A Comprehensive Evaluation Method for Shale Oil Mobility Based on ScCO2 Displacement and Multiple Technologies

Abstract

This invention relates to the field of unconventional oil and gas exploration and development technology, and discloses a comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and multiple technologies. The method includes: preparing samples to obtain oil-bearing shale core columns; performing in-situ characterization of the oil-bearing shale core columns before displacement, generating pre-displacement characterization results and post-test core columns; performing ScCO2 displacement experiments on the post-test core columns to generate post-displacement core columns; performing post-displacement in-situ characterization of the post-displacement core columns to generate post-displacement characterization results; combining the pre- and post-displacement characterization results for comprehensive data analysis and evaluation, generating analytical evaluation results that include quantitative mobility evaluation characteristics and component difference patterns; and outputting a comprehensive conclusion based on the analytical evaluation results. This invention utilizes a ScCO2 high-temperature and high-pressure displacement device, combining two-dimensional nuclear magnetic resonance (NMR T1-T2) with thermal evaporation gas chromatography (Tvap-GC) to simultaneously acquire fluid mobility and crude oil component spatial differentiation data, solving the problem of difficulty in quantifying shale oil mobility patterns.

Description

Technical Field

[0001] This invention relates to the field of unconventional oil and gas exploration and development technology, specifically to a comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and the combined use of multiple technologies. Background Technology

[0002] Shale reservoirs possess nanoscale pore-throat systems and complex fluid occurrence characteristics, posing technical bottlenecks such as injection difficulties and low recovery rates during conventional water or gas injection development. Supercritical CO2, combining the diffusivity of gases with the solubility of liquids, offers advantages in crude oil viscosity reduction, volume expansion, and light hydrocarbon extraction, and has become an important means of enhancing shale oil recovery. Accurately assessing the mobility of shale oil during supercritical CO2 displacement is crucial for studying displacement mechanisms and optimizing field engineering parameters.

[0003] Currently, nuclear magnetic resonance (NMR) technology is commonly used to characterize the occurrence state and mobilization degree of reservoir fluids in different pores, gas chromatography is mostly used to analyze the hydrocarbon composition of crude oil, while supercritical CO2 displacement technology is used to reproduce fluid flow and extraction behavior under formation temperature and pressure conditions. In actual evaluation, existing methods often rely on a single characterization technique, making it difficult to establish a correspondence between macroscopic fluid flow and microscopic component changes. Under supercritical CO2 displacement conditions, CO2 not only has a displacement effect on shale oil but also preferentially extracts and migrates lighter components. If only NMR testing is used, it can only obtain the increase or decrease in the total amount of fluid in the pores, but cannot reveal the migration law of different hydrocarbon components along the seepage path; if only component analysis methods are relied upon, it is usually necessary to crush the entire sample, which destroys the original structural and spatial information, making it impossible to quantify the mobilization degree of fluids under different occurrence states.

[0004] Due to the lack of in-situ, dynamic, multidimensional coupled analysis methods, existing technologies cannot simultaneously obtain information on changes in the occurrence state of crude oil before and after displacement, as well as the component differentiation at different spatial locations. This leads to a lack of understanding of the multiple control mechanisms in heterogeneous shale oil reservoirs. Current single evaluation models are insufficient to reflect the correlation between changes in fluid occurrence state and component changes, making it difficult to quantify the mobilization patterns of shale oil in different occurrence states. Furthermore, the dynamic response mechanisms during development are not adequately characterized, ultimately failing to accurately and comprehensively evaluate the overall mobility of shale oil. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and the combined use of multiple technologies. This method can evaluate shale oil mobility in situ, dynamically, and quantitatively, solving the problems of unclear understanding of multiple control mechanisms in heterogeneous shale oil reservoirs, difficulty in quantifying the oil movement and utilization patterns under different occurrence states, and insufficient characterization of dynamic response mechanisms during development.

[0006] To achieve the above objectives, the present invention provides a comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and the combined use of multiple technologies, comprising the following steps: Perform sample preparation to obtain oil-bearing shale core columns; The oil-bearing shale core column was characterized in situ before displacement, and the characterization results before displacement and the core column after testing were generated. A ScCO2 displacement experiment was performed on the core column after the test to generate a displaced core column; The core column after displacement was characterized in situ to generate the post-displacement characterization results. The pre-displacement characterization results and the post-displacement characterization results are combined for comprehensive data analysis and evaluation to generate analysis and evaluation results, which include quantitative mobility evaluation characteristics and component difference patterns. Based on the analysis and evaluation results, a comprehensive conclusion is output, providing a basis for the selection and development of desserts.

[0007] Preferably, the step of performing sample preparation to obtain an oil-bearing shale core column specifically includes: Oil-bearing shale samples harvested from closed core wells in the target block were subjected to cryopreservation to generate frozen oil-bearing shale samples. The frozen oil-bearing shale sample was drilled in a cold table operating environment of less than or equal to -20°C, and processed to generate the oil-bearing shale core column; Representative shale oil samples were obtained, and multi-gradient quantitative weighing and sealing were performed to generate calibration samples with multiple mass gradients. Using a low-field nuclear magnetic resonance (LF-NMR) spectrometer, two-dimensional nuclear magnetic resonance (NMR T1-T2) experiments were sequentially performed on the calibration samples of the multiple mass gradients to generate multiple sets of raw two-dimensional relaxation measurement data. Based on the multiple sets of two-dimensional relaxation measurement raw data, the signal intensity is generated by inversion fitting, and the calibration coefficient corresponding to the quantitative conversion relationship is calculated by linear regression formula in combination with crude oil quality.

