Low-permeability and ultra-low-permeability reservoir saturation model construction method based on digital core technology

Abstract

The invention relates to a low-permeability and ultra-low-permeability reservoir saturation model construction method based on a digital core technology, and the method comprises the steps: selecting a core sample of a target reservoir, carrying out a scanning experiment, and processing and reconstructing a scanning image into a three-dimensional digital core framework; the method comprises the following steps: constructing three-dimensional digital cores with different porosities, calculating the resistivity of the digital cores with different porosities when the digital cores are fully saturated with water, calculating formation factors, and constructing a relationship between the formation factors and the porosities. Three-dimensional digital cores with different water saturations are constructed, the resistivity of the digital cores with different water saturations is calculated, the resistivity index is calculated, and the relation between the resistivity index and the water saturations is constructed. And according to the relationship between the stratum factor and the porosity and the relationship between the resistivity index and the water saturation, constructing a new water saturation model suitable for the target reservoir. By implementing the technical scheme of the invention, the relationship between the reservoir resistivity and the water saturation, porosity and formation water resistivity can be deduced, and a more applicable model is provided for the calculation of the water saturation of the target reservoir.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas reservoir evaluation, and in particular to a method for constructing a low-permeability or ultra-low-permeability reservoir saturation model based on digital core technology. Background Art

[0002] The Archie saturation model has been widely used in reservoir saturation evaluation. With the development of oil and gas exploration, deep / ultra-deep reservoirs have gradually become the focus of exploration. The pore structure of such reservoirs is complex and heterogeneous. The saturation calculated by the Archie model constructed using traditional rock physics experiments is inaccurate and cannot meet the needs of oil and gas field exploration and development.

[0003] The Archie model and its derivative models need to determine the cementation index and saturation index in advance when calculating reservoir saturation, and both parameters come from the results of laboratory rock electrical experiments. For low-permeability and ultra-low-permeability reservoirs, the existing displacement technology in the laboratory cannot obtain low-water-saturation cores, and cannot measure the resistivity of cores through rock electrical experiments, so it is impossible to obtain accurate cementation index and saturation index, and it is impossible to build an accurate reservoir saturation model, resulting in inaccurate oil content evaluation of low-permeability and ultra-low-permeability reservoirs. Summary of the invention

[0004] The technical problem to be solved by the present invention is that, for low-permeability and ultra-low-permeability reservoir rocks, the existing laboratory displacement technology is difficult to completely displace the low-permeability rocks, resulting in the inability to measure the resistivity of low-water-saturation cores through rock electrical experiments, and it is also impossible to obtain accurate cementation index and water saturation index, and it is impossible to construct an accurate reservoir saturation model. A method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology is proposed.

[0005] The technical solution adopted by the present invention to solve the technical problem is: to provide a method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology, comprising:

[0006] Step S1, selecting a core sample of a target reservoir for a scanning experiment to obtain a scanned image, and processing and reconstructing the scanned image into a three-dimensional digital core grid;

[0007] Step S2, using the three-dimensional digital core framework to construct three-dimensional digital cores with different porosities and three-dimensional digital cores with different water saturations;

[0008] Step S3, respectively calculating the resistivity of the digital cores with different porosities when fully saturated with water, calculating the corresponding formation factors according to the resistivity, and constructing the relationship between the formation factors and the porosity;

[0009] Step S4, respectively calculating the resistivity of the digital cores with different water saturations, and constructing a relationship between the resistivity index and the water saturation;

[0010] Step S5: construct a new water saturation model of the target reservoir according to the relationship between the formation factors and porosity and the relationship between the resistivity index and water saturation.

