oil has not been effectively displaced and is still trapped in the pores, indicating crude oil enrichment. The lighter the color, the less residual oil in the area, indicating that the crude oil has been well displaced and the recovery rate is high.

[0049] The aforementioned simulation device for gas-water alternating displacement extraction process based on pulsed pressure differential includes a gas intermediate container, a formation water intermediate container, and a core holder placed inside an online nuclear magnetic resonance spectrometer. The core holder is connected to a confining pressure displacement pump via a pipeline.

[0050] The core holder is a core holder that is compatible with nuclear magnetic resonance, such as one made of a composite material of glass fiber and polyether ether ketone (PEEK), which can withstand experimental temperatures (25~100℃), confining pressure (0~60MPa) and pore pressure (0~50MPa).

[0051] A certain confining pressure is applied to the target core inside the core holder by a confining pressure displacement pump and a core holder to simulate the real stress environment of the formation, seal the displacement fluid, and prevent the fluid from flowing through the annular gap between the core and the rubber sleeve.

[0052] The temperature of the core holder can be adjusted through the main control terminal connected to the core holder to ensure that the formation temperature conditions are maintained throughout the experiment.

[0053] The inlet of the gas intermediate container is connected to the displacement pump via a pipeline; the outlet of the gas intermediate container is connected to the injection end of the core holder via a pipeline and a gas-water transfer valve; an inlet gas flow meter is installed on the pipeline between the gas intermediate container and the gas-water transfer valve.

[0054] The inlet of the formation water intermediate container is connected to the displacement pump via a pipeline; the outlet of the formation water intermediate container is connected to the injection end of the core holder via a pipeline and a gas-water transfer valve; a liquid flow meter is installed on the pipeline between the formation water intermediate container and the gas-water transfer valve.

[0055] A differential pressure controller and a pressure sensor are installed on the pipeline between the gas-water transfer valve and the core holder.

[0056] The intermediate gas container holds CO2, while the intermediate formation water container holds formation water. The injection lines for both gas and water are connected in parallel via gas-water transfer valves, then connected to the injection end of the core holder via pressure sensors and a differential pressure controller. By switching the gas-water transfer valves, alternating injection of CO2 and formation water can be achieved, simulating the alternating gas-water displacement process. The differential pressure controller controls the pressure difference between the injection and production ends, creating a pulsed differential pressure.

[0057] The core holder's extraction end is connected to a gas-liquid separation device via a pipeline. The gas-liquid separation device is connected to a liquid meter and a gas collection device, respectively. An outlet gas flow meter is installed between the gas collection device and the gas-liquid separation device.

[0058] A back pressure regulating valve is installed on the pipeline between the core holder extraction end and the gas-liquid separation device, and the back pressure regulating valve is connected to a back pressure displacement pump.

[0059] A differential pressure controller and a pressure sensor are installed on the pipeline between the core holder extraction end and the back pressure regulating valve.

[0060] Pressure sensors and differential pressure controllers are used to monitor the pressure at the injection and extraction ends in real time and to regulate the differential pressure between the two ends.

[0061] The pressure sensors at the injection and extraction ends of the core holder, the differential pressure controller, the back pressure regulating valve, the back pressure displacement pump, and the main control terminal constitute a pulsed differential pressure control system.

[0062] The differential pressure controller can control the fluid flow rate injected into the core holder, the fluid flow rate out of the core holder, and the opening of the back pressure regulating valve according to preset pulsed differential pressure parameters, thereby achieving pulsed periodic changes in the differential pressure at the inlet and outlet within a set range; the pulse period T (i.e., the time for a complete gas-water slug injection) and the pulse differential pressure amplitude Δ P max Total time T total All of these can be set, recorded, and adjusted in the data acquisition system at the central control terminal.

[0063] The pressure sensor can measure and display the pressure at both ends of the core holder inlet and outlet in real time, and summarize the pressure difference at both ends to the main control terminal.

[0064] The back pressure regulating valve is connected to the differential pressure controller and pressure sensor at the outlet of the core holder via a pipeline, and is also connected to a back pressure displacement pump to provide pore pressure to the core, thus achieving back pressure setting. Simultaneously, it works in conjunction with the differential pressure controller at the outlet to adjust the opening of the back pressure regulating valve, achieving dynamic opening adjustment.