[0008] Preferably, the steps of performing in-situ characterization of the oil-bearing shale core column before displacement and generating the characterization results before displacement and the core column after testing specifically include: Using LF-NMR, nuclear magnetic resonance tests were conducted on the oil-bearing shale core column to generate the NMR T1-T2 spectrum in the pre-displacement characterization results. The normalized content of adsorbed oil, bound oil and movable oil per unit sample mass was obtained by combining the calibration coefficients. Using a micro-area sampler and a cavity repair tool, a small amount of powder sample was obtained by sampling the oil-bearing shale core column. The sampling points included the bottom surface, i.e. the subsequent fluid injection end, and the axial side surface area. The cavity left after drilling was filled and repaired using an inert material without nuclear magnetic resonance signal to generate the tested core column that maintains the integrity of the sample structure. The powder sample was analyzed by thermal evaporation gas chromatography (Tvap-GC) using a micro-closed glass tube thermal simulation system (MSSV) to generate the original hydrocarbon components from the Tvap-GC analysis in the pre-displacement characterization results.

[0009] Preferably, the steps of performing a ScCO2 displacement experiment on the tested core column to generate a displaced core column specifically include: Using the ScCO2 high-temperature and high-pressure displacement device, the core column after the test is loaded into the high-pressure holder, radial confining pressure is applied and the temperature is raised to the set reservoir temperature, and carbon dioxide gas is injected under pressure to the target displacement pressure so that the carbon dioxide gas reaches the supercritical phase state and a phase equilibrium state is established. In the phase equilibrium state, supercritical carbon dioxide is continuously pumped from the injection end to perform isothermal and isobaric continuous displacement operation, and after the displacement operation is completed, a slow depressurization operation is performed with the internal pressure drop rate less than or equal to 0.1 MPa per minute to generate a sample that has undergone fluid action. After the system pressure is unloaded to atmospheric pressure, the sample that has undergone fluid action is taken out to generate the displacement core column.

[0010] Preferably, the step of performing in-situ characterization of the displaced core column to generate the characterization results specifically includes: The displacement-induced NMR T1-T2 spectrum was performed on the core column by LF-NMR to generate the NMR T1-T2 spectrum in the characterization results after displacement, and the residual oil content in different occurrence states after displacement was calculated. Using a micro-area sampler and a cavity repair tool, rock powder was drilled from the bottom and axial surfaces of the core column after displacement at locations adjacent to or corresponding to those before displacement, thus obtaining powder samples after displacement. The displaced powder sample was subjected to Tvap-GC analysis using MSSV to generate the residual hydrocarbon components from the Tvap-GC analysis in the characterization results after displacement.

[0011] Preferably, the step of combining the pre-displacement characterization results and the post-displacement characterization results to perform comprehensive data analysis and evaluation, and generating analysis and evaluation results, includes: Extract the fluid content data of each relaxation interval from the characterization results before and after displacement; Based on the fluid content data, the oil phase mobility is calculated using the formulas for the percentage of adsorbed oil displacement, the percentage of bound oil displacement, and the percentage of mobile oil displacement, thereby deriving the quantitative mobility evaluation characteristics in the analysis and evaluation results.

[0012] Preferably, the step of combining the pre-displacement characterization results and the post-displacement characterization results to perform comprehensive data analysis and evaluation, and generating analysis and evaluation results, further includes: Extract the response peak information of samples at different seepage locations from the characterization results before and after displacement; Based on the response peak information, the deviation of crude oil components along the flow path is evaluated using the light-heavy hydrocarbon ratio calculation formula. The spatial differentiation effect of hydrocarbon components along the seepage path caused by the dissolution and migration of different components in shale oil with fluid is analyzed, thereby obtaining the component difference characteristics in the analysis and evaluation results.

[0013] Preferably, the step of combining the pre-displacement characterization results and the post-displacement characterization results to perform comprehensive data analysis and evaluation, and generating analysis and evaluation results, further includes: Relevant parameters are extracted based on the pre-displacement characterization results and the post-displacement characterization results; The occurrence state utilization index calculated based on the movable percentage of movable oil displacement, and the light and heavy hydrocarbon differentiation index and spatial migration index calculated based on the light and heavy hydrocarbon ratio calculation formula are substituted into the comprehensive mobility index evaluation formula for coupling calculation to generate the shale oil comprehensive mobility index in the analysis and evaluation results.

[0014] Preferably, the step of outputting a comprehensive conclusion based on the analysis and evaluation results includes: evaluating the development potential of the target strata based on the comprehensive mobility index of shale oil.

[0015] This invention provides a comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and the combined use of multiple technologies. It has the following beneficial effects: 1. This invention obtains fluid content data in different relaxation ranges before and after displacement of oil-bearing shale core columns through two-dimensional nuclear magnetic resonance testing, calculates and quantifies the mobilization characteristics of adsorbed oil, bound oil and mobile oil, solves the problem of difficulty in quantifying the mobilization law of shale oil in different occurrence states, and realizes the quantitative evaluation of the oil phase mobilization capacity in each pore space inside the reservoir.