[0011] Preferably, in step S3, the formation factor refers to the ratio of the resistivity of the fully saturated core to the water resistivity. The relationship between the formation factor and the porosity is obtained by constructing a cross plot under double logarithmic coordinates and performing regression fitting. The relationship between the two is expressed by formula (1):

[0012]

[0013] In the formula,

[0014] F is the stratigraphic factor;

[0015] R0 is the resistivity of saturated rock;

[0016] R w is the formation water resistivity;

[0017] φ is the porosity, m is the cementation index;

[0018] a is a coefficient related to lithology;

[0019] Wherein, a and m are obtained by data fitting.

[0020] Preferably, in step S4, the resistivity index refers to the ratio of the resistivity of cores with different water saturations to the resistivity of cores with full water saturation. The relationship between the resistivity index and water saturation is obtained by constructing a cross plot under double logarithmic coordinates and performing regression fitting. The relationship between the two is expressed by formula (2):

[0021]

[0022] In step S5, the new water saturation model of the target reservoir is constructed according to the following formula (3):

[0023]

[0024] In the formula,

[0025] RI is the resistivity index;

[0026] R t is the formation resistivity;

[0027] S w is the water saturation;

[0028] b *is a coefficient related to lithology;

[0029] n * is the saturation index;

[0030] Wherein, the b* and n* are obtained through regression fitting.

[0031] Preferably, in step S1, processing and reconstructing the scanned image into a three-dimensional digital core grid comprises: using any one of Avizo, GMS, and Petrel to reconstruct the three-dimensional digital core grid.

[0032] Preferably, constructing the three-dimensional digital cores with different water saturations in step S2 includes: simulating the oil-water displacement process in the pores to construct the digital cores with different water saturations.

[0033] Preferably, the simulating the oil-water displacement process in the pores comprises: simulating the oil-water displacement process in the pores by using any one of mathematical morphology-open operation, phase field method, finite element method, and lattice Boltzmann method.

[0034] Preferably, the calculating the resistivity of the digital core that is completely saturated with water comprises: calculating the resistivity of the digital core using any one of a finite element numerical simulation method, a random walk algorithm, a lattice Boltzmann method, and a finite difference method.

[0035] Preferably, before step S1, the step further includes:

[0036] Step S0, determining whether the core sample can obtain a full range of water saturation cores through displacement technology, if not, executing steps S1-S5.

[0037] Preferably, after determining whether the core sample can obtain a full range of water saturation cores through displacement technology, the method further includes:

[0038] If yes, a rock electrical experiment is performed on the core sample, and the water saturation of the target reservoir is calculated using the Archie model according to the rock electrical experiment results; or, steps S1-S5 are executed.

[0039] Preferably, the Archie water saturation model of the core sample is constructed according to the following formula (4):

[0040]

[0041] In the formula,

[0042] S w is the water saturation;

[0043] n is the saturation index;

[0044] b is a coefficient related to lithology;

[0045] a is a coefficient related to lithology;

[0046] R w is the formation water resistivity;

[0047] R t is the formation resistivity;

[0048] φ is the porosity;

[0049] m is the cementation index.

[0050] The method for constructing a reservoir saturation model based on digital core technology of the present invention adopts digital rock physics technology and rock physics analysis method, which makes up for the deficiency that the existing displacement technology of rock electrical experiment cannot realize resistivity measurement under low water saturation. The constructed model covers a wide range of water saturation, and effectively solves the problem of large error in saturation prediction results of high oil-containing reservoirs (i.e. low permeability and ultra-low permeability reservoirs). BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0052] Figure 1 It is a flow chart of an embodiment provided by the present invention.

[0053] Figure 2 It is a formation factor-porosity intersection diagram of an embodiment provided by the present invention.

[0054] Figure 3 This is a resistivity index-water saturation cross-plot according to an embodiment of the present invention.

[0055] Figure 4 This is a water saturation evaluation result diagram of an embodiment provided by the present invention. DETAILED DESCRIPTION

[0056] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific implementation methods of the present invention are now described in detail with reference to the accompanying drawings. It should be noted that the terms "first", "second", "third", etc. are only for the convenience of describing the present technical solution, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second", "third", etc. may explicitly or implicitly include one or more of the features. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0057] In the following description, for the purpose of illustration rather than limitation, specific details such as specific system structures and technologies are provided to provide a thorough understanding of the embodiments of the present invention. However, it should be clear to those skilled in the art that the present invention may also be implemented in other embodiments without these specific details.

