Quantitative evaluation device and method for dissolution and precipitation effect of shale oil CO2 huff and puff

Abstract

This invention discloses a device and method for quantitatively evaluating the dissolution and precipitation effects of CO2 huff and puff in shale oil, belonging to the field of shale oil development technology. The invention includes: saturating a shale core with simulated formation water containing paramagnetic ions; completely saturating the core with shale oil to obtain a pre-huff and puff T2 NMR spectrum; injecting CO2 into the oil-saturated core, reaching a target pressure, then shutting off the well and releasing the CO2; repeating the above process of saturating the core with simulated formation water and shale oil to obtain a post-huff and puff T2 NMR spectrum; comparing the pre-huff and puff T2 NMR spectra with the post-huff and puff T2 spectra to calculate the intensity of dissolution and precipitation, respectively. This invention also discloses a device for quantitatively evaluating the dissolution and precipitation effects during CO2 huff and puff in shale oil. Using the device and method provided by this invention, the dissolution and precipitation effects and their intensity during the CO2 huff and puff process can be distinguished.

Description

Technical Field

[0001] This invention relates to the field of shale oil development technology, specifically to a device and method for quantitatively evaluating the CO2 huff and puff dissolution and precipitation effect in shale oil. Background Technology

[0002] Shale oil, as an important unconventional oil and gas resource, relies heavily on horizontal well drilling and large-scale hydraulic fracturing technology for its development. However, shale reservoirs generally possess extremely low porosity, ultra-low permeability, strong heterogeneity, and complex pore structures dominated by nanoscale pores, resulting in low natural productivity, primary recovery rates typically below 10%, and rapid production decline. Therefore, there is an urgent need for effective enhanced oil recovery (EOR) technologies to improve development outcomes. Against this backdrop, CO2 huff and puff technology has emerged as a highly promising shale oil EOR technology due to its effects of expanding crude oil and reducing its viscosity, extracting light components, reducing interfacial tension, and replenishing reservoir energy. It has attracted considerable attention in recent years.

[0003] When CO2 is injected into shale reservoirs rich in organic matter and minerals, it inevitably undergoes complex physicochemical reactions with formation water, crude oil, and rock minerals. Among these, dissolution and precipitation effects are the most critical geochemical processes. These two effects coexist and compete with each other, influencing the reservoir's pore structure and physical properties. Throughout the complete CO2 injection cycle (injection-well shut-in-production) and even multiple cycles, the same area may exhibit either a dominant dissolution or precipitation effect at different points in time. Furthermore, even at the same time point, both effects may coexist in adjacent microporous systems.

[0004] Therefore, the dissolution and precipitation effects triggered by CO2-water-oil-rock interactions are the core geochemical processes determining the dynamic evolution of reservoir properties and the final EOR (Energy Orientation Retention) effect. These two effects are complex, dynamic, coexisting, and competitive, and their net effect on reservoir properties is highly uncertain. Current evaluation methods struggle to effectively distinguish and simultaneously characterize dissolution and precipitation effects. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems in the prior art and provide a device and method for quantitatively evaluating the dissolution and precipitation effects of CO2 huff and puff in shale oil. Using the device and method provided by this invention, the dissolution and precipitation effects and their intensity during the CO2 huff and puff process can be distinguished, which has important guiding significance for the energy loss rate (EOR) of shale oil.

[0006] This invention provides a method for quantitatively evaluating the CO2 huff and puff dissolution and precipitation effect in shale oil, comprising: After cleaning and drying the shale core, simulated formation water containing paramagnetic ions was added to saturate it. Shale oil was injected into the core until it was completely saturated, and the pre-hook-out T2 NMR spectrum was measured. CO2 was injected into the oil-saturated core, and after reaching the target pressure, the well was shut off and then pumped out under reduced pressure. The core after hook-out was cleaned and dried again, and the above process of adding simulated formation water containing paramagnetic ions and injecting shale oil was repeated until the core was completely saturated, and the post-hook-out T2 NMR spectrum was measured. The intensity of the comprehensive dissolution and precipitation during the CO2 hook-out process was calculated based on the ratio of the difference between the total area of ​​the pre-hook-out and post-hook-out T2 spectra to the total area of ​​the pre-hook-out T2 spectra. When the intensity of the comprehensive dissolution and precipitation is positive, it indicates that the process is dominated by dissolution; when the intensity of the comprehensive dissolution and precipitation is negative, it indicates that the process is dominated by precipitation.