[0016] 2. This invention uses micro-area fixed-point sampling combined with thermal evaporation gas chromatography to obtain the distribution characteristics of hydrocarbon components at different spatial locations before and after supercritical carbon dioxide displacement. This avoids the loss of location information caused by the overall pulverization analysis method, preserves the spatial difference data of fluid along the displacement direction, reveals the selective migration law of different components of shale oil along the seepage path, and solves the problem of insufficient characterization of dynamic response mechanism during development.

[0017] 3. This invention combines supercritical carbon dioxide displacement with multiple characterization techniques to simultaneously extract parameters such as shale oil occurrence state mobilization indicators and light and heavy hydrocarbon differentiation indicators to construct a comprehensive mobility index. It couples macroscopic physical fluid test quantities with microscopic component differences, solving the problem of unclear understanding of multiple control mechanisms in heterogeneous shale oil reservoirs, improving the completeness of mobility evaluation, and providing a basis for judgment on shale oil reservoir development classification, block screening, and the formulation of enhanced oil recovery schemes. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the comprehensive evaluation method for shale oil mobility and compositional differences according to the present invention; Figure 2 This is a graph showing the relationship between crude oil quality and nuclear magnetic resonance signal intensity according to the present invention. Figure 3 The NMR T1-T2 spectra of the shale core sample before displacement according to the present invention; Figure 4 The NMR T1-T2 spectra of the shale core sample after displacement according to the present invention; Figure 5 This is a diagram showing the Tvap-GC test results of the shale sample before displacement according to the present invention; Figure 6 The figure shows the Tvap-GC test results of the shale samples after displacement according to the present invention. Detailed Implementation

[0019] The technical solutions in 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 embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] This invention provides a comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and the combined use of multiple technologies. This method operates using a specific hardware system architecture. This hardware system architecture includes a cryogenic sampling device, a low-field nuclear magnetic resonance (LF-NMR) spectrometer, a micro-area sampler and a cavity repair tool, a miniature sealed glass tube thermal simulation system (MSSV), and a ScCO2 high-temperature and high-pressure displacement device.

[0021] The cryogenic sampling device is used for cryopreservation and processing of oil-bearing shale cores. The cryogenic sampling module provides a cold stage operating environment of less than or equal to -20°C and liquid nitrogen freezing conditions to maintain the original fluid state inside the core sample.

[0022] LF-NMR is used to obtain two-dimensional relaxation distribution data of core samples before and after displacement. By inverting and fitting the two-dimensional relaxation distribution data, the quantitative characterization of different occurrence states of shale oil inside the core samples can be achieved.

[0023] The micro-area sampler and cavity repair tool are used to perform targeted micro-powder drilling in designated areas of core samples. After drilling the powder sample, an inert material without nuclear magnetic resonance signal (such as fully deuterated epoxy resin or polytetrafluoroethylene filler) is used to fill and repair the sampling cavity, maintain the structural integrity of the core sample, and avoid the cavity defect causing the core to puncture the high-pressure sleeve in the subsequent high-pressure test.

[0024] MSSV is used for thermal evaporation gas chromatography (Tvap-GC) analysis of powder samples, outputting the original hydrocarbon component distribution characteristics and residual hydrocarbon component distribution characteristics at different spatial locations of the core.

[0025] The ScCO2 high-temperature and high-pressure displacement device is used to carry out supercritical carbon dioxide displacement operations on core samples under set conditions to simulate the fluid phase and seepage process in the underground reservoir environment.

[0026] See attached document Figure 1 This invention provides a comprehensive evaluation method for the mobility of shale oil based on ScCO2 displacement and the combined use of multiple technologies, comprising the following steps: S1, Perform sample preparation to obtain an oil-bearing shale core column; S2, pre-displacement in-situ characterization of oil-bearing shale core column to generate pre-displacement characterization results and core column after testing. Pre-displacement characterization results include NMR T1-T2 spectra and micro-area sampling and Tvap-GC analysis of original hydrocarbon components. S3, Perform ScCO2 displacement experiment on the tested core column to generate a displaced core column; S4. The core column after displacement was characterized in situ to generate the post-displacement characterization results, which include NMR T1-T2 spectra and residual hydrocarbon components from micro-sampling and Tvap-GC analysis. S5. Combine the pre-displacement characterization results with the post-displacement characterization results to perform comprehensive data analysis and evaluation to generate analysis and evaluation results. The analysis and evaluation results include quantitative mobility evaluation characteristics and component difference patterns. S6 outputs a comprehensive conclusion based on the analysis and evaluation results, providing a basis for the selection and development of desserts.

[0027] The following will provide a detailed explanation of the specific implementation details of each of the above steps.

[0028] In the specific implementation of sample preparation to obtain an oil-bearing shale core column, namely step S1, the following steps are included: S101, Select oil-bearing shale samples harvested from closed core wells in the target block. To avoid disturbing the original state of the pore fluid within the samples due to changes in temperature and environmental pressure, the oil-bearing shale samples are placed in a liquid nitrogen tank for cryopreservation after core extraction. As a preferred method, the liquid nitrogen tank provides a low-temperature environment of approximately -196°C to perform cryopreservation of the oil-bearing shale samples, generating frozen oil-bearing shale samples.