[0065] The displacement pump, confining pressure displacement pump, core holder, pressure sensor, differential pressure controller, back pressure regulating valve, back pressure displacement pump, inlet gas flow meter, outlet gas flow meter, liquid flow meter, and online nuclear magnetic resonance spectrometer are all connected to the main control terminal.

[0066] The main control unit is connected to the high-precision displacement pump, pressure sensor, differential pressure controller, back pressure regulating valve, back pressure displacement pump, various flow meters, and online nuclear magnetic resonance spectrometer. It enables synchronous acquisition, real-time display, and automatic storage of multi-channel data. It controls the differential pressure controller to adjust parameters such as the maximum differential pressure, slug size, and total injection volume. It controls the online nuclear magnetic resonance spectrometer to perform different NMR spectrum tests. It controls the high-precision displacement pump and back pressure displacement pump to set and dynamically adjust the displacement pressure and displacement speed, thereby controlling the entire displacement device.

[0067] The gas-liquid separation device, outlet gas flow meter, liquid metering device, and gas collection device constitute the production metering system. All of them are connected to the back pressure regulating valve through pipelines. They can separate the produced CO2, crude oil, and water into gas and liquid components. The liquid metering device can measure the produced oil and water in real time to obtain experimental results such as the produced gas-oil ratio and water content. The crude oil recovery rate can be calculated through these results. The gas collection device is used to collect the gas produced in the experiment to prevent leakage.

[0068] The simulation device described in this invention combines a nuclear magnetic resonance (NMR) device with a pulsed pressure differential gas-water alternating displacement device. This allows for NMR testing during the pulsed pressure differential gas-water alternating displacement process, clarifying the pore fluid occurrence state, oil-water saturation distribution, and crude oil mobilization patterns at multiple scales. T2 testing can quantitatively evaluate the degree of mobilization and displacement efficiency of crude oil in different pore sizes by the gas-water alternating displacement. Online imaging testing can clarify the distribution of oil, gas, and water in a two-dimensional plane, observe the migration path and distribution pattern of fluids in pores, and intuitively study the microscopic mechanism of gas-water alternating displacement. It can clearly identify the enrichment areas of residual oil at different locations in the core, providing intuitive evidence for studying the effect of pulsed pressure differential gas-water alternating displacement.

[0069] In this invention, the simulation device for the gas-water alternating drive extraction process based on pulsed differential pressure is provided with an inlet gas drying pipe on the pipeline between the gas intermediate container and the inlet gas flow meter.

[0070] In this invention, the simulation device for the gas-liquid alternating drive extraction process based on pulsed differential pressure is provided with an outlet gas drying pipe on the pipeline between the gas-liquid separation device and the outlet gas flow meter.

[0071] The injected and produced CO2 gas is dried to ensure that the gas flowing into the gas flow meter is dry, thus achieving accurate measurement.

[0072] The beneficial effects of this invention are as follows: The gas-water alternating displacement and recovery process based on pulsed pressure differential described in this invention combines the pressure differential at the injection and production ends of the displacement system with the alternating pulsed changes of gas and water media to achieve a cycle of pressure differential pressurization and depressurization under artificial intervention. It utilizes the mechanical disturbance mechanism brought about by the periodic "increase-decrease" process of the pressure differential itself, such as pore elastic recoil and dominant channel blockage reconstruction, to effectively clear blocked pores, improve swept volume, suppress single dominant channels, delay crossflow time, and improve crude oil recovery rate.

[0073] Compared to conventional gas-water alternating flooding, the gas-water alternating flooding and extraction process based on pulsed differential pressure described in this invention combines dynamic energy management with alternating fluid injection, clearing blocked pore throats, helping to overcome the capillary resistance of oil droplets trapped in small pore throats (i.e., overcoming the Jamin effect), expanding the swept volume, increasing the crude oil utilization ratio and micro-displacement efficiency, better suppressing crossflow, delaying the formation of a single dominant channel for injected fluid, improving flow control, reducing ineffective injection cycles, triggering hysteresis effects to capture residual gas phase and seal crude oil, dynamically adjusting the injection-production differential pressure, adapting to heterogeneous reservoirs, enhancing the diffusion effect of CO2 and crude oil, and simulating the impact of periodic changes in reservoir pressure on displacement efficiency.