[0058] like Figure 1 As shown, in an embodiment of a method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology provided by the present invention, the method includes:

[0059] Step S1, select the core sample of the target reservoir for scanning experiment, obtain the scanned image, and process and reconstruct the scanned image into a three-dimensional digital core grid. In this step, it should be noted that the core sample comes from the rock sample of the target reservoir, and a small-size sample is drilled. The core grid usually refers to the microstructural framework of the rock, which can reflect the distribution characteristics of the pores, cracks and rock skeleton of the rock. This scheme adopts the method of obtaining two-dimensional pictures of the core, and then superimposing and reconstructing these pictures into a three-dimensional digital core through a modeling program or software to construct a three-dimensional digital core grid.

[0060] Various scanning techniques are used to analyze the core samples in detail to obtain their physical, chemical and structural properties. It is important to analyze the reservoir characteristics, pore structure, mineral composition and fluid flow characteristics of the rock through experiments. The core sample scanning experimental techniques used in this embodiment include but are not limited to: X-CT scanning, nuclear magnetic resonance imaging (NMR), optical microscope scanning, etc.

[0061] After obtaining the scanned image, the collected image is preprocessed by denoising, filtering, threshold segmentation, contrast enhancement, edge detection and other operations to improve the accuracy of subsequent digital core construction. Image processing technology is used to distinguish different areas in the core image, such as rock matrix, pores, cracks, etc. The segmented two-dimensional image sequence is converted into a three-dimensional model through a three-dimensional reconstruction algorithm. For details, please refer to the existing technology.

[0062] Optionally, in step S1, the scanning image is processed and reconstructed into a three-dimensional digital core grid, including: using any one of Avizo, GMS (Groundwater Modeling System), and Petrel to reconstruct the three-dimensional digital core grid. Specifically, Avizo software is a professional three-dimensional visualization and analysis tool, which is widely used in research fields such as earth geological science and material science. It has advanced three-dimensional visualization functions, 3D data survey and analysis, image data acquisition, three-dimensional reconstruction and other functions. The detailed steps of using Avizo for three-dimensional reconstruction include operations such as importing data, image segmentation, and pore analysis. GMS (Groundwater Modeling System) is a groundwater numerical simulation software that provides the function of building a three-dimensional geological model and supports modules such as TIN, Boreholes, and Solids to build a digital three-dimensional geological structure model. GMS can couple the three-dimensional geological structure model with models such as MODFLOW to realize the numerical simulation of groundwater flow. Petrel software is a professional reservoir management and geological modeling software. It quantitatively and vividly distributes the three-dimensional parameters and geometric morphology of the reservoir by establishing a three-dimensional reservoir geological model. Petrel software can observe and describe reservoirs from multiple angles, deepen the understanding of reservoir geological characteristics and reservoir parameter distribution, and provide important basis for detailed exploration of oil fields, formulation and adjustment of development plans. Avizo, GMS and Petrel all have the ability to reconstruct 3D digital core frameworks, and each plays an important role in geological research, groundwater simulation and reservoir management. Users can choose appropriate software for 3D digital core modeling and analysis according to their research needs and data characteristics.

[0063] By implementing the technical solution of this embodiment, a low-permeability and ultra-low-permeability digital core model can be created to obtain a saturation model suitable for low-permeability and ultra-low-permeability reservoirs that are difficult to displace, thereby effectively improving the calculation accuracy of the oil saturation of the target reservoir.