[0029] Based on the obtained frozen oil-bearing shale samples, S102, under a cold stage operating environment of less than or equal to -20°C, the frozen oil-bearing shale samples were drilled using a diamond core drill with a cryogenic sampling device.

[0030] Furthermore, it is processed into a cylindrical structure with dimensions of Φ25mm × 50mm. After processing and surface cleaning, the initial mass is weighed and recorded, thus completing the standard column processing to generate an oil-bearing shale core column. The oil-bearing shale core column will serve as the basis for subsequent in-situ characterization before displacement (i.e., step S2).

[0031] In the sample preparation stage, it is also necessary to establish a conversion benchmark between mass and signal. Specifically, in S103, representative shale oil samples from the target study area are obtained, and four clean, dry glass calibration bottles are prepared. Using a microsyringe, representative shale oil samples with masses of 0.1g, 0.3g, 0.5g, and 1.0g are respectively transferred into the corresponding glass calibration bottles and sealed. Then, the representative shale oil samples are quantitatively weighed at multiple gradients and sealed to generate calibration samples with multiple mass gradients.

[0032] For the calibration samples prepared above, in step S104, LF-NMR is used to sequentially perform NMR T1-T2 experiments with consistent test parameters on calibration samples of multiple mass gradients, generating corresponding sets of two-dimensional relaxation measurement raw data. In this embodiment, the set test parameters include: echo time of 0.2 to 0.6 milliseconds, waiting time of 2000 to 3000 milliseconds, and number of accumulations of 32 to 64. As a preferred parameter configuration, the T1-T2 two-dimensional relaxation measurement sequence is performed with an echo time of 0.3 milliseconds, a waiting time of 2500 milliseconds, and a number of accumulations of 32.

[0033] To establish a quantitative relationship between signal and quality, in step S105, the signal intensity of each calibration sample is generated by inversion fitting based on multiple sets of two-dimensional relaxation measurement raw data, and the calibration coefficient corresponding to the quantitative conversion relationship is calculated by combining the crude oil quality of each calibration sample.

[0034] See attached document Figure 2 In the figure, the horizontal axis represents signal intensity (μA), and the vertical axis represents crude oil mass (g). Linear regression was performed on the test data points of four calibration samples corresponding to multiple mass gradients to obtain the corresponding linear fitting trend line. The data in the figure shows that the crude oil mass of the calibration samples exhibits a high degree of linear positive correlation with the signal intensity.

[0035] In the calculation, the calibration coefficients are obtained using the linear regression formula, which is as follows: ; in, Indicates crude oil quality, Indicates signal strength. This represents the calibration coefficient.

[0036] According to the appendix Figure 2 The fitting results, linear fitting equation Therefore, the specific value of the calibration coefficient was determined to be 0.024. The calibration coefficient is used to quantitatively convert the two-dimensional nuclear magnetic resonance signals obtained in the multi-stage in-situ testing into actual oil-bearing quality characteristics.

[0037] After obtaining the basic core sample, step S2 is performed to conduct in-situ characterization of the oil-bearing shale core column before displacement, generating pre-displacement characterization results and the core column after testing. In this embodiment, the pre-displacement characterization results include NMR T1-T2 spectra and micro-area sampling and Tvap-GC analysis of the original hydrocarbon components. To achieve a comprehensive quantification of the fluid state inside the core, this embodiment uses a combination of physical scanning and chemical analysis, specifically including the following operational steps: In the initial fluid distribution data acquisition stage, S201, LF-NMR was used to conduct nuclear magnetic resonance (NMR) tests on the oil-bearing shale core column to obtain the two-dimensional relaxation distribution state of the initial fluid. In this embodiment, the test parameters were set consistently with those in the aforementioned calibration stage, namely, the echo time was set to 0.3 milliseconds (ms), the waiting time to 2500 ms, and the number of iterations to 32. By acquiring spin echo train data from inside the core, NMR T1-T2 spectra were constructed.

[0038] See attached document Figure 3Based on the inversion data, the occurrence state of shale oil is divided using pre-defined relaxation intervals. In practical operation, shale oil is divided according to the T2 relaxation time as follows: regions with T2 less than 10 ms and a T1 / T2 ratio between 1 and 5 are defined as adsorbed oil; regions with T2 between 10 and 100 ms and a T1 / T2 ratio between 2 and 10 are defined as bound oil; and regions with T2 greater than 100 ms are defined as mobile oil.

[0039] Based on this classification, signal integration is performed on each region in the spectrum, and the signal intensity of each region is converted into the normalized content of adsorbed oil, bound oil, and movable oil per unit rock sample mass using pre-determined calibration coefficients. It should be noted that the T1 and T2 cutoff thresholds and their ratio ranges used to classify the occurrence state are set based on the calibration results of rock samples from the target block. In practical applications, these can be dynamically adjusted based on the core pore structure characteristics and fluid system properties of different regions.