[0007] Preferably, the intensity of dissolution and the intensity of precipitation are calculated by comparing the nuclear magnetic resonance T2 spectra before and after the ingestion and swallowing. The intensity of the dissolution effect is expressed as a percentage of the difference between the T2 spectrum area corresponding to the rising part of the nuclear magnetic resonance T2 spectrum curve after swallowing and the T2 spectrum area corresponding to the pore space of the rising part of the nuclear magnetic resonance T2 spectrum curve before swallowing and the T2 spectrum area corresponding to the pore space of the rising part of the nuclear magnetic resonance T2 spectrum curve before swallowing and the T2 spectrum area corresponding to the pore space of the rising part of the nuclear magnetic resonance T2 spectrum curve before swallowing and the T2 spectrum area. The precipitation intensity is expressed as a percentage of the difference between the T2 spectral area corresponding to the descending portion of the nuclear magnetic resonance T2 spectrum curve after swallowing and the T2 spectral area corresponding to the pore space of the descending portion of the nuclear magnetic resonance T2 spectrum curve before swallowing, relative to the T2 spectral area corresponding to the pore space of the descending portion of the nuclear magnetic resonance T2 spectrum curve before swallowing.

[0008] Preferably, the CO2 injection pressure is stabilized at 10 MPa to 20 MPa; and the well shut-in time is 12h to 48h.

[0009] Preferably, during the depressurization and discharge process, the pressure is controlled within a range of 2 MPa to 5 MPa lower than the CO2 injection pressure.

[0010] Preferably, the core being fully saturated means that the amplitude error of the nuclear magnetic resonance T2 spectrum signal in two consecutive tests is less than 1%.

[0011] The present invention also discloses an apparatus for implementing the above-mentioned quantitative evaluation method of CO2 huff and puff dissolution and precipitation effect of shale oil, including a displacement system, which includes a displacement pump and intermediate containers for containing CO2, shale oil and simulated formation water respectively, wherein the displacement pump is used to provide displacement power. A CO2 huff and puff simulation system includes a core holder, a confining pressure pump, and a backpressure system. The inlet and outlet ends of the core holder are connected to the displacement system and the backpressure system, respectively, via valves. The confining pressure pump applies confining pressure to the core holder. The backpressure system includes a backpressure valve and a backpressure pump. The backpressure valve is connected to a valve at the outlet end of the core holder to control the outlet pressure of the core holder, thereby simulating the CO2 emission process. The nuclear magnetic resonance testing system includes a nuclear magnetic resonance spectrometer and a computer. The nuclear magnetic resonance spectrometer is used to perform in-situ scanning of the core in the core holder, and the computer is used to receive and process nuclear magnetic signals. A temperature control device is used to house the CO2 throughput simulation system and provide a constant temperature environment. The displacement system, CO2 huff and puff simulation system, and backpressure system are connected by pipelines to simulate the entire process of CO2 "injection-well shut-out". The nuclear magnetic resonance T2 spectrum of the core samples before and after huff and puff is detected by the nuclear magnetic resonance testing system to quantitatively evaluate the dissolution and precipitation effects.

[0012] Preferably, a gas-liquid separator is also connected to the back pressure valve.

[0013] Preferably, pressure gauges are provided at the outlet of the displacement pump, the inlet of the core holder, and upstream of the back pressure valve.