[0074] In the simulation device of the gas-water alternating displacement and extraction process based on pulsed pressure difference, a nuclear magnetic resonance device is set up to monitor the changes in pore fluid distribution at different displacement stages, quantify the displacement efficiency at different pore sizes and displacement locations, visualize the distribution of residual oil, and verify the improvement of sweep efficiency. This provides a characterization means for clarifying the crude oil mobilization law and quantifying the extraction effect of gas-water alternating displacement based on pulsed pressure difference at multiple scales. Attached Figure Description

[0075] Figure 1 This is a schematic diagram of a simulation device for the gas-water alternating displacement extraction process based on pulsed differential pressure described in this invention.

[0076] Among them, 1 is the displacement pump, 2 is the gas intermediate container, 3 is the formation water intermediate container, 4 is the inlet gas drying pipe, 5 is the inlet gas flow meter, 6 is the liquid flow meter, 7 is the gas-water transfer valve, 8 is the differential pressure controller, 9 is the pressure sensor, 10 is the core, 11 is the core holder, 12 is the online nuclear magnetic resonance spectrometer, 13 is the confining pressure displacement pump, 14 is the back pressure regulating valve, 15 is the back pressure displacement pump, 16 is the gas-liquid separation device, 17 is the liquid meter, 18 is the outlet gas drying pipe, 19 is the outlet gas flow meter, 20 is the gas collection device, and 21 is the main control terminal.

[0077] Figure 2 This is an enlarged schematic diagram of the internal structure of the gas-water transfer valve in the simulation device described in this invention.

[0078] Figure 3 This is a diagram showing the installation position of the core holder in the nuclear magnetic resonance device of the simulation apparatus described in this invention.

[0079] Figure 4 The images are online two-dimensional nuclear magnetic resonance images of the water-drive baseline experiment and the alternating displacement of CO2 and water by pressure pulse in Example 1.

[0080] Figure 5 The images show the T2 NMR spectra after the water-drive baseline experiment and the pressure-pulse CO2-water alternating displacement in Example 1.

[0081] Figure 6 The graph shows the relationship between pressure difference and PV number during the pulsed pressure difference CO2-water alternating drive process of the extraction process described in Example 1.

[0082] Figure 7 This is a graph showing the relationship between oil recovery rate and PV number for baseline water displacement and pulsed differential pressure CO2-water alternating flooding in Example 1. Detailed Implementation

[0083] The technical solution of the present invention will be described in detail below.

[0084] Example 1 The simulation device for the gas-water alternating displacement extraction process based on pulsed pressure difference includes a gas intermediate container 2, a formation water intermediate container 3, and a core holder 11 placed in an online nuclear magnetic resonance spectrometer 12. The core holder 11 is connected to a confining pressure displacement pump 13 via a pipeline.

[0085] The inlet of the gas intermediate container 2 is connected to the displacement pump 1 via a pipeline; the outlet of the gas intermediate container 2 is connected to the injection end of the core holder 11 via a pipeline and a gas-water transfer valve 7; an inlet gas flow meter 5 is installed on the pipeline between the gas intermediate container 2 and the gas-water transfer valve 7.

[0086] The inlet of the formation water intermediate container 3 is connected to the displacement pump 1 via a pipeline; the outlet of the formation water intermediate container 3 is connected to the injection end of the core holder 11 via a pipeline and a gas-water transfer valve 7; a liquid flow meter 6 is installed on the pipeline between the formation water intermediate container 3 and the gas-water transfer valve 7.

[0087] A differential pressure controller 8 and a pressure sensor 9 are installed on the pipeline between the gas-water transfer valve 7 and the core holder 11.