[0040] Based on the above non-destructive scanning results, to further obtain the microscopic chemical substances in the corresponding area, step S202 was executed. After completing the NMR scanning data acquisition, a micro-area sampler and a cavity repair tool were used to perform micro-point sampling on the oil-bearing shale core column. To prevent the light hydrocarbon components inside the sample from being lost due to heat volatilization during sampling, the sampling operation was arranged to be performed in a cold stage operating environment of less than or equal to -20°C. Two sampling points were selected on the bottom surface of the oil-bearing shale core column, i.e., the subsequent fluid injection end, and two sampling points were selected on the side surface of its axial direction, thus forming four spatially representative sampling positions. Using a micro drill bit with a diameter of 1 mm, a drilling depth of approximately 1.5 mm was made at each set position to obtain approximately 10 mg of shale powder. The obtained powder samples were quickly transferred to pre-weighed micro glass tubes for sealing and were labeled as samples A1, A2, A3, and A4, respectively.

[0041] After obtaining the powder, the holes left after drilling were filled and repaired using an inert material without nuclear magnetic resonance signal, relying on a micro-area sampler and hole repair tools.

[0042] In this embodiment, fully deuterated epoxy resin is used as the inert material to fill the pores and smooth the surface, thereby maintaining the structural integrity of the sample.

[0043] To analyze the specific chemical composition characteristics of the sampled powder, in step S203, MSSV was used to perform Tvap-GC analysis on the obtained powder sample, generating micro-area sampling and Tvap-GC analysis of the original hydrocarbon components. Micro-glass tubes containing samples A1 to A4 were sequentially placed into the injector of the thermal evaporative gas chromatograph. Under a predetermined heating program, the glass tubes were crushed by the built-in hammer, releasing the hydrocarbons contained within the sample. These hydrocarbons were then purged by high-purity nitrogen carrier gas and transported to a U-shaped trap placed in a liquid nitrogen environment for enrichment.

[0044] Specifically, the heating and purging program was set to maintain a constant temperature of 300°C for 5 minutes; the carrier gas used was high-purity helium, with a flow rate set at 28.5 mL per minute. After the enrichment process was completed, the cold trap environment was removed, allowing the U-shaped trap to rapidly heat to 300°C, which facilitated the transfer of the captured analytes to the chromatographic column along with the carrier gas for separation and detection.

[0045] See attached document Figure 5 This refers to the operation of qualitative and quantitative analysis of major hydrocarbon components such as n-alkanes and isoalkanes using the external standard method after obtaining chromatographic data.

[0046] Through the modular scanning and micro-area chemical testing process described above, a comprehensive pre-displacement characterization result reflecting the initial distribution state of the fluid is constructed, thereby establishing a basic data benchmark and providing a comparative basis for measuring the changes in the micro-mobilization of the reservoir interior by subsequent high-pressure fluid displacement behavior.

[0047] After completing the aforementioned in-situ characterization tests, in order to examine the flow response characteristics of crude oil under reservoir conditions, step S3 is performed on the tested core column to generate a displaced core column. To ensure that the experimental process closely matches the actual formation environment, this embodiment relies on corresponding hardware support and environmental simulation mechanisms, and specifically includes the following operational steps: To simulate the fluid phase state of the underground reservoir environment, in step S301, the ScCO2 high-temperature and high-pressure displacement device was used to construct and set the parameters for high-temperature and high-pressure supercritical carbon dioxide displacement. In this embodiment, the core column obtained after the aforementioned micropore repair treatment was loaded into the high-pressure holder of the ScCO2 high-pressure fluid displacement module.

[0048] As a preferred method, to simulate the pressure state of the overlying strata in the target block and prevent the injected fluid from flowing through the core casing, a predetermined radial confining pressure is applied to the sealing sleeve inside the holder. In this embodiment, a confining pressure of 25 MPa is applied. After sample loading and sealing tests are completed, the temperature-controlled heating jacket is activated to raise the entire system environment to the set reservoir temperature. Considering the phase transformation boundary of supercritical carbon dioxide and the geothermal gradient of actual deep shale oil reservoirs, the experimental temperature is preferably set between 60°C and 90°C; in this embodiment, it is set to 90°C. Subsequently, carbon dioxide gas is pressurized and injected into the system pipeline using a high-pressure advection pump, and the system fluid pressure is gradually increased to the target displacement pressure.

[0049] The displacement pressure is preferably controlled between 10 MPa and 30 MPa, and is set to 20 MPa in this embodiment. Under this temperature and pressure conditions, the carbon dioxide entering the system is converted into a supercritical phase, utilizing its high diffusion and dissolving capabilities to achieve crude oil viscosity reduction, volume expansion, and selective extraction of light hydrocarbon components.

[0050] After the system temperature and pressure conditions reach equilibrium, the process proceeds to the long-term fluid seepage stage. S302, the constant temperature and pressure continuous displacement operation and shutdown pressure holding and reduction process are executed. In this embodiment, the system settings of 90℃ and 20MPa are maintained, and supercritical carbon dioxide is continuously pumped into the core column after testing. Simultaneously, a backpressure valve is configured and adjusted at the production end of the core to stabilize the outlet pressure, creating a stable displacement pressure difference between the two ends of the core. As a preferred method, since the carbon dioxide changes from a supercritical state to a gaseous state after the produced fluid is depressurized by the backpressure valve, a gas-liquid two-phase separation device is needed to separately capture and measure the liquid phase produced oil and the gaseous phase produced products to ensure the accuracy of the produced volume measurement. Due to the high fluid mass transfer resistance in the tight porous system of shale reservoirs, the displacement process is relatively slow; the displacement operation time is preferably set to 15 to 30 days. In this embodiment, the continuous displacement process lasts for 20 days.