[0014] Compared with existing technologies, the advantages of this invention are as follows: This invention utilizes the shielding effect of paramagnetic ions on hydrogen nuclei signals, effectively suppressing hydrogen signals in formation water during subsequent nuclear magnetic resonance (NMR) detection. The obtained T2 spectra are almost entirely derived from crude oil in the core. This provides conditions for subsequent accurate and interference-free observation of fluid distribution changes in pore space, ensuring the accuracy and reliability of experimental data. This invention obtains pre- and post-swallowing NMR T2 spectra and characterizes and judges dissolution and precipitation effects by comprehensively assessing the intensity of dissolution and precipitation. The comprehensive dissolution and precipitation intensity is calculated based on the ratio of the difference between the total area of ​​the pre- and post-swallowing T2 spectra to the total area of ​​the pre-swallowing T2 spectra. A positive comprehensive dissolution and precipitation intensity indicates that the process is dominated by dissolution, while a negative comprehensive dissolution and precipitation intensity indicates that the process is dominated by precipitation. This solves the problem of existing technologies being unable to effectively distinguish and simultaneously characterize dissolution and precipitation effects.

[0015] This invention also uses local variations in the nuclear magnetic resonance T2 spectrum to provide precise numerical intensity for the dissolution and precipitation effects. This not only distinguishes between the dissolution and precipitation effects during CO2 huff and puff, but also quantifies the impact of CO2 huff and puff parameters such as CO2 injection pressure, well shut-in time, and huff and puff cycles on the dissolution and precipitation effects. This enables accurate evaluation of the dissolution and precipitation effects during shale oil CO2 huff and puff, providing theoretical support for optimizing shale oil huff and puff development schemes.

[0016] The device provided by this invention, which can comprehensively evaluate the dissolution and precipitation effects during the CO2 huff and puff process of shale oil, is equipped with a combination displacement pump, intermediate container, core holder, back pressure valve, and nuclear magnetic resonance spectrometer. It can simulate the shale oil huff and puff process, monitor the fluid transport law during the CO2 huff and puff process, and quantitatively evaluate the occurrence characteristics of various fluids in pore spaces of different scales by combining the evaluation method of this invention. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of the device according to an embodiment of the present invention.

[0018] Figure 2 This is a schematic diagram of the method flow according to an embodiment of the present invention.

[0019] Figure 3 The T2 NMR spectra are before and after CO2 huff and puff in shale oil.

[0020] Explanation of reference numerals in the attached figures: 1. Displacement pump working medium container; 2. Displacement pump; 3. Pressure gauge 1; 4. Nuclear magnetic resonance spectrometer; 5. Computer; 6. Valve 1; 7. Three-way valve 1; 8. Three-way valve 2; 9. Valve 2; 10. Valve 3; 11. Valve 4; 12. Intermediate container 1; 13. Intermediate container 2; 14. Intermediate container 3; 15. Valve 5; 16. Valve 6; 17. Valve 7; 18. Three-way valve 3; 19. Three-way valve 20. Valve 4; 21. Pressure gauge 2; 22. Six-way valve; 23. Three-way valve 5; 24. Valve 8; 25. Back pressure valve; 26. Back pressure pump; 27. Pressure gauge 3; 28. Confining pressure pump; 29. ​​Valve 10; 30. Valve 11; 31. Core holder; 32. Pressure gauge 4; 33. Valve 12; 34. Gas-liquid separator bottle; 35. Drying bottle; 36. Degreased cotton; 37. Thermostatic device. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0022] Unless otherwise defined, the technical or scientific terms used herein shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms “first,” “second,” and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as “comprising” or “including” indicate that the elements or objects preceding “comprising” or “including” encompass the elements or objects listed following “comprising” or “including” and their equivalents, and do not exclude other elements or objects. Terms such as “connected” or “linked” are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as “upper,” “lower,” “left,” and “right” are used only to indicate relative positional relationships; when the absolute position of the described objects changes, the relative positional relationship may also change accordingly.

