Quantitative characterization method for connectivity of carbon dioxide in shale cover layer

Abstract

The invention relates to the technical field of carbon capture and storage, in particular to a quantitative characterization method for connectivity of carbon dioxide in a shale cover layer, which comprises the following steps: simulating a formation in-situ environment for the shale cover layer, carrying out a stepped injection pressure displacement test, and collecting transverse relaxation time T2 spectrums under different displacement pressures; and constructing a T2-Pc two-dimensional joint function based on the capillary pressure Pc and the T2 spectrum, and calculating the carbon dioxide phase connectivity coefficient of each pore throat unit in the T2-Pc two-dimensional spectrum. According to the method, a formation in-situ environment is simulated through a multi-field coupling test system, and synchronous acquisition of the occurrence state and the fluidity of the fluid under in-situ stress is realized by combining injection pressure displacement and nuclear magnetic resonance technologies; by constructing a T2-Pc two-dimensional joint map, a pore structure and fluid migration characteristics are synergistically represented, pore throat units which are geometrically communicated but hydraulically isolated are accurately distinguished, a calculation result is remarkably related to microCT test and CO2 phase endpoint relative permeability, and the representation precision is greatly improved.

Description

Technical Field

[0001] This invention relates to the field of carbon capture and storage technology, specifically to a quantitative characterization method for the connectivity of carbon dioxide in shale caprock. Background Technology

[0002] With the advancement of global carbon neutrality goals, carbon capture and storage (CCS) technology has become a key means of controlling carbon dioxide emissions. Among these technologies, deep saline aquifers, with their enormous storage capacity, have become the most promising geological storage carriers. Shale caprocks serve as the core containment barrier for CO2 storage, and their airtightness directly determines the long-term safety of the storage system. Supercritical CO2 migrates upward under buoyancy and needs to be bound by the pore network and capillary forces of the shale caprock. However, if the pore connectivity is too high, it will lead to the risk of CO2 leakage.

[0003] However, existing methods for characterizing CO2 connectivity in shale caprock have several problems: traditional methods (such as microCT and high-pressure mercury intrusion porosimetry) calculate connectivity based solely on pore geometry and topology, neglecting hydrodynamic factors such as fluid viscous resistance, gas-liquid interface effects, and inlet resistance, which can easily overestimate the actual CO2 connectivity; conventional tests (such as centrifugation) cannot simulate in-situ formation stress conditions, and high centrifugal forces may damage the integrity of shale pore structure, leading to significant deviations between test results and the actual geological environment; one-dimensional nuclear magnetic resonance and other techniques can only characterize pore size distribution and cannot effectively reflect the coupling relationship between pore structure and fluid flow, nor can they accurately distinguish geometrically connected but hydraulically isolated pore units; the nanoscale pore-throat structure of low-permeability shale is complex, and existing methods cannot quantify the differentiated contributions of multi-scale pores to CO2 connectivity, resulting in a lack of reliable basis for assessing the safety of CO2 storage.

[0004] Therefore, developing a quantitative characterization method for CO2 connectivity that can couple in-situ stress conditions, hydrodynamic resistance, and pore structure characteristics has become an urgent need to ensure the safe implementation of CCS projects. Summary of the Invention

[0005] To address the aforementioned deficiencies and problems, this invention provides a quantitative characterization method for carbon dioxide connectivity in shale caprock.

[0006] The solution adopted by this invention to solve its technical problem is: a quantitative characterization method for the connectivity of carbon dioxide in shale caprock, comprising the following steps: S1. Pre-treat the shale caprock samples to obtain standard samples that meet the test requirements; S2. The pretreated sample was placed in a multi-field coupling test system to simulate the in-situ environment of the formation and carry out a step-injection pressure displacement test. Simultaneously, the transverse relaxation time T2 spectrum under different displacement pressures was acquired in real time using nuclear magnetic resonance technology. S3, Capillary pressure P based on different displacement pressures c Using the T2 spectrum obtained in step S2, a T2-P spectrum characterizing the pore fluid occurrence state and flowability is constructed. c Two-dimensional joint functions yield T2-P c Two-dimensional map; S4. Based on the variable diameter capillary model, considering capillary resistance, viscous resistance, and inlet resistance, calculate T2-P. c Carbon dioxide connectivity coefficient of each pore throat unit in the two-dimensional spectrum; S5, with T2-P c The signal intensity of each pore throat unit in the two-dimensional map is used as the weight. The carbon dioxide connectivity coefficient of the corresponding pore throat unit is weighted and averaged to obtain the carbon dioxide connectivity in the shale caprock.