[0051] After a prolonged period of phase miscibility and physical displacement, the injection operation of the high-pressure advection pump is stopped. For the depressurization phase after the experiment, those skilled in the art can perform depressurization by adjusting the backpressure valve. As a preferred method, a step-by-step slow depressurization strategy is adopted, controlling the rate of pressure decrease within the system to less than or equal to 0.1 MPa per minute until it returns to atmospheric pressure. The purpose of this slow depressurization operation is to prevent the high-pressure fluid in the tiny pores inside the core from undergoing a drastic volume expansion effect due to excessively rapid depressurization, thereby avoiding induced microcracks or structural damage to the core skeleton.

[0052] After the system pressure is unloaded to atmospheric pressure, the temperature-controlled heating jacket is turned off, allowing the environment to cool naturally to room temperature. The high-pressure clamp is then opened to remove the sample that has undergone fluid action, thus generating a displaced core column. This physical sample preserves the residual material distribution after fluid displacement, providing reliable physical support for subsequent NMR and micro-area composition re-analysis.

[0053] After completing the high-pressure supercritical fluid displacement operation, in order to obtain information on the dynamic changes in the fluid state inside the core, step S4 is entered to perform in-situ characterization of the core column after displacement to generate characterization results. The characterization results include NMR T1-T2 spectra and residual hydrocarbon components from micro-area sampling and Tvap-GC analysis.

[0054] To obtain evolution data on the macroscopic fluid occurrence state, S401, using a low-field nuclear magnetic resonance spectrometer, performed post-displacement NMR T1-T2 spectral measurements, referring to the attached... Figure 4 , and calculate the residual oil content.

[0055] After completing the non-destructive NMR detection, to further investigate the molecular composition and spatial distribution of the residual fluid, S402, a micro-area sampler and a cavity repair tool were used in conjunction with MSSV to sample adjacent locations in the post-displacement micro-area and to test the residual hydrocarbon components in the Tvap-GC. As a preferred method, to ensure the spatial correspondence of the test data before and after displacement, the micro-area sampler and the cavity repair tool were used to select adjacent or corresponding spatial locations on the bottom and axial surfaces of the post-displacement core column, and drilled an appropriate amount of rock powder. During the drilling process, the drill bit penetration depth was controlled to be consistent with that before displacement (e.g., set to 1.5 mm) by a micro-limiting device to eliminate the interference of radial depth differences on component analysis; then, the cavities left by the sampling were filled with an inert material without NMR signal. The powder samples obtained from each point were properly sealed and transferred to the Tvap-GC component detection module.

[0056] See attached document Figure 6 , Figure 6 The "min" indicates minutes. This figure shows the gas chromatographic peaks of the residual sample after displacement, with the coordinate axis settings remaining the same as before displacement. A comparison with the pre-displacement values ​​shows that... Figure 6 The height of most signal peaks decreased and tended to the baseline. In the light component region on the left with a shorter peak time, the higher peaks receded, leaving behind low peaks; the peak heights in the heavy component region in the middle and later stages also decreased accordingly.

[0057] After completing step S4 to perform in-situ characterization of the core column after displacement and generate the characterization results, in order to establish the engineering connection between macroscopic pore fluid data and microscopic molecular component data, step S5 is entered. The pre-displacement characterization results and post-displacement characterization results are combined for comprehensive data analysis and evaluation to generate analysis and evaluation results that include quantitative mobility evaluation and component difference characteristics.

[0058] To analyze the correlation between fluid occurrence state and utilization potential, step S501 is executed to calculate the quantitative mobility evaluation characteristics of shale oil under different occurrence states. In this embodiment, based on the fluid content data of each relaxation interval before and after displacement, the utilization capacity of the oil phase in the nanoscale confined space is calculated using the adsorbed oil displacement mobility percentage formula. The adsorbed oil displacement mobility percentage formula is as follows: ; in, Indicates the percentage of movable displacement by adsorbed oil; Indicates the absolute content of adsorbed oil before displacement; This indicates the absolute content of adsorbed oil after displacement. If any of the following occurs during calculation... In the case of a value of 0, as a preferred error prevention mechanism, the division operation is not performed; instead, the result is directly... Record it as 0.

[0059] The production degree of capillary-confined oil phase in medium-porosity environments was calculated using the formula for the movable percentage of bound oil displacement. The formula for the movable percentage of bound oil displacement is as follows: ; in, Indicates the percentage of movable parts displaced by the binding oil; Indicates the absolute content of binding oil before displacement; This indicates the absolute content of bound oil after displacement. Similarly, if If it is 0, then Record it as 0.

[0060] The displacement efficiency of the free oil phase in the macropore space is calculated using the movable percentage formula for movable oil displacement. The movable percentage formula for movable oil displacement is as follows: ; in, Indicates the percentage of movable oil that can be displaced; Indicates the absolute content of mobile oil before displacement; This indicates the absolute content of movable oil after displacement. If If it is 0, then Record it as 0.

