Rock porosity prediction method
By conducting loading and unloading experiments on rock core samples and applying fractal geometry theory, combined with the pore volume deformation of shallow rock layers, the problem of inaccurate porosity measurement in deep rocks was solved, achieving low-cost and efficient porosity prediction and improving the accuracy and reliability of the prediction.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-01-23
- Publication Date
- 2026-04-07
AI Technical Summary
Existing technologies struggle to recreate the rock stress state in deep environments when measuring the porosity of rocks in deep formations, leading to inaccurate porosity measurements and high drilling and transportation costs.
By conducting loading and unloading experiments on core samples, the pore volume deformation of shallow rock layers is obtained. Combined with fractal geometry theory, it is transformed into the porosity of the target rock layer, avoiding in-situ sampling and using the characteristics of the same set of sedimentary strata for prediction.
It improves the accuracy and reliability of porosity prediction, reduces costs, decreases reliance on large equipment, and more realistically reflects the mechanical response of rocks at different depths, avoiding prediction bias caused by sample differences.
Smart Images
Figure CN121805002A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of oil extraction technology, and more specifically to a method for predicting rock porosity. Background Technology
[0002] Porosity, as a key physical parameter of rocks, has a crucial impact on global ecosystems, hydrological cycles, climate change research, and human activities such as weather forecasting and water resource management. Porosity in deep rocks is an important parameter in rock mechanics and engineering. The porosity of deep rock strata under in-situ stress conditions has theoretical and engineering significance for assessing the safety of CO2 geological storage, evaluating oil and gas reserves, and developing coalbed methane in goaf areas. Therefore, studying the porosity of deep rock strata under in-situ stress conditions is one of the important tasks in the field of rock mechanics.
[0003] Currently, methods for analyzing porosity in deep formations include in-situ core sampling, laboratory photoelectric radiation techniques (such as nano-CT technology for characterizing porosity in cement slurry), nuclear magnetic resonance monitoring, electron microscopy, and fluid flow methods (such as the helium method GB / T 34533), high-pressure mercury intrusion porosimetry, and warm gas adsorption methods. While these methods offer advantages in terms of accuracy and intuitiveness, they heavily rely on in-situ rock samples. Deep core drilling is not only costly, but the stress state of the sample in the deep environment is difficult to recreate during drilling, transportation, and testing. This leads to discrepancies between the measured porosity and the in-situ rock condition, thus affecting the accuracy of porosity measurements. Summary of the Invention
[0004] To address the problems existing in the above-mentioned fields, the method proposed in this invention detects the volumetric deformation of the pores in the rock core sample by performing loading and unloading experiments on the rock core sample. This transforms the porosity of the target rock layer deep in the test area into the porosity of a rock layer that is part of the same sedimentary strata as the target rock layer but shallower in burial depth. This method enables the acquisition of porosity under in-situ formation stress conditions without the need for in-situ sampling.
[0005] To address the aforementioned technical problems, this invention discloses a method for predicting rock porosity, comprising the following steps: Obtain a rock stratum that is in the same sedimentary stratum as the target rock stratum but is shallower than the target rock stratum. Take a sample from this shallow rock stratum to obtain a core sample and test the pore volume within the core sample. Based on the geological structure of the area to be tested, the burial depth of the target rock layer is determined; the volumetric deformation of the pores in the rock core sample is detected by loading and unloading experiments; when the burial depth of the target rock layer is greater than or equal to the preset burial depth threshold, the volumetric deformation of the pores when the triaxial stress is equal is taken as the volumetric deformation of the target rock layer; when the burial depth of the target rock layer is less than the preset burial depth threshold, the volumetric deformation of the pores when the triaxial stress is unequal is taken as the volumetric deformation of the target rock layer. Determine the rate of change of volume of the target rock stratum based on its volumetric deformation. Based on the pore volume and the volume change rate of the target rock layer, and using fractal geometry theory, the volume of the pore portion of the target rock layer after deformation under stress is determined. Obtain the volume of the matrix portion of the core sample under stress; determine the porosity of the target rock layer under stress based on the volume of the pore portion of the target rock layer after deformation under stress and the volume of the matrix portion of the core sample.
[0006] Preferably, the step of determining the burial depth of the target rock strata based on the geological structure of the area to be measured specifically includes: By obtaining geological survey reports, mineral exploration reports, and hydrogeological reports of the area to be measured, the geological structure of the area to be measured is determined; based on the geological structure, the burial depth range of the area to be measured is determined, and the burial depth of the target rock strata is measured.