[0023] This embodiment provides a quantitative evaluation method for the CO2 huff and puff dissolution and precipitation effect of shale oil, comprising: cleaning and drying shale cores, then saturating them with simulated formation water containing paramagnetic ions; the simulated formation water needs to reach the salinity of the formation water in the mining area, and injecting shale oil into the cores until the cores are completely saturated, wherein complete saturation means that the core mass no longer changes with the injection of simulated formation water and shale oil. In this embodiment, 40% MnCl2 by mass is added to the simulated formation water to shield the hydrogen signal in the simulated formation water; the shale oil is shale oil produced from the oil field, and the T2 NMR spectrum before huff and puff is obtained; CO2 is injected into the saturated oil core, and after reaching the target pressure, the well is shut in, followed by depressurization and pumping out. The core is then washed and dried again, and the process of adding simulated formation water with paramagnetic ions and injecting shale oil is repeated until the core is completely saturated. The nuclear magnetic resonance T2 spectrum after the pumping is obtained. The intensity of the comprehensive dissolution and precipitation during the CO2 pumping process of the shale oil is calculated based on the ratio of the difference between the total area of ​​the T2 spectrum before and after the pumping to the total area of ​​the T2 spectrum before the pumping. When the intensity of the comprehensive dissolution and precipitation is positive, it indicates that the process is dominated by dissolution; when the intensity of the comprehensive dissolution and precipitation is negative, it indicates that the process is dominated by precipitation.

[0024] In another preferred embodiment of the present invention, the intensity of dissolution and the intensity of precipitation are calculated by comparing the NMR T2 spectrum before and after the spitting-out process. The intensity of dissolution is expressed as the percentage of the area difference between the T2 spectrum area corresponding to the rising portion of the NMR T2 spectrum curve caused by the dissolution effect after spitting-out and the area corresponding to the pore space of the rising portion of the NMR T2 spectrum curve before spitting-out, relative to the area corresponding to the pore space of the rising portion of the NMR T2 spectrum curve before spitting-out. The intensity of precipitation is expressed as the percentage of the area difference between the area of ​​the T2 spectrum area corresponding to the falling portion of the NMR T2 spectrum curve caused by the precipitation effect after spitting-out and the area corresponding to the pore space of the falling portion of the NMR T2 spectrum curve before spitting-out, relative to the area corresponding to the pore space of the falling portion of the NMR T2 spectrum curve before spitting-out.

[0025] More specifically, in this embodiment, the decrease in the post-swallowing NMR T2 spectrum compared to the pre-swallowing curve is the result of the precipitation effect, and the increase in the post-swallowing NMR T2 spectrum compared to the pre-swallowing curve is the result of the dissolution effect. The ratio of the area of ​​T2 spectrum increase caused by the dissolution effect to the area of ​​T2 spectrum corresponding to the pore space of the increased portion of the pre-swallowing NMR T2 spectrum is the intensity of the dissolution effect. D i (Equation 1) The ratio of the area of ​​the NMR T2 spectrum decrease caused by the precipitation effect to the area of ​​the T2 spectrum corresponding to the pore space of the decreased NMR T2 spectrum curve before swallowing and puffing is the precipitation intensity. S i (Equation 2) The ratio of the difference in T2 spectrum area before and after huff and puff to the T2 spectrum area before huff and puff is the intensity of the comprehensive dissolution and precipitation effect. R (Equation 3) If the value is positive, it indicates that the CO2 huff and puff process is dominated by dissolution. If the value is negative, it indicates that the CO2 huff and puff process is dominated by precipitation. This allows for a quantitative evaluation of the intensity of dissolution and precipitation effects during the CO2 huff and puff process of shale oil.