[0061] To visually demonstrate the above calculation results and facilitate state comparison, the relevant test and calculation data are summarized in Table 1.

[0062] Table 1. Shale oil content and movable percentage in different occurrence states before and after displacement in Example 1 To investigate the evolution of the residual fluid system at the microscopic molecular level, step S502 was performed to analyze the spatial compositional differences of shale oil along the seepage path. Based on chromatographic data from different spatial locations, response peak information of samples at different seepage positions was extracted to evaluate the phase transformation status.

[0063] To quantify the changes in crude oil composition caused by extraction, the light-heavy hydrocarbon ratio ( ) was used. The calculation formula assesses the degree of deviation of crude oil composition along the route. The formula for calculating the light-heavy hydrocarbon ratio is: ; in, Indicates the ratio of light to heavy hydrocarbons; Indicates the light hydrocarbon components in the chromatogram ( The sum of the areas of the corresponding chromatographic peaks; Indicates the heavy hydrocarbon component in the chromatogram ( The sum of the areas of the corresponding chromatographic peaks; This indicates the summation symbol. To avoid the denominator being zero, which would prevent calculation, when measured... When the value is 0, it indicates that the content of heavy components is lower than the lower limit of the detection instrument. At this time, the residual crude oil components at the current test location are directly recorded as being in a light state, and the ratio division operation is not performed.

[0064] To reflect the evolution characteristics of the components along the route, the test indicators obtained at different spatial locations are presented in Table 2.

[0065] Table 2. Hydrocarbon component differentiation results of samples at different locations before and after displacement in Example 1. To comprehensively evaluate the exploitation potential of the target reservoir and avoid the limitations of a single index in characterizing fluid dynamic response, step S503 is performed to construct a comprehensive mobility index based on the coupling of exploitation state and component differentiation. In this embodiment, by combining the state and component evolution patterns obtained from the aforementioned analysis, the comprehensive mobility index evaluation formula is used to output the final quantitative result for the measured shale layer. The comprehensive mobility index evaluation formula is: ; in, Indicates the overall mobility index of shale oil; The index representing the allocated state is selected as a specific implementation method, based on the calculated value. The value of (percentage of movable oil displacement) is directly used as... Substitute the value; The light and heavy hydrocarbon separation index is specifically calculated by subtracting the light and heavy hydrocarbon ratio after displacement from the light and heavy hydrocarbon ratio before displacement in the injection end area, and then dividing by the light and heavy hydrocarbon ratio before displacement, in order to characterize the intensity of crude oil extraction. The spatial migration index is specifically calculated by the ratio of the light to heavy hydrocarbon ratio after displacement at the axial position to the light to heavy hydrocarbon ratio after displacement at the injection end position, and is used to characterize the ability to maintain the component gradient along the displacement path. , , All represent dimensionless weighting coefficients, and must satisfy the following conditions: As a preferred embodiment, the equilibrium conditions are such that a, b, and c are 0.5, 0.3, and 0.2, respectively.

[0066] The values ​​of these three dimensionless weighting coefficients can be obtained by those skilled in the art through constructing a judgment matrix using the analytic hierarchy process or by multivariate regression fitting based on historical development data of the target block's core samples. Their values ​​are all within the range of... Through the coupled calculation of various physical indicators, the final shale oil comprehensive mobility index can more multidimensionally and objectively reflect the comprehensive fluid production efficiency of the core target area under the set high-pressure carbon dioxide operation mode. Then, in the subsequent step S6, a comprehensive conclusion is output based on the analysis and evaluation results, providing a basis for sweet spot selection and development scheme.

[0067] After completing step S5, in order to apply the evaluation data to the exploration and development of actual engineering blocks, step S6 is executed to output a comprehensive conclusion based on the analysis and evaluation results, providing a basis for the selection of sweet spots and development plans.

Claims

1. A comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and the combined use of multiple technologies, characterized in that, Includes the following steps: Perform sample preparation to obtain oil-bearing shale core columns; The oil-bearing shale core column was characterized in situ before displacement, and the characterization results before displacement and the core column after testing were generated. A ScCO2 displacement experiment was performed on the core column after the test to generate a displaced core column; The core column after displacement was characterized in situ to generate the post-displacement characterization results. The pre-displacement characterization results and the post-displacement characterization results are combined for comprehensive data analysis and evaluation to generate analysis and evaluation results, which include quantitative mobility evaluation characteristics and component difference patterns. Based on the analysis and evaluation results, a comprehensive conclusion is output, providing a basis for the selection and development of desserts.

2. The comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and multiple technologies as described in claim 1, characterized in that, The specific steps of performing sample preparation to obtain oil-bearing shale core columns include: Oil-bearing shale samples harvested from closed core wells in the target block were subjected to cryopreservation to generate frozen oil-bearing shale samples. The frozen oil-bearing shale sample was drilled in a cold table operating environment of less than or equal to -20°C, and processed to generate the oil-bearing shale core column; Representative shale oil samples were obtained, and multi-gradient quantitative weighing and sealing were performed to generate calibration samples with multiple mass gradients. Using a low-field nuclear magnetic resonance (LF-NMR) spectrometer, two-dimensional nuclear magnetic resonance experiments (NMR T1-T2) were sequentially performed on the calibration samples of the multiple mass gradients to generate multiple sets of raw two-dimensional relaxation measurement data. Based on the multiple sets of two-dimensional relaxation measurement raw data, the signal intensity is generated by inversion fitting, and the calibration coefficient corresponding to the quantitative conversion relationship is calculated by linear regression formula in combination with crude oil quality.