[0007] Furthermore, the preprocessing in step S1 includes the following steps: S11. The shale sample is processed into a standard cylindrical sample, the geometric dimensions of which are φ25mm×20mm. S12. The processed sample was dried in an oven at 105℃ to constant weight, and then degassed under a negative pressure of -0.1MPa for at least 6 hours using a vacuum saturation device. Then, 15MPa hydrostatic pressure was applied and distilled water was injected for saturation for 24 hours. S13. After saturation, the sample is sealed to ensure airtightness during the test.

[0008] Furthermore, in step S2, the in-situ environment of the formation is simulated, specifically by setting the test temperature to 50℃, the confining pressure and axial pressure to 15MPa, and simulating the formation pressure conditions corresponding to a CO2 sequestration depth of 800~2300m; the injection pressure gradient of the stepped injection pressure displacement test is 1MPa, the injection pressure range is 8~14MPa, and each pressure level is maintained for 12 hours to reach stress equilibrium before collecting the T2 spectrum.

[0009] Furthermore, the T2-P mentioned in step S3 c The construction process of a two-dimensional joint function is as follows: S31. Subtract the T2 spectra under different displacement pressures to eliminate interference from the rock matrix signal and obtain the T2 distribution of the displaced mobile water in different displacement pressure ranges. S32. Based on the Laplace equation, the displacement pressure is converted into the corresponding pore throat radius, and the mapping relationship between the displacement pressure and the pore throat radius is established. S33. Perform spline interpolation on the T2 distribution of movable water and the corresponding pore throat radius to construct the T2-P distribution. c Two-dimensional matrix, forming T2-P cTwo-dimensional spectra, where signal intensity represents the movable water volume of the corresponding pore throat unit.

[0010] Furthermore, the calculation basis for the carbon dioxide connectivity coefficient in step S4 is as follows: Capillary resistance P c The calculation is based on the Young-Laplace equation, and the formula is as follows: (1) Where σ is the gas-liquid interfacial tension, θ is the contact angle, and r t The radius of the throat; Viscous resistance P v Considering the thickness of the adsorbed water film on the pore wall, and based on the fluid flow equation in a variable-diameter capillary, the expression for viscous resistance is derived as follows: (2) In the formula, v is the flow velocity of the fluid in the capillary, and r g It is the gas flow radius, μ g δ is the gas phase viscosity, δ is the thickness of the bound water film, and Q is the gas phase viscosity. vd This is the flow rate of the capillary model with equal radius, μ is the fluid viscosity, and L is the pore length; Inlet resistance P e Based on the momentum loss caused by changes in fluid flow direction, the calculation formula is derived as follows: (3) In the formula, C=3; The total resistance is the sum of capillary resistance, viscous resistance, and inlet resistance: .

[0011] Furthermore, the carbon dioxide connectivity within the pore-throat unit is defined as the ratio of the aqueous flux calculated using a constant radius capillary model to that calculated using a variable diameter capillary model: (4) In the formula, α is the angle of change of streamline direction, and r pj Let r represent the radius of the j-th pore. tr Let CI represent the radius of the r-th pore throat. jr Indicates a pore radius of r pj And the radius of the throat is r tr The carbon dioxide interconnectivity in the pore throat unit.

[0012] Furthermore, the overall carbon dioxide connectivity index (CI) of the shale sample is calculated using the following formula: (5) (6) In the formula, f jrTwo-dimensional T2-P c Each coordinate point (T2, P) in the distribution map c The corresponding signal strength represents the number or volume weight of aperture throat units of a specific size, D. jr Indicates T2-P c Figure 2 The product of carbon oxide phase connectivity and signal strength matrix data.

[0013] The beneficial effects of this invention are: This invention provides a quantitative characterization method for CO2 connectivity in shale caprocks that is precisely adapted to in-situ CO2 sequestration scenarios, effectively solving the characterization bias problems caused by traditional methods neglecting hydrodynamic resistance and detachment from formation stress conditions. This method simulates formation temperature, confining pressure, and axial pressure environments at depths of 800–2300 m using a multi-field coupled experimental system. Combined with stepped injection pressure displacement and nuclear magnetic resonance (NMR) technology, it achieves simultaneous acquisition of fluid storage state and flowability under in-situ stress. By constructing a T2-P... c Two-dimensional joint mapping is used to collaboratively characterize pore structure and fluid transport characteristics, accurately distinguishing geometrically connected but hydraulically isolated pore throat units. Based on a variable diameter capillary model, the influence of pore throat heterogeneity on CO2 flow is quantified by comprehensively considering capillary resistance, viscous resistance and inlet resistance. The calculation results are significantly correlated with microCT test and the relative permeability of CO2 phase endpoints (R²≥0.98), greatly improving the characterization accuracy.