[0064] Step S2, using a three-dimensional digital core grid, construct three-dimensional digital cores with different porosities and three-dimensional digital cores with different water saturations. In this step, the three-dimensional digital core grid can be used as a model for microscopic numerical simulation of resistivity, and the pore structure can be adjusted using a preset algorithm to generate digital cores with different porosities; a preset algorithm is used to simulate the oil-water displacement process to construct digital cores with different water saturations. The two cores are constructed separately, and the specific porosity and water saturation are constructed according to the actual situation of the core sample and the operation of the laboratory. For example, for a sample obtained at a certain time, digital cores with different porosities (8.6%, 9.9%, 10.3%, 11.3%, 12.1%, 14.8%) are constructed in the laboratory, and digital cores with different water saturations (20.7%, 27.4%, 33.5%, 43.4%, 58.3%, 78.3%, 100%) are constructed. The constructed core porosity values ​​and water saturation values ​​fluctuate within the preset range.

[0065] In a specific embodiment, any one of mathematical morphology-open operation, phase field method, finite element method, and lattice Boltzmann method can be used to simulate the oil-water displacement process in the pore. In this embodiment, by simulating digital cores with different water saturations, a suitable simulation method is selected according to the specific conditions of the reservoir, the required accuracy, computing resources, and the purpose of the simulation.

[0066] Step S3, respectively calculating the resistivity of digital cores with different porosities when fully saturated with water, calculating the corresponding formation factors according to each resistivity, and constructing the relationship between the formation factors and the porosity.

[0067] In this step, the resistivity of the digital cores with different porosities constructed in step S2 is calculated in a state of being completely saturated with water to obtain the resistivity of the saturated rock, so as to obtain the relationship between the resistivity and the porosity. Alternatively, any one of the finite element numerical simulation method, the random walk algorithm, the lattice Boltzmann method, and the finite difference method can be used to calculate the resistivity of the digital core.

[0068] In practical applications, the choice of method depends on the specific research objectives, computing resources, and physical properties of rocks. The finite element method is often used for conductivity simulation of digital cores because of its advantages in dealing with complex geological structures and anisotropic media. The random walk algorithm and the lattice Boltzmann method have attracted attention due to their potential application in simulating fluid diffusion and seepage. The finite difference method is suitable for scenarios that require fast solutions and have relatively simple geological structures.

[0069] In a specific implementation, one or more methods can be selected to calculate the resistivity according to actual conditions. A suitable calculation method can be selected according to the computing resources and the purpose of the simulation.

[0070] Furthermore, the ratio of different rock resistivity to formation water is used to obtain the formation factor, and each core corresponds to a formation factor value. The formation factor refers to the ratio of the resistivity of the fully saturated core to the water resistivity. The relationship between the formation factor and the porosity is obtained by constructing a cross plot under double logarithmic coordinates and regression fitting. The relationship between the two is expressed by formula (1):

[0071]

[0072] Where F is the formation factor; R0 is the resistivity of saturated rock; R w is the formation water resistivity; φ is the porosity, m is the cementation index; a is a coefficient related to lithology; a and m are obtained through data fitting.

[0073] In other embodiments, fitting techniques (such as the least squares method) may be used to establish a relationship model between porosity and formation factors, and the model may be optimized to fit the experimental data by adjusting the cementation index m and other related parameters.

[0074] Step S4, respectively calculating the resistivity of the digital cores with different water saturations, and constructing the relationship between the resistivity index and the water saturation. In this step, it should be noted that, because the constructed water saturation range is between 0-100%, a digital core with low water saturation can be constructed according to demand, so it can make up for the deficiency that the traditional laboratory displacement technology cannot effectively reduce the water saturation in the core to the required low level, and provide technical support for calculating the core resistivity under low water saturation, so as to simulate a more accurate relationship between the resistivity index and the water saturation.

[0075] Step S5: construct a new water saturation model of the target reservoir according to the relationship between formation factors and porosity and the relationship between resistivity index and water saturation.