[0088] The core holder 11 is connected to the gas-liquid separator 16 via a pipeline. The gas-liquid separator 16 is connected to a liquid meter 17 and a gas collection device 20. An outlet gas flow meter 19 is installed between the gas collection device 20 and the gas-liquid separator 16.

[0089] A back pressure regulating valve 14 is installed on the pipeline between the core holder 11 extraction end and the gas-liquid separation device 16. The back pressure regulating valve 14 is connected to a back pressure displacement pump 15.

[0090] A differential pressure controller 8 and a pressure sensor 9 are installed on the pipeline between the core holder 11 extraction end and the back pressure regulating valve 14.

[0091] The displacement pump 1, confining pressure displacement pump 13, core holder 11, pressure sensor 9, differential pressure controller 8, back pressure regulating valve 14, back pressure displacement pump 15, inlet gas flow meter 5, outlet gas flow meter 19, liquid flow meter 6, and online nuclear magnetic resonance spectrometer 12 are all connected to the main control terminal 21.

[0092] An inlet gas drying pipe 4 is installed on the pipeline between the gas intermediate container 2 and the inlet gas flow meter 5.

[0093] An outlet gas drying pipe 18 is installed on the pipeline between the gas-liquid separator 16 and the outlet gas flow meter 19.

[0094] 99.9% pure CO2 is pressurized and stored in intermediate gas container 2. The outlet of intermediate gas container 2 is connected to inlet gas flow meter 5. The selected inlet gas flow meter 5 has a range of 0~500mL / min and an accuracy of ±0.5%FS.

[0095] Simulated formation water, prepared according to formation conditions, was stored in intermediate formation water container 3. The simulated formation water composition was: NaCl 8000 mg / L; CaCl2 1200 mg / L; heavy water (D2O) was used to eliminate NMR signal interference.

[0096] Formation water, after passing through liquid flow meter 6, merges with the gas branch through gas-water transfer valve 7, and is then connected to the injection end of core holder 11 via pressure sensor 9 and differential pressure controller 8 on the injection end side.

[0097] The MacroMR12-150H-VTHP large-aperture magnetic resonance analysis and visualization system was used. The core holder was made of non-magnetic material with an outer diameter of 60mm. The high-performance probe optimized to match the core holder had a gain greater than 20dB, a bandwidth greater than 30kHz, a maximum withstand pressure of 40MPa, a maximum withstand temperature of 80℃, and a magnetic field strength of 0.05T.

[0098] The injection end and extraction end of the core holder 11 are respectively connected to pressure sensors 9 and differential pressure controllers 8 via pipelines. Pressure sensors 9 can monitor the pressure at both ends in real time, and the differential pressure is monitored in real time through the main control terminal 21. The differential pressure controllers 8 at both ends can cooperate to adjust the differential pressure at both ends of the core holder 11, and the differential pressure is uniformly adjusted and controlled through the main control terminal 21. Related parameters include the pulse differential pressure period T (i.e., the time of a complete gas-water slug injection) and the pulse differential pressure amplitude ΔP. max Total displacement time T of air-water alternating drive total The back pressure is set and adjusted by connecting the differential pressure controller 8 and pressure sensor 9 at the extraction end to the back pressure regulating valve 14 and the back pressure displacement pump 15.

[0099] The production pipeline connects to the gas-liquid separator 16 via pressure sensor 9, differential pressure controller 8, back pressure regulating valve 14, and then to the gas-liquid separator 16. The gas outlet of the gas-liquid separator 16 is connected in sequence to the outlet gas drying pipe 18 (filled with molecular sieve desiccant, 150g), the outlet gas flow meter 19, and the gas collection device 20. The liquid outlet of the gas-liquid separator 16 is connected to the liquid metering device 17 to measure the amount of oil and water produced, thereby calculating the corresponding oil recovery rate.

[0100] Example 2 A tight sandstone reservoir (porosity φ=12.3%, permeability K=0.85mD) was used as the experimental object. The diameter of the experimental core was 25mm and the length was 70mm. Nuclear magnetic resonance (NMR) testing and a simulation experiment of pulsed differential CO2-water alternating flooding extraction process were conducted using the apparatus described in Example 1.