[0014] This method does not require damaging the sample structure and can be adapted to different types of shale with high and low connectivity. It can capture the connectivity characteristics of major seepage channels such as the mesopore throat and reflect the dynamic influence of confining pressure changes on CO2 connectivity. It provides accurate quantitative basis for CCS engineering site selection, shale caprock sealing performance assessment and leakage risk early warning. At the same time, it enriches the application of multiphase flow theory in unconventional reservoirs and has important theoretical value and engineering practice significance. Attached Figure Description

[0015] Figure 1 Schematic diagram of a stepped injection pressure displacement test; Figure 2 A flowchart for calculating the connectivity of CO2 in shale samples; Figure 3 A schematic diagram of the hydrodynamic resistance of CO2-water two-phase flow; Figure 4 The signal strength matrix T2-P c Atlas; Figure 5 T2-P is the connectivity matrix of CO2. c Atlas; Figure 6 A CO2 connectivity map of the pore throat unit; Figure 7 For microCT and T2-P c A comparison chart of connectivity results; Detailed Implementation

[0016] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0017] Please see Figure 1-7 This invention provides a technical solution for a quantitative characterization method of carbon dioxide connectivity in shale caprock: Example

[0018] This embodiment selects a shale sample from Luzhou City (LZ) from the disclosed materials. The sample mainly consists of quartz (49.2%), dolomite (15.7%), and illite (18.6%). The porosity was measured to be 2.92% by nuclear magnetic resonance (NMR) and verified to be 2.97% by saturated weighing (error ≤ 0.05%). The contact angle was 40.9°, and the interfacial tension of the CO2-water system was 35.1 mN / m. This sample is dominated by interconnected macropores, with an average pore radius of 144.22 μm, an average coordination number of 2.71, and good pore connectivity, making it suitable as a typical case to verify the effectiveness of the method of this invention. The specific implementation steps are as follows: Step S1: Pretreatment of shale samples aims to ensure uniform initial state of the samples, eliminate interference from residual moisture and air on the test results, and provide standardized samples for subsequent displacement and testing. ① Sample processing: The original shale sample is processed into a standard cylindrical sample of φ25mm×20mm using a core cutting device. During the processing, microcracks are avoided in the sample to ensure the integrity of the pore structure.

[0019] ② Drying and Saturation: The processed sample was placed in a 105℃ forced-air oven to dry for 24 hours until constant weight, removing free and adsorbed water from the pores to avoid residual moisture interfering with subsequent NMR signal identification. Then, the dried sample was placed in a vacuum saturation device, the exhaust valve was closed and the vacuum pump was started to maintain a negative pressure of -0.1MPa for 6 hours to fully remove air from the pores and create an unobstructed channel for distilled water injection. After the vacuum pump was turned off, the exhaust valve was opened and a hydrostatic pressure of 15MPa was applied to continuously inject distilled water for 24 hours to ensure that the distilled water fully penetrated the nanoscale pores and achieved complete saturation of the pore network, providing a uniform initial fluid distribution for subsequent displacement experiments.

[0020] ③ Encapsulation treatment: After saturation, wipe the excess moisture on the sample surface with a damp towel, and use heat shrink tubing to fully encapsulate the sample. Use a hot air gun (temperature 120℃) to set the temperature and ensure that the encapsulation layer is tightly attached to the sample to prevent fluid from leaking from the sample surface during subsequent displacement tests. At the same time, it ensures that the confining pressure, axial pressure and other stress conditions are stably transmitted to the inside of the sample.

[0021] Step S2: Stepped injection pressure displacement and nuclear magnetic resonance (NMR) test. The pretreated LZ sample is installed in a multi-field coupling test system to carry out the test. The multi-field coupling test system consists of a low-field NMR system, a confining pressure / axial pressure control system, and a displacement pressure control system.

[0022] ① Simulating the in-situ formation environment: The test temperature was kept constant at 50℃ using a water bath heating system, and both the confining pressure and axial pressure were set to 15MPa, corresponding to the formation pressure conditions at CO2 sequestration depths of 800~2300m, ensuring that CO2 was in a supercritical state; a stepped injection pressure displacement scheme was set, such as... Figure 1 As shown, the injection pressure range is 8~14MPa, with a pressure gradient of 1MPa. Each pressure level is maintained for 12 hours to ensure internal stress balance and fluid distribution stability of the sample, and to avoid distortion of the fluid distribution reflected in the T2 spectrum due to unstable pressure.

[0023] ② NMR Test Parameter Settings: NMR testing was performed using a CPMG pulse sequence with the following parameters: wait time (Tw) 3000ms, echo time (Te) 0.3ms, number of echoes (NE) 10000, and number of scans (NS) 64. The spin echo signal was converted into a transverse relaxation time (T2) spectrum using the Butler-Reeds-Dawson multi-exponential inversion algorithm. 200 inversion data points were used, with relaxation times ranging from 0.01 to 10000ms. The CPMG sequence is suitable for acquiring relaxation signals from porous fluids in porous media. Multi-exponential inversion can accurately analyze the relaxation characteristics of pores with different diameters. The peak position and amplitude of the T2 spectrum correspond to the pore radius and pore volume ratio, respectively, providing basic data for subsequent T2-Pc construction.