3. The comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and multiple technologies as described in claim 2, characterized in that, The specific steps for performing in-situ characterization of the oil-bearing shale core column before displacement and generating the characterization results before displacement and the core column after testing include: Using LF-NMR, nuclear magnetic resonance tests were conducted on the oil-bearing shale core column to generate the NMR T1-T2 spectrum in the pre-displacement characterization results. The normalized content of adsorbed oil, bound oil and movable oil per unit sample mass was obtained by combining the calibration coefficients. Using a micro-area sampler and a cavity repair tool, a small amount of powder sample was obtained by sampling the oil-bearing shale core column. The sampling points included the bottom surface, i.e. the subsequent fluid injection end, and the axial side surface area. The cavity left after drilling was filled and repaired using an inert material without nuclear magnetic resonance signal to generate the tested core column that maintains the integrity of the sample structure. The powder sample was analyzed by thermal evaporation gas chromatography (Tvap-GC) using a micro-closed glass tube thermal simulation system (MSSV) to generate the original hydrocarbon components from the Tvap-GC analysis in the pre-displacement characterization results.

4. The comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and multiple technologies as described in claim 3, characterized in that, The specific steps for performing a ScCO2 displacement experiment on the tested core column to generate a displaced core column include: Using the ScCO2 high-temperature and high-pressure displacement device, the core column after the test is loaded into the high-pressure holder, radial confining pressure is applied and the temperature is raised to the set reservoir temperature, and carbon dioxide gas is injected under pressure to the target displacement pressure so that the carbon dioxide gas reaches the supercritical phase state and a phase equilibrium state is established. In the phase equilibrium state, supercritical carbon dioxide is continuously pumped from the injection end to perform isothermal and isobaric continuous displacement operation, and after the displacement operation is completed, a slow depressurization operation is performed with the internal pressure drop rate less than or equal to 0.1 MPa per minute to generate a sample that has undergone fluid action. After the system pressure is unloaded to atmospheric pressure, the sample that has undergone fluid action is taken out to generate the displacement core column.

5. The comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and multiple technologies as described in claim 1, characterized in that, The specific steps for performing in-situ characterization of the displaced core column and generating the characterization results include: The displacement-induced NMR T1-T2 spectrum was performed on the core column by LF-NMR to generate the NMR T1-T2 spectrum in the characterization results after displacement, and the residual oil content in different occurrence states after displacement was calculated. Using a micro-area sampler and a cavity repair tool, rock powder was drilled from the bottom and axial surfaces of the core column after displacement at locations adjacent to or corresponding to those before displacement, thus obtaining powder samples after displacement. The displaced powder sample was subjected to Tvap-GC analysis using MSSV to generate the residual hydrocarbon components from the Tvap-GC analysis in the characterization results after displacement.

6. The comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and multiple technologies as described in claim 1, characterized in that, The steps for generating analysis and evaluation results by combining the pre-displacement and post-displacement characterization results include: Extract the fluid content data of each relaxation interval from the characterization results before and after displacement; Based on the fluid content data, the oil phase mobility is calculated using the formulas for the percentage of adsorbed oil displacement, the percentage of bound oil displacement, and the percentage of mobile oil displacement, thereby deriving the quantitative mobility evaluation characteristics in the analysis and evaluation results.

7. The comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and multiple technologies as described in claim 6, characterized in that, The step of combining the pre-displacement characterization results with the post-displacement characterization results to perform comprehensive data analysis and evaluation, and generating analysis and evaluation results, further includes: Extract the response peak information of samples at different seepage locations from the characterization results before and after displacement; Based on the response peak information, the deviation of crude oil components along the flow path is evaluated using the light-heavy hydrocarbon ratio calculation formula. The spatial differentiation effect of hydrocarbon components along the seepage path caused by the dissolution and migration of different components in shale oil with fluid is analyzed, thereby obtaining the component difference characteristics in the analysis and evaluation results.

8. The comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and multiple technologies as described in claim 7, characterized in that, The analysis and evaluation steps, which combine the pre-displacement and post-displacement characterization results for comprehensive data analysis and evaluation, also include: Relevant parameters are extracted based on the pre-displacement characterization results and the post-displacement characterization results; The occurrence state utilization index calculated based on the movable percentage of movable oil displacement, and the light and heavy hydrocarbon differentiation index and spatial migration index calculated based on the light and heavy hydrocarbon ratio calculation formula are substituted into the comprehensive mobility index evaluation formula for coupling calculation to generate the shale oil comprehensive mobility index in the analysis and evaluation results.

9. The comprehensive evaluation method for shale oil mobility based on ScCO2 displacement and multiple technologies as described in claim 8, characterized in that, The steps for outputting a comprehensive conclusion based on the analysis and evaluation results include: evaluating the development potential of the target strata based on the comprehensive mobility index of shale oil.

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