[0101] The gas-water alternating displacement extraction process based on pulsed differential pressure has the following specific steps: (1) Pretreatment of target core: S1. The core was washed and dried in accordance with the standard GB / T 29172-2012 "Core Analysis Methods".

[0102] S2. Obtain pore volume using a helium porosimeter. V pore =4.23 mL, confirming a porosity φ = 12.3%. The core permeability K was measured to be 0.85 mD using a permeability testing device. The matrix T2 spectrum was obtained by scanning the initial T2 relaxation spectrum using nuclear magnetic resonance.

[0103] S3. Evacuate the displacement system to a pressure <50Pa and continue for 12 hours; pressurize and saturate the formation water until saturation is complete after 48 hours.

[0104] (2) Saturated crude oil: S1. Set the confining pressure of the core holder to 30 MPa, the experimental temperature of the core holder to 74℃, and the back pressure to 20 MPa.

[0105] S2. The pressure at the extraction end of the core holder is controlled to 20MPa by the back pressure displacement pump and the back pressure regulating valve.

[0106] S3. Inject simulated formation crude oil (density 0.832 g / mL, viscosity 4.2 mPa·s, 80℃) at a constant rate of 0.05 mL / min until oil is continuously produced at the extraction end and the water cut is less than 2%.

[0107] S4. T2 spectra were scanned using nuclear magnetic resonance (NMR) and online core imaging was performed; the bound water saturation was calculated. S wi =28.6%, initial oil saturation S oi =71.4%, initial crude oil volume V oil,initial =3.02mL.

[0108] (3) Baseline water displacement: S1. Water drive is performed on the target core after it has been saturated with crude oil in step (2). Simulated formation water is injected at a constant pressure of 25 MPa. The liquid flow meter at the production end of the core holder measures the produced liquid. After oil-water separation, the amount of produced oil and water is recorded. The water drive is stopped when the water cut at the production end of the core holder is ≥98%, and the water drive is completed.

[0109] S2. T2 spectra were scanned using nuclear magnetic resonance (NMR) and online core imaging tests were performed; cumulative oil production during the waterflooding stage. V oil,water =1.32 mL, cumulative water production during the water drive phase V water =3.42 mL, water drive recovery rate RF water =43.7%.

[0110] S3. Adjust the differential pressure controller at both ends of the core holder to reset and stabilize the differential pressure at the injection end and extraction end of the core holder at 0MPa, which serves as the starting point for the pulse differential pressure cycle, and prepare to carry out the pulse differential pressure gas-water alternating displacement.

[0111] (4) Pulse differential pressure setting: The maximum pressure difference (pulse-type pressure difference amplitude) between the injection end and the extraction end of the core holder is set to ΔP using the differential pressure controller and pressure sensor at both ends of the main control terminal and the core holder. max =10MPa.

[0112] The size of a slug for a single fluid medium is set to 0.2 PV; that is, a slug for CO2 is 0.2 PV and a slug for formation water is 0.2 PV.

[0113] The time T for each pulsed differential pressure cycle is set by the main control terminal to be the time required to inject 0.4 PV of fluid; that is, the CO2 slug time of 0.2 PV + the water slug time of 0.2 PV.

[0114] In the displacement system, the total fluid injection volume for gas-water alternating displacement is 2PV.

[0115] Total displacement time T of air-water alternating drive total =Time taken to inject 2PV of fluid.

[0116] In the overall displacement process of gas-water alternating drive, the number of complete pulse differential pressure cycles k= =5. That is, a total of 5 complete pulse differential pressure cycles are performed.

[0117] (5) Inject CO2 into the sluice gate: S1. Open the intermediate gas container containing CO2 gas, and continuously inject CO2 into the injection end of the core holder at a constant rate of 0.02 ml / min through the displacement pump connected to it.

[0118] S2. Real-time monitoring of the pressure at both the injection and production ends of the displacement system, and real-time calculation of the pressure difference between the two ends. The pressure sensors at both the injection and production ends of the core holder continuously monitor the pressure at both ends, and the pressure difference between the two ends is calculated through the main control terminal.