[0026] (1); In the formula, A i The area of ​​the T2 spectrum corresponding to the rising portion of the nuclear magnetic resonance T2 spectrum curve caused by the dissolution effect after throughput. B i The T2 spectral area corresponding to the pore space in the rising portion of the nuclear magnetic resonance T2 spectrum curve before throughput; (2); In the formula, The area of ​​the T2 spectrum corresponding to the decrease in the T2 spectrum curve caused by the precipitation effect after swallowing is shown. The T2 spectral area corresponding to the pore space in the descending portion of the nuclear magnetic resonance T2 spectrum curve before throughput; (3); In the formula, A The area of ​​the T2 NMR spectrum after throughput. B The area of ​​the nuclear magnetic resonance T2 spectrum curve before spitting out; it should be understood that the intensity of dissolution and precipitation effects under different spitting out parameters can also be compared by setting different spitting out pressures, well-closing times and spitting out cycles.

[0027] This embodiment utilizes low-field nuclear magnetic resonance (NMR) technology to directly observe T2 spectrum changes during shale oil huff and puff. The T2 spectrum reflects the pore structure and fluid occurrence characteristics of shale, with the transverse relaxation time T2 being directly proportional to the pore structure, and the signal amplitude reflecting changes in pore volume. During shale oil huff and puff, the dissolution effect improves shale properties, manifested as an increase in the T2 spectrum amplitude; conversely, the precipitation effect deteriorates shale properties, manifested as a decrease in the T2 spectrum amplitude. Therefore, this embodiment quantitatively evaluates the dissolution and precipitation effects by observing changes in the shape and amplitude of the T2 spectrum curve.

[0028] In a preferred embodiment, the CO2 injection pressure is stabilized at 10 MPa to 20 MPa; the well-closing time is 12h to 48h, used to simulate the actual injection pressure and well-closing process.

[0029] In a preferred embodiment, during the depressurization and discharge process, the pressure is controlled within a range of 2 MPa to 5 MPa lower than the CO2 injection pressure. This pressure not only protects the core and prevents damage to the core due to excessive pressure difference, but also avoids premature and excessive CO2 production, thereby improving the utilization efficiency of CO2.

[0030] In a preferred embodiment, the core being fully saturated means that the amplitude error of the nuclear magnetic resonance T2 spectrum signal in two consecutive tests is less than 1%.

[0031] This embodiment also discloses an apparatus for implementing the above-mentioned quantitative evaluation method of CO2 huff and puff dissolution precipitation effect of shale oil. The apparatus includes a displacement system, including a displacement pump 2 and intermediate containers for containing CO2, shale oil and simulated formation water, respectively. The displacement pump 2 is used to provide displacement power. The CO2 huff and puff simulation system includes a core holder 31, a confining pressure pump 27, and a back pressure system. The inlet and outlet ends of the core holder 31 are connected to the displacement system and the back pressure system, respectively, via valves. The confining pressure pump 27 is used to apply confining pressure to the core holder 31. The back pressure system includes a back pressure valve 24 and a back pressure pump 25. The back pressure valve 24 is connected to the valve at the outlet end of the core holder 31 and is used to precisely control the outlet pressure of the core holder 31 to simulate the CO2 emission process. The nuclear magnetic resonance testing system includes a nuclear magnetic resonance instrument 4 and a computer 5. The nuclear magnetic resonance instrument 4 is used to perform in-situ scanning of the core in the core holder 31, and the computer 5 is used to receive and process nuclear magnetic signals. Thermostat 37 is used to house the CO2 throughput simulation system and provide a constant temperature environment; The displacement system, CO2 huff and puff simulation system, and backpressure system are connected by pipelines to simulate the entire process of CO2 "injection-well shut-out". The nuclear magnetic resonance T2 spectrum of the core samples before and after huff and puff is detected by the nuclear magnetic resonance testing system to quantitatively evaluate the dissolution and precipitation effects.

[0032] In a preferred embodiment, a gas-liquid separation device is also connected to the back pressure valve 24.

[0033] In a preferred embodiment, pressure gauges are provided at the outlet of the displacement pump 2, the inlet of the core holder 31, and upstream of the back pressure valve 24.