[0007] Preferably, when the burial depth of the target rock stratum is greater than or equal to a preset burial depth threshold, the volumetric deformation of the pores when the triaxial stress is equal is used as the volumetric deformation of the target rock stratum, specifically including: The elastic modulus and Poisson's ratio of the matrix portion of the rock core sample were obtained by conducting an equal load cyclic loading and unloading test on the rock core sample. When the burial depth of the target rock layer is greater than or equal to a preset burial depth threshold, based on the thin-walled theory of hollow spheres, equal triaxial stress is applied to the pores of the rock core sample. The volumetric deformation of the hollow sphere when the triaxial stress is equal is taken as the volumetric deformation Δ of the target rock layer under stress. V 1: ; In the formula: r Let be the radius of the hollow sphere; t The thickness of the hollow sphere; E This represents the elastic modulus of the matrix portion of the rock core sample. v Δ is the Poisson's ratio of the matrix portion of the core sample; p This indicates the magnitude of the external pressure exerted on a hollow sphere under stress.
[0008] Preferably, when the burial depth of the target rock layer is less than a preset burial depth threshold, the volumetric deformation of the pores when the triaxial stresses are unequal is used as the volumetric deformation of the target rock layer, specifically including: The mean values of the triaxial stresses of the hollow sphere under its original state and under stress were obtained when the triaxial stresses were unequal. and ; The change in average stress caused by external pressure on a hollow sphere is obtained as follows: ; When the burial depth of the target rock stratum is less than the preset burial depth threshold, t The magnitude Δ of the external pressure exerted on the hollow sphere under stress at any given time p Equivalent to Δ s m ; The volumetric deformation of a hollow sphere under unequal triaxial stresses is taken as the volumetric deformation Δ of the target rock layer under stress. V 2: .
[0009] Preferably, determining the volume change rate of the target rock layer based on its volume deformation specifically includes: Based on the volumetric deformation of the target rock strata under stress and the volume of the hollow sphere V The ratio of the two values yields the volume change rate of the target rock layer: ; ; in, i The value can be 1 or 2; when i When Δ = 1, it indicates that when the burial depth of the target rock layer is greater than or equal to the preset burial depth threshold, the volumetric deformation Δ of the target rock layer under stress is... V 1; when i When Δ = 2, it indicates that when the burial depth of the target rock layer is less than the preset burial depth threshold, the volumetric deformation Δ of the target rock layer under stress is... V 2.
[0010] Preferably, the process of obtaining the pore volume within the core sample specifically includes: The relationship between the number of pores and the pore radius in a rock core sample is obtained as follows: ; In the formula: N ( L ≥ R ( ) represents the pore size of the core sample. L Greater than or equal to pore radius R The cumulative number;R max λ represents the maximum value of the pore radius, and λ is the volume coefficient. The volume factor λ is: ; In the formula: V m To obtain the core sample volume through equal load cyclic loading and unloading experiments, The volume of the core sample obtained through mercury intrusion porosimetry or gas isothermal adsorption experiments; Based on the relationship between the number of pores and the pore radius of a rock core sample, the formula for calculating the total number of pores inside the rock core sample is as follows: ; In the formula: N t ( L ≥ R min ( ) represents the pore size of the core sample. L Greater than or equal to R min The cumulative number, that is, the total number of pores in the core sample; R min This represents the minimum value of the pore radius; Differentiating the formula for calculating the total quantity, we obtain the pore radius in [...]. R , R + dR The number of pores within the range is: ; In the formula: - dN For pore radius in [ R , R + dR The number of pores within the range, D p It is the fractal dimension; Dividing the number of pores by the total number of pores inside the core sample yields the probability density function of the pore radius distribution in the core sample: ; when R max , R min When the following equation is satisfied, the probability density function of the pore radius distribution satisfies the normalization principle: ; When the pore radius is distributed in [ R , R + dR When the probability density function within the range satisfies the normalization principle, we get: ; Then aperture L Less than or equal to R The cumulative pore volume is: ; In the formula: c 1 is a constant related to the pore shape; when the pore is spherical, c 1=4 π / 3; Based on the cumulative pore volume, the pore volume within the core sample is obtained as follows: V p0 for: .
[0011] Preferably, determining the volume of the target rock layer after deformation of the pore portion under stress, based on fractal geometry theory and the pore volume and the volume change rate of the target rock layer, specifically includes: Based on the pore volume within the core sample V p0 To obtain the volume of a specific pore within a rock core sample. V b Integrating the rate of change of volume of the target rock layer, we obtain: ; Assuming that the wall thickness of pores with different diameters inside the core sample is proportional to the radius of the hollow sphere, that is: ; In the formula: k It is a constant; Based on the integration result, we obtain V b and radius r The relation is: ; according to V b and radius r The relationship between the number of pores and the distribution law of pore radius, in [ R , R + dR Within the specified range, based on fractal geometry theory, the porosity calculation model is constructed as follows: ; Based on the porosity calculation model, the following is obtained under stress: R , R + dR The total volume of rock mass pores within the range after overall deformation V p aboutR The function is: ; Based on total volume V p about R The function is used to obtain the volume of the pore portion of the core sample after deformation under stress. V p1 for: .