[0024] ③ Data acquisition: After each injection pressure level (8MPa, 9MPa, ..., 14MPa) stabilizes, NMR testing is initiated to acquire T2 spectra. Simultaneously, the instantaneous flow rates of gas and water at the outlet are recorded through a gas-liquid separation device to establish the correspondence between displacement pressure, fluid flow rate and T2 spectra, providing a basis for the analysis of movable water distribution.

[0025] Step S3: Based on the T2 spectrum and displacement pressure obtained in step S2, construct T2-P c A two-dimensional joint function that simultaneously characterizes pore structure (T² reflection) and fluid flowability (P² reflection). c reflect).

[0026] This study constructs a step function H(P) based on a one-dimensional T² distribution. cr -P s ), and then proposed T2-P c Two-dimensional distribution functions accurately characterize the distribution and mobility of fluids within shale pores. When the pore throat capillary pressure (P...)... cr Less than the displacement pressure (P) s When P is sufficient to overcome capillary resistance, the pore water will be effectively displaced; conversely, when P is insufficient... cr >P s At this time, the corresponding pores remain water-saturated, and this saturated water contributes to the NMR signal in nuclear magnetic resonance (NMR) detection. Therefore, the inversion of the two-dimensional Laplace algorithm can be simplified to a first-kind Fredholm integral equation, whose discrete form is expressed as: (1) In the formula, M is f is the intensity of the i-th echo under the s-th displacement pressure. jr (P cr ,T 2j ) indicates that the lateral relaxation time is T. 2j The capillary pressure is P cr The signal strength of the movable fluid, kernel function exp(-t) i / T 2j H(P) represents the attenuation term of the echo intensity, and the kernel function is H(P). cr -P s ) is a step function, m is the number of transverse relaxation times, which is logarithmically distributed over 200 intervals, ranging from 0.01 to 1000 ms, and p is 7 injection pressures: 8, 9, 10, 11, 12, 13, and 14 MPa.

[0027] ① Extraction of T2 distribution of mobile water: The T2 spectra under different displacement pressures were subtracted from the matrix signal spectrum of the dried LZ sample. The T2 spectrum of the dried sample only reflects the signal of the mineral matrix. The remaining signal after subtraction is the relaxation response of the fluid in the pores. The signal difference in different pressure ranges corresponds to the T2 distribution of the displaced mobile water at that pressure, effectively eliminating the interference of the rock matrix on the NMR signal and ensuring the purity of the fluid signal.

[0028] ② Mapping of Displacement Pressure and Orifice Throat Radius: Based on the Laplace equation, the injection pressure is converted into the corresponding orifice throat radius: (2) Where σ is the interfacial tension of the CO2-water system (35.1 mN / m), θ is the contact angle of the LZ sample (40.9°), and r tThe value is the pore throat radius. This equation is a classic correlation formula between capillary pressure and pore throat radius. Different injection pressures correspond to the minimum pore throat radius that can be broken through (e.g., 8 MPa corresponds to a pore throat radius of 1.03 μm, and 14 MPa corresponds to a pore throat radius of 0.029 μm), providing a quantitative basis for pore throat unit division.

[0029] ③ Spline interpolation is performed on the T2 distribution of movable water and the corresponding pore throat radius to construct T2-P. c Two-dimensional matrix, forming T2-P c Two-dimensional maps, such as Figure 4 As shown, the signal intensity of each grid in the spectrum represents the movable water volume of the corresponding pore-throat unit. The T2-P of the LZ sample... c The spectrum shows that the movable water signal is mainly concentrated in the long T2 region (corresponding to large pores), indicating that a large amount of movable water can be displaced even at low displacement pressure, consistent with the good pore connectivity characteristic of the LZ sample. This spectrum realizes the correlation between pore structure (T2 reflection) and fluid flowability (P). c The collaborative representation of (reflection).

[0030] Step S4: Calculation of the connectivity coefficient of carbon dioxide in the pore-throat unit. Based on the variable diameter capillary model, the connectivity coefficient is calculated by comprehensively considering capillary resistance, viscous resistance, and inlet resistance, as follows: Figure 3 As shown, the flow connectivity of CO2 in actual pores is accurately quantified: ① Capillary resistance (P) c ) Calculation: Using the Young-Laplace equation, i.e. , where r t The throat radius obtained in step S3 is used to ensure that the connectivity calculation takes into account the geometric constraints of the pore structure.