[0076] In step S4, the resistivity index refers to the ratio of the resistivity of the core with different water saturation to the resistivity of the core with full water saturation. The relationship between the resistivity index and the water saturation is obtained by constructing a cross plot under double logarithmic coordinates and performing regression fitting. The relationship between the two is expressed by formula (2):

[0077]

[0078] In step S5, a new water saturation model of the target reservoir is constructed according to the following formula (3):

[0079]

[0080] Where RI is the resistivity index; R t is the core resistivity; S w is the water saturation; b *is a coefficient related to lithology; n * is the saturation index; b* and n* are obtained through regression fitting.

[0081] The technical solution of the present invention adopts digital rock physics technology and rock physics analysis method, which makes up for the deficiency that the existing displacement technology of rock electrical experiment cannot realize resistivity measurement under low water saturation. The constructed model covers a wide range of water saturation, and effectively solves the influence of low saturation results of Archie model calculation of high oil-bearing reservoir. It has significant theoretical research significance and practical application value.

[0082] Furthermore, before step S1, the following steps are also included:

[0083] Step S0, judging whether the core sample can obtain a full range of water saturation cores through displacement technology. In this step, it should be noted that the full range refers to multiple specific values ​​in the range of water saturation 0-100%. These specific values ​​are not limited in the present invention, and are determined according to experimental requirements, as long as they can meet the requirements of reservoir saturation model construction.

[0084] If not, execute steps S1-S5.

[0085] If yes, perform a rock electrical test on the core sample, and calculate the water saturation of the target reservoir using the Archie model based on the rock electrical test results; or, execute steps S1-S5.

[0086] Because when traditional rock electrical experiments are performing oil-water displacement, if a certain low water saturation is reached, further displacement cannot be performed, and the rock resistivity below the water saturation point cannot be calculated. Therefore, the present invention divides the reservoir based on whether the full range of water saturation cores can be obtained through displacement technology. The low water saturation cores that cannot be obtained by existing displacement technology are low-permeability and ultra-low permeability reservoirs (high oil saturation reservoirs), and the existing displacement technology can obtain low water saturation cores and high permeability reservoirs (high water saturation reservoirs). When the reservoir core sample cannot be further displaced, the method of constructing a digital core with low water saturation can be used to calculate the core resistivity, which makes up for the deficiency that the traditional rock electrical experiment cannot complete the calculation of the resistivity of the low water saturation core, and the saturation model of the low permeability and ultra-low permeability reservoir is obtained by executing steps S1-S5.

[0087] Furthermore, for high-permeability reservoirs, cores with low water saturation can be obtained through oil-water displacement experiments. Therefore, the Archie saturation model can be applied to directly construct cores with different porosities and cores with different water saturations, and the resistivity of cores with different porosities when fully saturated with water is calculated respectively. The corresponding formation factors are calculated according to each resistivity, and the relationship between the formation factors and porosity of the core is constructed; the resistivity of cores with different water saturations is calculated, and the relationship between the resistivity index and water saturation of the core is constructed. According to the relationship between formation factors and porosity and the relationship between resistivity index and water saturation, an Archie water saturation model of the target reservoir is constructed.

[0088] The Archie water saturation model of the core sample is constructed according to the following formula (4):

[0089]

[0090] In the formula, S w is water saturation; n is saturation index; b is coefficient related to lithology; a is coefficient related to lithology; R w is the formation water resistivity; R t is the formation resistivity; φ is the porosity; m is the cementation index. The above parameters are all derived from laboratory rock electrical test results or calculated from rock electrical test results, and do not need to be simulated by digital core technology.

[0091] When the target reservoir is a high permeability reservoir, the method of constructing digital cores with different water saturations can also be used to calculate the core resistivity. The digital core of this embodiment can also replace the traditional oil-water displacement experiment to simulate digital cores with different water saturations, and then construct the relationship between the formation factors and porosity of the digital core of the high permeability reservoir, and the relationship between the resistivity index and water saturation of the digital core of the high permeability reservoir. According to the relationship between the formation factors and porosity and the relationship between the resistivity index and water saturation, a new water saturation model of the high permeability reservoir is constructed. The technical solution of this embodiment provides a new method for constructing a saturation model of a high permeability reservoir. Technicians can choose the traditional Archie water saturation model or the new water saturation model according to needs to calculate the saturation model of the high permeability reservoir. The present invention has a wider range of applications.