[0119] S3. During the injection process, the main control end will continuously monitor the pressure difference between the two ends and dynamically adjust the flow rate of the CO2 medium injected into the core holder through the pressure difference controller at the injection end of the core holder. At the same time, it will also adjust the pressure difference controller at the extraction end of the core holder, thereby adjusting the pressure difference between the two ends of the core holder to ensure a continuous and stable increase at a certain pressure difference increase frequency.

[0120] The differential pressure rise frequency increases steadily according to the differential pressure rise frequency shown in formula (1); (1); In the formula, F Rise-pressure differential rise frequency, MPa / PV; —The maximum pressure difference between the injection end and the extraction end, in MPa.

[0121] S4. When the cumulative injection volume of the CO2 slug reaches the set value of 0.2PV, the pressure difference simultaneously reaches the set maximum value ΔP. max =10MPa, stop injecting CO2, stop injecting CO2 by adjusting the gas-water switching valve, and switch to injecting formation water.

[0122] (6) Injecting formation water slugs: S1. The displacement pump connected to the intermediate formation water container is turned on to continuously inject formation water into the injection end of the core holder at a constant rate of 0.01 ml / min. The initial pressure difference between the injection end and the production end is ΔP. max =10MPa.

[0123] S2. Monitor the pressure at both ends of the displacement system (injection and extraction) in real time, and calculate the pressure difference between the two ends in real time.

[0124] S3. As the injection volume continues to increase, the main control end dynamically adjusts the injection speed of formation water into the core holder through the differential pressure controller based on the real-time differential pressure feedback from the pressure sensors at both ends of the core holder. At the same time, it also adjusts the differential pressure controller at the core holder's production end, so that the pressure setpoints at both ends are gradually adjusted at a dynamic rate, thereby causing the differential pressure at both ends to continuously and stably decrease at a certain differential pressure decrease frequency.

[0125] The frequency of pressure drop is calculated according to formula (2): (2); In the formula, F Decrease—Frequency of pressure differential decrease, MPa / PV; —The maximum pressure difference between the injection end and the extraction end, in MPa.

[0126] S4. When the cumulative injection volume of the formation water slug reaches the set 0.2PV, the pressure difference simultaneously reaches 0MPa, and the gas-water transfer valve is adjusted to stop the injection of formation water.

[0127] (7) Circulating injection of gas and water sluice gate: Continue to inject CO2 slug and formation water slug in sequence according to steps (5) and (6) to carry out gas-water alternating drive; CO2 and simulated formation water are injected alternately through the gas-water switching valve, and the differential pressure controller synchronously controls the fluid flow rate injected into the core holder and the opening of the back pressure regulating valve to realize the periodic change of the differential pressure at both ends of the core holder inlet and outlet.

[0128] Stop injecting fluid when the total injection volume reaches 2PV.

[0129] Complete nuclear magnetic resonance T2 and online imaging tests of core samples, and end the gas-water alternating displacement system; real-time dynamic analysis during displacement: T2 and online imaging tests can be performed during displacement, and the production metering system measures the volume of gas, water and oil phases produced in real time.

[0130] (8) Data processing: During the experiment, all instruments recorded data synchronously. The main experimental parameters and results are shown in Table 1 below.

[0131] Table 1 Experimental parameters and results

[0132] Based on the above experimental results, in this embodiment, under the condition of tight and low-permeability sandstone reservoir, pulsed pressure differential CO2-water alternating displacement increased oil production by 0.85 mL on the basis of water drive, improved crude oil recovery rate by 28.2 percentage points, and finally achieved a cumulative oil recovery rate of 71.9%.

[0133] Experimental results show that the gas-water alternating displacement extraction process based on pulsed pressure difference described in this invention can realize pulsed pressure difference CO2-water alternating displacement, which enhances the synergistic effect of CO2 diffusion mass transfer and sweep efficiency. Compared with the constant pressure / constant speed continuous displacement method, it effectively reduces the risk of crossflow and increases the swept volume.