[0031] ② Viscous resistance (P) v Calculation: Considering the thickness of the adsorbed water film on the pore wall, the formula is derived based on the fluid flow equation in a variable diameter capillary: (3) Where μ is the viscosity of CO2 (5.2 × 10⁻⁻⁶). 5 Pa·s), L is the pore length, μ g Here, v is the gas phase viscosity, and r is the average fluid velocity. p Where is the pore radius, δ is the thickness of the bound water film (0.7 nm), and Q is the pore radius. vd This is the flow rate of the constant-radius capillary model. The introduction of this resistance avoids the overestimation of connectivity caused by neglecting the water film effect in traditional methods.

[0032] ③Inlet resistance (P) e Calculation: Based on the momentum loss caused by the change in fluid flow direction, the formula is derived as follows: (4) Where C is a constant, C=3. The LZ sample has a geometrical abrupt change in the pore-throat structure (difference between pore and throat radii), and the deflection of fluid streamlines generates additional momentum loss. The introduction of this resistance further improves the comprehensive consideration of flow resistance.

[0033] Calculation of total resistance and connectivity coefficient: Total resistance .

[0034] Therefore, combining equations (2)-(4), the volumetric flow rate in a capillary with a variable diameter is expressed as: (5) Fluid resistance weakens CO2 connectivity. Therefore, the CO2 connectivity coefficient within the pore-throat unit is defined as the ratio of the water flux calculated using a constant radius capillary model to that calculated using a variable diameter capillary model, as shown in the formula: (6) Where α is the angle of change of streamline direction, r pj Let r represent the radius of the j-th pore. tr Let CI represent the radius of the r-th pore throat. jr Indicates a pore radius of r pj And the radius of the throat is r tr Carbon dioxide interconnectivity in the pore throat unit, such as Figure 6 As shown. In summary, the connectivity of carbon dioxide within a single pore-throat unit is closely related to the pore size and pore-throat radius. This coefficient quantifies the influence of the pore-throat structural heterogeneity on CO2 flow. The CI coefficient of the pore-throat unit in the LZ sample... jr The values ​​are relatively high, reaching a maximum of 0.74, indicating that CO2 flows smoothly within these units.

[0035] Step S5: Quantitative calculation of carbon dioxide connectivity in shale caprock. Using T2-P... c The signal intensity of each aperture throat unit in the two-dimensional spectrum is used as a weight, and a weighted average is calculated, such as... Figure 2 As shown: Weight determination: T2-P c Each coordinate point (T2, P) in the two-dimensional atlas c The corresponding signal strength f jr This represents the number or volumetric weight of pore throat units of that size. The higher signal intensity of the pore throat units in the LZ sample indicates that these units account for a larger proportion of the sample and contribute more to the overall connectivity.

[0036] Weighted average calculation: (7) (8) In the formula, f jr Two-dimensional T2-P c Each coordinate point (T2, P) in the distribution map c The corresponding signal strength represents the number or volume weight of aperture throat units of a specific size, D. jr Indicates T2-P c Figure 2 The product of carbon oxide phase connectivity and signal intensity matrix data. The calculation results show that the CI of the LZ sample is 0.111. This result quantifies the overall flow connectivity of CO2 in the LZ sample, providing a precise indicator for subsequent storage safety assessment.

[0037] This embodiment successfully obtained the carbon dioxide connectivity of the LZ sample by fully executing the above steps, and the calculated CI was 0.111. Micro-focus computed tomography (microCT) technology was used to verify the pore connectivity of the LZ sample. Figure 7 As shown, the three-dimensional pore network reconstructed by microCT shows that the LZ sample is consistent with the CO2 connectivity order calculated by the method of this invention. Example

[0038] This embodiment selects a shale sample from Changde City (CD) for comparison with the LZ sample from Example 1. This sample is mainly composed of dolomite (83.7%) and calcite (14.7%). Nuclear magnetic resonance (NMR) analysis revealed a porosity of 1.44%, a contact angle of 73.4°, and an interfacial tension of 27.5 mN / m for the CO2-water system. The CD sample has an average pore radius of 21.71 μm and an average coordination number of 1.75. The pore space consists of discretely distributed micropores and nanoscale pore throats, indicating poor pore connectivity. This sample is used to verify the characterization ability of the method of this invention for samples with low connectivity. The specific implementation steps are consistent with Example 1, with only differences in core parameters and results, which are highlighted below: Differences between core parameters and key steps: Pretreatment and test parameters: The drying, saturation, and packaging process of CD samples is the same as in Example 1. The NMR test parameters, displacement pressure range, and holding time are also the same as in Example 1 to ensure the consistency and comparability of test conditions.