[0092] The following is the verification process for the new water saturation model of the present invention:

[0093] Well A is an exploration well in a basin in the South my country Sea. The porosity of the target layer of the well is about 13.2%, and the permeability is about 1mD, which is a typical low-permeability reservoir. The reservoir saturation parameters are calculated using the Archie formula and the new water saturation model method of the present invention.

[0094] Specifically, step S1 is executed to reconstruct a three-dimensional digital core frame by filtering, threshold segmentation and other processing to obtain a scanned image obtained by scanning a core sample; step S2 is executed to construct digital cores with different porosities (8.6%, 9.9%, 10.3%, 11.3%, 12.1%, 14.8%), and mathematical morphology-opening operations are used to simulate the oil-water displacement process in the pores to construct digital cores with different water saturations (20.7%, 27.4%, 33.5%, 43.4%, 58.3%, 78.3%, 100%); step S3 is executed to construct the digital cores as shown in FIG. Figure 2 The formation factor-porosity cross-plot shown in FIG. The fitting lithology coefficient a=0.96, the cementation index m=1.68. Execute step S4 to construct Figure 3 The resistivity index-water saturation cross-plot shown is compared with the rock electrical test results in step 1. Figure 4 As shown in Figure 2, when Sw < 40%, that is, in a high oil saturation reservoir, RI and Sw are nonlinear in double logarithmic coordinates, reflecting the new conductivity characteristics of the low permeability reservoir. The new mathematical form of RI and Sw is derived (Formula 2), b * =14.18,n * =2.77. Execute step S5 to construct a new water saturation model for the target reservoir.

[0095] In step S4, the resistivity index refers to the ratio of the resistivity of the core with different water saturation to the resistivity of the core with full water saturation. The relationship between the resistivity index and the water saturation is obtained by constructing a cross plot under double logarithmic coordinates and performing regression fitting. The relationship between the two is expressed by formula (2):

[0096]

[0097] In step S5, a new water saturation model of the target reservoir is constructed according to the following formula (3):

[0098]

[0099] Where RI is the resistivity index; R t is the formation resistivity; S w is the water saturation; b * is a coefficient related to lithology; n * is the saturation index.

[0100] Figure 4 This is the result diagram of water saturation evaluation of the target layer of the well. Figure 4As shown, the first track is the depth track, the second track is the gamma logging while drilling and the borehole diameter, the third track is the resistivity logging while drilling, the fourth track is the density logging while drilling, the neutron logging and the photoelectric absorption cross-section index, the fifth track is the formation permeability, the sixth track is the formation porosity, the seventh track is the saturation evaluation result, and the eighth track is the mineral profile. In the saturation evaluation results, red is the calculation result of the new model, blue is the calculation result of the Archie model, and the black scattered points are the results of the closed coring saturation experiment. It can be seen that in high water saturation reservoirs, the saturation results calculated by the two models are consistent. In high oil saturation reservoirs, the oil saturation calculated by the new model is more consistent with the core experiment results, while the Archie model calculation results are low, and the higher the oil saturation, the greater the error of the Archie model calculation results. Applications show that the saturation model established by the present invention is effective in the saturation calculation of low permeability reservoirs, and effectively solves the influence of the low saturation results of the Archie model in calculating high oil reservoirs.

[0101] It can be understood that the above embodiments only express the preferred implementation modes of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the patent scope of the present invention. It should be pointed out that, for ordinary technicians in this field, the above technical features can be freely combined without departing from the concept of the present invention, and several deformations and improvements can be made, which all belong to the protection scope of the present invention. Therefore, all equivalent changes and modifications made to the scope of the claims of the present invention should belong to the coverage of the claims of the present invention.