[0134] Simultaneously, it can achieve multi-method joint monitoring of the entire process using nuclear magnetic resonance T2 and online imaging, making up for the shortcomings of traditional simulation devices that can only perform outlet-end measurement and cannot visualize internal displacement dynamics. Through multi-source data fusion, it can achieve accurate quantitative determination of oil enhancement rate and recovery rate, providing reliable experimental basis for the optimization design of pulse parameters (pressure difference amplitude, cycle, total injection volume) and the formulation of field application schemes. The device has a high degree of integration and can complete displacement, monitoring and data processing in one unit, providing new processes and corresponding simulation devices for crude oil extraction under different permeability and heterogeneous reservoir conditions.

Claims

1. A gas-water alternating displacement extraction process based on pulsed differential pressure, characterized in that, Includes the following steps: (1) Pretreatment of target core: S1. Wash and dry the target core. S2. Obtain the physical properties of the target core, including pore volume. V pore Porosity φ, core permeability K, and core matrix T2 spectrum; S3. Evacuate the displacement system to a pressure <50Pa and continue for 12 hours; pressurize and saturate the formation water until saturation is complete after 48 hours. (2) Saturated crude oil: S1. Set the temperature of the displacement system to the formation temperature, and apply confining pressure to the core placed in the displacement system after pretreatment in step (1) according to the formation pressure. S2, The pressure at the production end of the displacement system is adjusted to the formation pressure; S3. Using the displacement saturation method, simulated formation crude oil is injected into the displacement system at a constant rate or pressure until oil is continuously produced at the production end of the displacement system, and the water cut is <2%; S4. T2 spectra were scanned using nuclear magnetic resonance (NMR) and online core imaging tests were performed to obtain the bound water saturation of the target core. S wi Initial oil saturation S oi Initial crude oil volume V oil,initial ; (3) Baseline water displacement: S1. Water drive is performed on the target core after it has been saturated with crude oil in step (2). Formation water is injected at constant pressure or constant rate and the water drive is stopped when the water cut at the production end of the displacement system is ≥98%. The water drive is then complete. S2. T2 spectrum was scanned using nuclear magnetic resonance instrument, and online core imaging test was performed; S3. Reset and stabilize the pressure difference between the injection end and the extraction end of the displacement system at 0MPa, which serves as the starting point for the pulsed differential pressure cycle. (4) Pulse differential pressure setting: The maximum pressure difference between the injection and production ends in the displacement system is set to ΔP. max ; Let the size of a single slug in a single fluid medium be nPV; Each pulse differential pressure cycle T = the time taken to inject fluid of 2nPV; In the displacement system, the total fluid injection volume for gas-water alternating displacement is mPV; where m is a positive integer multiple of 2n. Total displacement time T of air-water alternating drive total =The time taken to inject fluid into mPV; In the overall displacement process of gas-water alternating drive, the number of complete pulse differential pressure cycles k= ; (5) Inject CO2 into the sluice gate: S1. CO2 is continuously injected into the displacement system from the injection end of the displacement system at a constant rate; S2. Monitor the pressure at both ends of the displacement system (injection and extraction) in real time, and calculate the pressure difference between the two ends in real time. S3. By dynamically adjusting the flow rate of CO2 medium in the displacement system in real time, and simultaneously adjusting the pressure at the extraction end of the displacement system, the pressure difference at both ends is controlled to rise steadily according to the pressure difference rise frequency shown in formula (1). （1）； In the formula, F Rise-pressure differential rise frequency, MPa / PV; —The maximum pressure difference between the injection end and the extraction end, in MPa; S4. When the cumulative injection volume of the CO2 slug reaches the set nPV, the pressure difference simultaneously reaches the set maximum value △P. max Stop injecting CO2 and switch to injecting formation water; (6) Injecting formation water into the sluice: S1. Formation water is continuously injected into the displacement system from the injection end at a constant rate. The initial pressure difference between the injection end and the production end is ΔP. max ; S2. Monitor the pressure at both ends of the displacement system (injection and extraction) in real time, and calculate the pressure difference between the two ends in real time. S3. By dynamically adjusting the flow rate of the formation water medium in the displacement system in real time, and simultaneously adjusting the pressure at the production end of the displacement system, the pressure difference at both ends is controlled to continuously and stably decrease according to the pressure difference decrease frequency shown in formula (2). （2）； In the formula, F Decrease—Frequency of pressure differential decrease, MPa / PV; —The maximum pressure difference between the injection end and the extraction end, in MPa; S4. When the cumulative injection volume of the formation water slug reaches the set nPV, the pressure difference simultaneously reaches 0MPa, and the injection of formation water is stopped. (7) Circulating injection of gas and water sluice gate: Continue to inject CO2 slug and formation water slug in sequence according to steps (5) and (6) to carry out gas-water alternating drive; Stop injecting fluid when the total injected volume reaches mPV; Completed online imaging tests of nuclear magnetic resonance T2 and core samples, and ended the gas-water alternating displacement system; (8) Data processing: The baseline water displacement recovery rate is calculated according to formula (3): （3）； In the formula, RF water —Baseline water displacement recovery, % V oil,water —Baseline water displacement oil production, mL; V oil,initial —Initial saturated oil volume, mL; S water —The sum of signal intensities in the T2 NMR spectrum after baseline water displacement; S initial —The sum of signal intensities from the matrix NMR T2 spectrum; S oil —The sum of signal intensities in the T2 NMR spectrum after saturating crude oil; The final recovery rate is calculated according to formula (4): （4）； In the formula, RF finial —Final recovery rate, % V oil,finial —Final oil production, mL; V oil,initial —Initial saturated oil volume, mL; S finial —The sum of signal intensities in the T2 NMR spectrum after gas-water alternating drive; S initial —The sum of signal intensities from the matrix NMR T2 spectrum; S oil —The sum of signal intensities in the T2 NMR spectrum after saturating crude oil; The oil yield increase of gas-water alternating drive is calculated according to formula (5): （5）； In the formula, RF increase —Oil increase rate of gas-water alternating drive, % RF finial —Final recovery rate, % RF water —Baseline water displacement recovery rate, %.