[0039] T2-P c Two-dimensional joint function construction: The T2 spectrum of the CD sample exhibits a double-peaked characteristic, with a trough signal intensity of approximately 135 a.u. Subtraction reveals that the movable water T2 distribution extends across the entire vertical axis of the spectrum, indicating that water is more difficult to displace by CO2 and that the residual water saturation is high. The pore throat radius derived from the Laplace equation (σ=27.5 mN / m, θ=73.4°) shows that the CD sample requires a higher displacement pressure to overcome the pore throat resistance. The constructed matrix T2-P... cIn the two-dimensional spectrum, the intensity of the movable water signal is generally below 100 a.u., reflecting the small volume of movable water in its pore throat unit.

[0040] Calculation of connectivity coefficient and overall connectivity: The CD sample has a smaller pore throat radius, and its capillary resistance is significantly greater than that of the LZ sample. Under the same pore throat radius, the P of CD is... c It is 1.8 times that of LZ; the viscous resistance and inlet resistance also increase due to the stronger heterogeneity of the pore structure, and the CI of the pore throat unit... jr The maximum value is 0.68, lower than LZ's 0.74; based on signal strength f jr After weighted averaging, the overall carbon dioxide connectivity CI of the CD sample was 0.056, which was significantly lower than that of the LZ sample (0.111).

[0041] like Figure 7 As shown, the CI of the CD sample is 0.056, which is consistent with the low porosity connectivity results of the microCT test, and it has an exponential relationship with the relative permeability of the CO2 phase endpoint (0.35) (R²=0.98), verifying the accurate characterization capability of the method of the present invention for low-connectivity samples. Example

[0042] This embodiment selects a shale sample from Yibin City (YB). The sample is mainly composed of quartz (62.1%), dolomite (11.5%), and illite (16.2%). The porosity was measured to be 2.01% by nuclear magnetic resonance (NMR) and verified to be 2.03% by saturated weighing (error ≤ 0.05%). The contact angle was 17.9°, and the interfacial tension of the CO2-water system was 41.3 mN / m. The YB sample exhibits well-developed mesopores with an average pore radius of 95.24 μm, forming a distribution characteristic of local mesopore enrichment and overall pore dispersion. The average coordination number is 2.43, and the pore connectivity is between that of the LZ and CD samples. This sample is used to verify the applicability of the method of this invention for characterizing samples with moderate connectivity. The specific implementation steps are consistent with Example 1, with the following differences in core parameters and key steps: Regarding pretreatment and testing parameters, the processing, drying (105℃, 24h), vacuum saturation (-0.1MPa degassing for 6h, 15MPa saturation for 24h), and heat shrink tubing packaging process of YB samples were completely consistent with those in Example 1, ensuring a unified test standard. The NMR test used the same CPMG pulse sequence parameters (Tw=3000ms, Te=0.3ms, etc.), and the stepped injection pressure (8~14MPa, gradient 1MPa, stabilization for 12h per stage) and formation environment simulation conditions (temperature 50℃, confining pressure / axial pressure 15MPa) remained unchanged, ensuring comparability between different samples.

[0043] T2-P cIn the two-dimensional joint function construction stage, the initial T2 spectrum of the YB sample exhibited a bimodal characteristic, with the left peak signal intensity close to 800 a.u. and the right peak exceeding 150 a.u., while the trough signal intensity was 0, reflecting that the connectivity between the mesopores and macropores was weaker than that of the LZ sample. After subtracting the T2 spectra under different displacement pressures, the movable water signal was concentrated in the medium-long T2 interval, with a total signal intensity of 10434.6 au, between LZ (22122.8 au) and CD (6066.0 au). Based on the Laplace equation (σ=41.3 mN / m, θ=17.9°), the displacement pressure was converted into the pore throat radius, with 8 MPa corresponding to a pore throat radius of 1.12 μm and 14 MPa corresponding to 0.032 μm. The constructed T2-P c The two-dimensional map shows that the distribution of movable water signals extends to the lower part of the map, indicating that some pore throat units require higher displacement pressure to displace water, which is consistent with the structural characteristics of its dispersed pore distribution.

[0044] In the calculation of connectivity coefficient and overall connectivity, the degree of geometric abrupt change in the pore throat structure of the YB sample is between LZ and CD. The combined effect of capillary resistance, viscous resistance, and inlet resistance leads to the CO2 connectivity coefficient CI of the pore throat unit. jr The maximum value is 0.71, lower than LZ's 0.74 and higher than CD's 0.68; with T2-P c The signal intensity f of each pore throat unit in the spectrum jr As the weight (the signal intensity of the pore throat unit accounts for the highest proportion), the overall carbon dioxide connectivity CI of the YB sample was calculated to be 0.103 using the weighted average formula.