[0034] like Figure 1The device shown is a quantitative evaluation device for the CO2 huff and puff dissolution and precipitation effect of shale oil, including a displacement system, a CO2 huff and puff system, a back pressure system, a constant temperature device 37, and a nuclear magnetic resonance testing system. The displacement system includes a displacement pump working medium container 1, a displacement pump 2, an intermediate container 12, an intermediate container 2 13, and an intermediate container 3 14 connected in sequence by pipelines. More specifically, the displacement pump working medium container 1 is filled with distilled water and is connected to the inlet pipeline of the displacement pump 2. The outlet pipeline of the displacement pump 2 is equipped with a valve 1 6, a three-way valve 3 18, and a three-way valve 4 19 in sequence. The three-way valves 3 18 and 4 19, as well as the end of the outlet pipeline, are connected in sequence to the lower interfaces of the intermediate containers 12, 23, and 34. The upper interfaces of the intermediate containers 12, 23, and 34 are respectively connected to the upper interface output main pipe. The upper and lower interfaces of the intermediate container 12 are also connected to valves 2 9 and 5 15, respectively. The intermediate container 12 contains simulated formation water. The upper and lower interfaces of intermediate container 2 13 are respectively connected to valve 3 10 and valve 6 16, and intermediate container 2 13 contains shale oil; the upper and lower interfaces of intermediate container 3 14 are respectively connected to valve 4 11 and valve 7 17, and intermediate container 3 14 contains CO2; the outlet of valve 2 9 at the upper interface of intermediate container 12 is connected to the upper interface output main pipe, the outlet of valve 3 10 of intermediate container 2 13 is connected to the upper interface output main pipe through three-way valve 1 7, and valve 4 11 of intermediate container 3 14 is connected to the upper interface output main pipe through three-way valve 2 8. A pressure gauge 1 3 is installed on the pipeline between the displacement pump 2 and intermediate container 1 12 to monitor the injection pressure of the displacement pump 2; the output end of the upper interface output main pipe is connected to a six-way valve 21, and a pressure gauge 2 20 is installed on the six-way valve 21 to monitor the pressure at the injection end of the core holder 31.

[0035] The CO2 huff and puff system includes a core holder 31 and a confining pressure pump 27 connected to the core holder 31 via a pipeline. The connecting pipeline between the core holder 31 and the confining pressure pump 27 is also equipped with a pressure gauge 26 and a valve 29. The pressure gauge 26 is used to monitor the confining pressure of the core holder 31. The back pressure system includes a back pressure valve 24 and a back pressure pump 25. The CO2 huff and puff system is located within a thermostat 37. More specifically, the inlet end of the core holder 31 is connected to a six-way valve 21 via an inlet pipeline, and the outlet pipeline of the core holder 31 is connected to valve 30. The inlet pipeline of the core holder 31 is sequentially equipped with a three-way valve 22 and a valve 28. The three-way valve 22 connects to the back pressure valve 24 via valve 23. One end of the pipeline is connected, and the other end of the back pressure valve 24 is connected to the back pressure pump 25; the back pressure valve 24 is also connected to the gas-liquid separation bottle 34 and the drying bottle 35 in sequence. A pressure gauge 4 32 and a valve 12 33 are installed in sequence on the pipeline connecting the back pressure valve 24 and the gas-liquid separation bottle 34; the pressure gauge 4 32 is used to monitor the back pressure; the mouths of the gas-liquid separation bottle 34 and the drying bottle 35 are connected by a rubber stopper pipeline. The drying bottle 35 contains degreased cotton 36, which is used to absorb the residual liquid carried out in the gas-liquid separation bottle 34. As a preferred embodiment, a gas flow meter is connected to the outlet of the drying bottle 35; the nuclear magnetic resonance testing system includes a nuclear magnetic resonance spectrometer 4 and a computer 5. The nuclear magnetic resonance spectrometer 4 is connected to the computer 5 through a data cable.