Claims

1. A method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology, characterized in that: include: Step S1, selecting a core sample of a target reservoir for a scanning experiment to obtain a scanned image, and processing and reconstructing the scanned image into a three-dimensional digital core grid; Step S2, using the three-dimensional digital core framework to construct three-dimensional digital cores with different porosities and three-dimensional digital cores with different water saturations; Step S3, respectively calculating the resistivity of the digital cores with different porosities when fully saturated with water, calculating the corresponding formation factors according to the resistivity, and constructing the relationship between the formation factors and the porosity; Step S4, respectively calculating the resistivity of the digital cores with different water saturations, and constructing a relationship between the resistivity index and the water saturation; Step S5: construct a new water saturation model of the target reservoir according to the relationship between the formation factors and porosity and the relationship between the resistivity index and water saturation.

2. The method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology according to claim 1, characterized in that: In step S3, the formation factor refers to the ratio of the resistivity of the fully saturated core to the water resistivity. The relationship between the formation factor and the porosity is obtained by constructing a cross plot under double logarithmic coordinates and performing regression fitting. The relationship between the two is expressed by formula (1): In the formula, F is the stratigraphic factor; R0 is the resistivity of saturated rock; R w is the formation water resistivity; is the porosity, m is the cementation index; a is a coefficient related to lithology; Wherein, a and m are obtained by data fitting.

3. The method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology according to claim 2, characterized in that: In step S4, the resistivity index refers to the ratio of the resistivity of cores with different water saturations to the resistivity of cores with full water saturation. The relationship between the resistivity index and water saturation is obtained by constructing a cross plot under double logarithmic coordinates and performing regression fitting. The relationship between the two is expressed by formula (2): In step S5, the new water saturation model of the target reservoir is constructed according to the following formula (3): In the formula, RI is the resistivity index; R t is the formation resistivity; S w is the water saturation; b * is a coefficient related to lithology; n * is the saturation index; Among them, the b * 、n * It is obtained through regression fitting.

4. The method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology according to claim 1, characterized in that: The step S1 of processing and reconstructing the scanned image into a three-dimensional digital core grid includes: using any one of Avizo, GMS, and Petrel to reconstruct the three-dimensional digital core grid.

5. The method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology according to claim 1, characterized in that: The step S2 of constructing the three-dimensional digital cores with different water saturations includes: simulating the oil-water displacement process in the pores to construct the digital cores with different water saturations.

6. The method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology according to claim 5, characterized in that: The simulation of the oil-water displacement process in the pores includes: using any one of mathematical morphology-open operation, phase field method, finite element method, and lattice Boltzmann method to simulate the oil-water displacement process in the pores.

7. The method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology according to claim 1, characterized in that: The calculating the resistivity of the digital core that is completely saturated with water includes: calculating the resistivity of the digital core using any one of a finite element numerical simulation method, a random walk algorithm, a lattice Boltzmann method, and a finite difference method.

8. The method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology according to claim 1, characterized in that: Before step S1, the following steps are also included: Step S0, determining whether the core sample can obtain a full range of water saturation cores through displacement technology, if not, executing steps S1-S5.

9. The method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology according to claim 8, characterized in that: After determining whether the core sample can obtain a full range of water saturation cores through displacement technology, the following steps are further included: If yes, a rock electrical experiment is performed on the core sample, and the water saturation of the target reservoir is calculated using the Archie model according to the rock electrical experiment results; or, steps S1-S5 are executed.

10. The method for constructing a low-permeability and ultra-low-permeability reservoir saturation model based on digital core technology according to claim 9, characterized in that: The Archie water saturation model of the core sample is constructed according to the following formula (4): In the formula, S w is the water saturation; n is the saturation index; b is a coefficient related to lithology; a is a coefficient related to lithology; R w is the formation water resistivity; R t is the formation resistivity; φ is the porosity; m is the cementation index.

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