[0045] The verification results show that the CI of the YB sample is 0.103, which is consistent with the connectivity of the three-dimensional pore network reconstructed by microCT, and has a significant exponential relationship with the relative permeability of the CO2 phase endpoint (0.63) (R²=0.98). This confirms the accuracy of the method of the present invention in characterizing medium-connectivity shale samples and can accurately capture the influence of the mesopore dispersion distribution on CO2 connectivity. Example

[0046] This embodiment uses a shale sample from Guiyang City (GY). The sample is mainly composed of dolomite (34.8%), calcite (57.7%), and siderite (3.1%). The porosity was measured to be 1.76% by nuclear magnetic resonance (NMR) and verified to be 1.78% by saturated weighing (error ≤ 0.05%). The contact angle was 46.0°, and the interfacial tension of the CO2-water system was 27.5 mN / m. The GY sample exhibits a predominantly small-pore structure with locally developed macropores. The average pore radius is 42.32 μm, containing numerous isolated pores. The average coordination number is 2.19, and the pore connectivity is lower than that of samples LZ and YB but higher than that of sample CD. This sample is used to further verify the universality of the method of this invention. The specific implementation steps are consistent with those in Example 1. The differences in core parameters and key steps are as follows: Pretreatment and testing parameters strictly followed the standard procedure of Example 1. The sample was processed into a standard cylinder of φ25mm×20mm, and the drying, vacuum saturation and packaging operations were identical. NMR testing parameters, displacement pressure scheme and formation environment simulation conditions were kept consistent to ensure the comparability of test data and avoid deviations in characterization results due to parameter differences.

[0047] T2-P c During the construction of the two-dimensional joint function, the initial T2 spectrum of the GY sample exhibited a bimodal characteristic, with the left peak signal intensity close to 800 a.u., the right peak signal intensity exceeding 150 a.u., and the trough signal intensity being 0, reflecting poor connectivity between the small, medium, and large pores. The total movable water signal intensity obtained after subtraction was 8326.0 au, falling between YB (10434.6 au) and CD (6066.0 au). Based on the Laplace equation (σ=27.5 mN / m, θ=46.0°), the pore throat radius was transformed; 8 MPa corresponds to a pore throat radius of 1.08 μm, and 14 MPa corresponds to 0.031 μm. T2-P c Two-dimensional spectra show that the movable water signals are discretely distributed, and there is still a small amount of signal response at the bottom of the spectrum, indicating that there is a lot of residual water that is difficult to displace, which matches the structural characteristics of a large number of isolated pores in the sample.

[0048] In the calculation of connectivity coefficient and overall connectivity, the smaller and unevenly distributed pore throat radius of the GY sample resulted in significantly higher capillary resistance and inlet resistance compared to the LZ and YB samples. The CO2 connectivity coefficient CI of the pore throat unit was also higher. jr The maximum value is 0.69, lower than LZ (0.74) and YB (0.71); based on the signal strength f of each aperture throat unit... jr The weighted average was calculated to obtain the overall carbon dioxide connectivity CI of the GY sample as 0.089.

[0049] The verification results show that the CI of the GY sample is 0.089, which is consistent with the pore connectivity results of the microCT test. Furthermore, the exponential correlation R² with the relative permeability (0.55) of the CO2 phase endpoint is 0.98, which further confirms that the method of the present invention can accurately characterize the CO2 connectivity of shale samples with different connectivity levels and is suitable for the storage safety assessment of diverse shale caprocks.

[0050] The above embodiments successfully obtained the carbon dioxide connectivity of each sample by fully executing the above steps. Micro-focus computed tomography (microCT) technology was used to verify the pore connectivity of the samples. Figure 7 As shown, the three-dimensional pore network reconstructed by microCT shows that the CO2 connectivity ranking of the sample is consistent with that calculated by the method of this invention. Furthermore, the CO2 connectivity of the sample exhibits a significant exponential relationship with the relative permeability of the CO2 phase endpoints (R²=0.98), verifying the scientific validity of the method of this invention. Simultaneously, this method captures the characteristic that mesopore throats are the main seepage channels, providing technical support for the targeted assessment of the shale caprock's storage potential.

[0051] The above description is only a preferred embodiment of the present invention and does not limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for quantitatively characterizing the connectivity of carbon dioxide in shale caprock, characterized in that, Includes the following steps: S1. Pre-treat the shale caprock samples to obtain standard samples that meet the test requirements; S2. The pretreated sample was placed in a multi-field coupling test system to simulate the in-situ environment of the formation and carry out a step-injection pressure displacement test. Simultaneously, the transverse relaxation time T2 spectrum under different displacement pressures was acquired in real time using nuclear magnetic resonance technology. S3, Capillary pressure P based on different displacement pressures c Using the T2 spectrum obtained in step S2, a T2-P spectrum characterizing the pore fluid occurrence state and flowability is constructed. c Two-dimensional joint functions yield T2-P c Two-dimensional map; S4. Based on the variable diameter capillary model, considering capillary resistance, viscous resistance, and inlet resistance, calculate T2-P. c Carbon dioxide connectivity coefficient of each pore throat unit in the two-dimensional spectrum; S5, with T2-P c The signal intensity of each pore throat unit in the two-dimensional map is used as the weight. The carbon dioxide connectivity coefficient of the corresponding pore throat unit is weighted and averaged to obtain the carbon dioxide connectivity in the shale caprock.