[0036] This embodiment also discloses a method for quantitatively evaluating the CO2 huff and puff dissolution and precipitation effect of shale oil, including the following steps: Step 1: The core was placed in the extraction solvent for cleaning. After cleaning, it was placed in a constant temperature oven. After taking it out, the porosity and permeability of the core were tested and found to be 6.44% and 0.0668 mD, respectively. Step 2: Prepare simulated formation water to achieve a mineralization of 54.9 g / L in the mine. Place the core into the core holder 31 and continuously saturate it with simulated formation water. When the core mass remains constant, the simulated formation water is completely saturated. Add 40% MnCl2 by mass to the simulated formation water to shield the hydrogen signal in the simulated formation water. Step 3: Use a core holder to extract shale oil from the 31 saturated oil field. After the shale oil is fully saturated, collect nuclear magnetic resonance T2 spectra. Fully saturated shale oil means that there is no significant difference between two consecutive nuclear magnetic resonance T2 spectra.

[0037] Step 4: After the core is saturated with shale oil, close all valves on the pipeline, open valve 16, valve 717, valve 41, and valve 928, start displacement pump 2, and inject CO2 from intermediate container 314 into core holder 31 to simulate CO2 injection. Monitor the CO2 injection pressure through pressure gauge 20. When the CO2 injection pressure stabilizes at 15MPa, close valve 928 to simulate the well shut-in process. Step 5: After the well has been sealed for 24 hours, adjust the back pressure pump 25 to control the back pressure to be lower than the CO2 injection pressure, and open valves 9 28, 8 23, and 12 33 to allow the fluid in the core to flow out, simulating the CO2 release process; Step Six: After CO2 huff and puff, the core is washed again in the extractant. After washing, the shale oil is resaturated, and the core's T2 NMR spectrum is tested. Figure 3 As shown, comparing the changes in NMR T2 spectra before and after the throughput, the decrease in the T2 spectrum after throughput compared to the curve before throughput is the result of the precipitation effect, while the increase in the T2 spectrum after throughput compared to the curve before throughput is the result of the dissolution effect. The ratio of the area of ​​T2 spectrum rise caused by the dissolution effect to the area of ​​the T2 spectrum corresponding to the pore space of the rise portion of the pre-swallowing NMR T2 spectrum curve is the intensity of the dissolution effect. D 1 is 10.04%, which is the ratio of the area of ​​the NMR T2 spectrum decrease caused by the precipitation effect to the area of ​​the T2 spectrum corresponding to the pore space of the decreased part of the NMR T2 spectrum before swallowing and puffing, i.e., the intensity of precipitation. S 1 and S The percentages were 13.02% and 33.01%, respectively. The ratio of the difference in T2 spectral area before and after huff and puff to the T2 spectral area before huff and puff was the intensity of the comprehensive dissolution and precipitation effect. R The value was 4.77%, which is positive, indicating that the CO2 huff and puff process is mainly characterized by dissolution.

[0038] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for quantitatively evaluating the CO2 huff and puff dissolution and precipitation effect in shale oil, characterized in that, include: After cleaning and drying the shale core, simulated formation water containing paramagnetic ions was added to saturate it. Shale oil was injected into the core until it was completely saturated, and the pre-hook-out T2 NMR spectrum was measured. CO2 was injected into the oil-saturated core, and after reaching the target pressure, the well was shut off and then pumped out under reduced pressure. The core after hook-out was cleaned and dried again, and the process of adding simulated formation water containing paramagnetic ions and injecting shale oil was repeated until the core was completely saturated, and the post-hook-out T2 NMR spectrum was measured. The intensity of the comprehensive dissolution and precipitation during the CO2 hook-out process was calculated based on the ratio of the difference between the total area of ​​the pre-hook-out and post-hook-out T2 spectra to the total area of ​​the pre-hook-out T2 spectra. When the intensity of the comprehensive dissolution and precipitation is positive, it indicates that the process is dominated by dissolution; when the intensity of the comprehensive dissolution and precipitation is negative, it indicates that the process is dominated by precipitation.