2. A simulation device for the gas-water alternating displacement extraction process based on pulsed differential pressure as described in claim 1, characterized in that, It includes a gas intermediate container, a formation water intermediate container, and a core holder placed inside an online nuclear magnetic resonance spectrometer, which is connected to a confining pressure displacement pump via a pipeline. The inlet of the gas intermediate container is connected to the displacement pump via a pipeline; the outlet of the gas intermediate container is connected to the injection end of the core holder via a pipeline and a gas-water transfer valve; an inlet gas flow meter is installed on the pipeline between the gas intermediate container and the gas-water transfer valve. The inlet of the formation water intermediate container is connected to the displacement pump via a pipeline; the outlet of the formation water intermediate container is connected to the injection end of the core holder via a pipeline and a gas-water transfer valve; a liquid flow meter is installed on the pipeline between the formation water intermediate container and the gas-water transfer valve. A differential pressure controller and a pressure sensor are installed on the pipeline between the gas-water transfer valve and the core holder; The core holder's extraction end is connected to a gas-liquid separation device via a pipeline. The gas-liquid separation device is connected to a liquid meter and a gas collection device, respectively. An outlet gas flow meter is installed between the gas collection device and the gas-liquid separation device. A back pressure regulating valve is installed on the pipeline between the core holder extraction end and the gas-liquid separation device. The back pressure regulating valve is connected to a back pressure displacement pump. A differential pressure controller and a pressure sensor are installed on the pipeline between the core holder extraction end and the back pressure regulating valve. The displacement pump, confining pressure displacement pump, core holder, pressure sensor, differential pressure controller, back pressure regulating valve, back pressure displacement pump, inlet gas flow meter, outlet gas flow meter, liquid flow meter, and online nuclear magnetic resonance spectrometer are all connected to the main control terminal.

3. The simulation device for gas-water alternating displacement extraction process based on pulsed differential pressure as described in claim 2, characterized in that, An inlet gas drying pipe is installed on the pipeline between the gas intermediate container and the inlet gas flow meter.

4. The simulation device for gas-water alternating displacement extraction process based on pulsed differential pressure as described in claim 2, characterized in that, An outlet gas drying pipe is installed on the pipeline between the gas-liquid separation device and the outlet gas flow meter.

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