2. The method for quantitatively characterizing the connectivity of carbon dioxide in shale caprock according to claim 1, characterized in that, The preprocessing in step S1 includes the following steps: S11. The shale sample is processed into a standard cylindrical sample, the geometric dimensions of which are φ25mm×20mm. S12. The processed sample was dried in an oven at 105℃ to constant weight, and then degassed under a negative pressure of -0.1MPa for at least 6 hours using a vacuum saturation device. Then, 15MPa hydrostatic pressure was applied and distilled water was injected for saturation for 24 hours. S13. After saturation, the sample is sealed to ensure airtightness during the test.

3. The method for quantitatively characterizing the connectivity of carbon dioxide in shale caprock according to claim 1, characterized in that, In step S2, the in-situ environment of the formation is simulated, specifically: the test temperature is set to 50℃, the confining pressure and axial pressure are 15MPa, and the formation pressure conditions corresponding to the CO2 sequestration depth of 800~2300m are simulated; the injection pressure gradient of the stepped injection pressure displacement test is 1MPa, the injection pressure range is 8~14MPa, and each pressure level is maintained for 12 hours to reach stress equilibrium before collecting the T2 spectrum.

4. The method for quantitatively characterizing the connectivity of carbon dioxide in shale caprock according to claim 1, characterized in that, T2-P in step S3 c The construction process of a two-dimensional joint function is as follows: S31. Subtract the T2 spectra under different displacement pressures to eliminate interference from the rock matrix signal and obtain the T2 distribution of the displaced mobile water in different displacement pressure ranges. S32. Based on the Laplace equation, the displacement pressure is converted into the corresponding pore throat radius, and the mapping relationship between the displacement pressure and the pore throat radius is established. S33. Perform spline interpolation on the T2 distribution of movable water and the corresponding pore throat radius to construct the T2-P distribution. c Two-dimensional matrix, forming T2-P c Two-dimensional spectra, where signal intensity represents the movable water volume of the corresponding pore throat unit.

5. The method for quantitatively characterizing the connectivity of carbon dioxide in shale caprock according to claim 1, characterized in that, The calculation basis for the carbon dioxide connectivity coefficient in step S4 is as follows: Capillary resistance P c The calculation is based on the Young-Laplace equation, and the formula is as follows: （1） Where σ is the gas-liquid interfacial tension, θ is the contact angle, and r t The radius of the throat; Viscous resistance P v Considering the thickness of the adsorbed water film on the pore wall, and based on the fluid flow equation in a variable-diameter capillary, the expression for viscous resistance is derived as follows: （2） In the formula, v is the flow velocity of the fluid in the capillary, and r g It is the gas flow radius, μ g δ is the gas phase viscosity, δ is the thickness of the bound water film, and Q is the gas phase viscosity. vd This is the flow rate of the capillary model with equal radius, μ is the fluid viscosity, and L is the pore length; Inlet resistance P e Based on the momentum loss caused by changes in fluid flow direction, the calculation formula is derived as follows: （3） In the formula, C=3; The total resistance is the sum of capillary resistance, viscous resistance, and inlet resistance: .

6. The method for quantitative characterizing the connectivity of carbon dioxide in shale caprock according to claim 5, characterized in that, The carbon dioxide connectivity within a pore-throat unit is defined as the ratio of the aqueous flux calculated using a constant radius capillary model to that calculated using a variable diameter capillary model. （4） In the formula, α is the angle of change of streamline direction, and r pj Let r represent the radius of the j-th pore. tr Let CI represent the radius of the r-th pore throat. jr Indicates a pore radius of r pj And the radius of the throat is r tr The carbon dioxide interconnectivity in the pore throat unit.

7. The method for quantitatively characterizing the connectivity of carbon dioxide in shale caprock according to claim 6, characterized in that, The overall carbon dioxide connectivity index CI of the shale sample was calculated using the following formula: （5） （6） In the formula, f jr Two-dimensional T2-P c Each coordinate point (T2, P) in the distribution map c The corresponding signal strength represents the number or volume weight of aperture throat units of a specific size, D. jr Indicates T2-P c The figure shows the product of carbon dioxide phase connectivity and signal strength matrix data.

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