2. The method for quantitatively evaluating the CO2 huff and puff dissolution and precipitation effect of shale oil as described in claim 1, characterized in that, By comparing the nuclear magnetic resonance T2 spectra before and after the blotting, the intensity of dissolution and precipitation were calculated respectively. The intensity of the dissolution effect is expressed as the percentage of the T2 spectrum area corresponding to the rising part of the nuclear magnetic resonance T2 spectrum curve after swallowing and the T2 spectrum area corresponding to the pore space of the rising part of the nuclear magnetic resonance T2 spectrum curve before swallowing and to the T2 spectrum area corresponding to the pore space of the rising part of the nuclear magnetic resonance T2 spectrum curve before swallowing and to the T2 spectrum area corresponding to the pore space of the rising part of the nuclear magnetic resonance T2 spectrum curve before swallowing and to the T2 spectrum area. The precipitation intensity is expressed as a percentage of the T2 spectral area corresponding to the descending portion of the nuclear magnetic resonance T2 spectrum curve after swallowing and the T2 spectral area corresponding to the pore space of the descending portion of the nuclear magnetic resonance T2 spectrum curve before swallowing, relative to the T2 spectral area corresponding to the pore space of the descending portion of the nuclear magnetic resonance T2 spectrum curve before swallowing.

3. The method for quantitatively evaluating the CO2 huff and puff dissolution and precipitation effect of shale oil as described in claim 1, characterized in that, The CO2 injection pressure is stabilized at 10 MPa to 20 MPa; the well shut-in time is 12h to 48h.

4. The method for quantitatively evaluating the CO2 huff and puff dissolution and precipitation effect of shale oil as described in claim 1, characterized in that, During the depressurization and discharge process, the pressure is controlled within a range of 2 MPa to 5 MPa lower than the CO2 injection pressure.

5. The method for quantitatively evaluating the CO2 huff and puff dissolution and precipitation effect of shale oil as described in claim 1, characterized in that, The core being fully saturated means that the amplitude error of the nuclear magnetic resonance T2 spectrum signal in two consecutive tests is less than 1%.

6. The apparatus for quantitative evaluation of the CO2 huff-and-puff dissolution and precipitation effect of shale oil as described in claim 1, characterized in that, include: The displacement system includes a displacement pump and intermediate containers for containing CO2, shale oil and simulated formation water, respectively, wherein the displacement pump is used to provide displacement power; A CO2 huff and puff simulation system includes a core holder, a confining pressure pump, and a backpressure system. The inlet and outlet ends of the core holder are connected to the displacement system and the backpressure system, respectively, via valves. The confining pressure pump applies confining pressure to the core holder. The backpressure system includes a backpressure valve and a backpressure pump. The backpressure valve is connected to a valve at the outlet end of the core holder to control the outlet pressure of the core holder, thereby simulating the CO2 emission process. The nuclear magnetic resonance testing system includes a nuclear magnetic resonance spectrometer and a computer. The nuclear magnetic resonance spectrometer is used to perform in-situ scanning of the core in the core holder, and the computer is used to receive and process nuclear magnetic signals. A temperature control device is used to house the CO2 throughput simulation system and provide a constant temperature environment. The displacement system, CO2 huff and puff simulation system, and backpressure system are connected by pipelines to simulate the entire process of CO2 "injection-well shut-out". The nuclear magnetic resonance T2 spectrum of the core samples before and after huff and puff is detected by the nuclear magnetic resonance testing system to quantitatively evaluate the dissolution and precipitation effects.

7. The apparatus as claimed in claim 6, characterized in that, A gas-liquid separator is also connected to the back pressure valve.

8. The apparatus as claimed in claim 6, characterized in that, Pressure gauges are provided at the outlet of the displacement pump, the inlet of the core holder, and upstream of the back pressure valve.

Images