[0012] Preferably, obtaining the volume of the matrix portion of the core sample under stress specifically includes: The volume of the core sample obtained through the constant load cyclic loading and unloading experiment is V m : ; In the formula: V g0 This indicates the volume of the matrix portion of the rock core sample; Based on the volume of the matrix portion of the core sample and using the generalized Hooke's law, the volume of the matrix portion of the core sample under stress is obtained as follows: in, s m This represents the average of the triaxial stresses in different directions of the hollow sphere.
[0013] Preferably, the porosity of the target rock layer under stress is determined based on the volume of the pore portion of the target rock layer after deformation under stress and the volume of the matrix portion of the core sample: .
[0014] Compared with the prior art, the present invention has the following beneficial effects: The rock porosity prediction method proposed in this invention obtains core samples from rock strata located within the same sedimentary stratum as the target rock stratum but at a shallower depth. The same sedimentary stratum implies similarities in sedimentary environment and material origin. Shallower strata are more similar to the target rock stratum in initial characteristics such as mineral composition and grain structure. This sampling method provides a reasonable basis for subsequent inference of the target rock stratum's properties based on shallow rock samples, effectively enhancing the reliability of the prediction results and avoiding prediction bias caused by excessive sample differences. The burial depth of the target rock stratum is determined based on the geological structure of the area to be tested, and this is used as a standard to classify different stress conditions. The product deformation of the target rock stratum is determined separately when the burial depth is in a state of equal and unequal triaxial stress. This detailed differentiation and accurate simulation of stress states at different burial depths fully considers the complexity of stress on rocks in actual geological environments, enabling the prediction method to more realistically reflect the mechanical response of the target rock stratum at different depths, thereby improving the accuracy of porosity prediction. The volume change rate directly reflects the degree of volume change of a target rock stratum under stress. Accurate calculation of this parameter helps to accurately grasp the deformation of the pore portion under stress, laying the foundation for constructing a reasonable porosity calculation model. By constructing a porosity calculation model, the volume of the pore portion of the target rock stratum after deformation under stress is determined. This result provides important data for the final porosity calculation, more realistically reflecting the state of pores under actual geological conditions, avoiding errors caused by simplified assumptions or empirical formulas, and improving the accuracy of the prediction results. Attached Figure Description
[0015] Figure 1 This is a flowchart of the rock porosity prediction method proposed in this invention; Figure 2 This is a schematic diagram of rock sampling provided in an embodiment of the present invention; Figure 3 The rock full stress-strain curve provided for the embodiments of the present invention; Figure 4 This is an example of an equal load cyclic loading and unloading experiment provided in an embodiment of the present invention. Detailed Implementation
[0016] The following will refer to the appendices in the embodiments of the present invention. Figure 1-Figure 4 The technical solutions in the embodiments of the present invention will be clearly and completely described. It should be understood that the terminology used in the present invention is only for describing particular implementation methods and is not intended to limit the present invention.
[0017] Example like Figure 1 The figure shows a method for predicting rock porosity proposed in this invention, which includes the following steps: S1: Obtain a rock layer that is in the same sedimentary stratum as the target rock layer but is shallower than the target rock layer. Take a sample from this shallow rock layer to obtain a core sample and test the pore volume within the core sample. S2: Determine the burial depth of the target rock layer based on the geological structure of the area to be tested; detect the volumetric deformation of the pores in the rock core sample by loading and unloading experiments; when the burial depth of the target rock layer is greater than or equal to the preset burial depth threshold, the volumetric deformation of the pores when the triaxial stress is equal is taken as the volumetric deformation of the target rock layer; when the burial depth of the target rock layer is less than the preset burial depth threshold, the volumetric deformation of the pores when the triaxial stress is unequal is taken as the volumetric deformation of the target rock layer. S3: Determine the rate of change of volume of the target rock layer based on the volume deformation of the target rock layer; S4: Based on the pore volume and the volume change rate of the target rock layer, and using fractal geometry theory, determine the volume of the pore portion of the target rock layer after deformation under stress. S5: Obtain the volume of the matrix portion of the core sample under stress; determine the porosity of the target rock layer under stress based on the volume of the pore portion of the target rock layer after deformation under stress and the volume of the matrix portion of the core sample.
[0018] Specifically, in step S1, the geological survey report, mineral exploration report and hydrogeological report of the area to be measured are obtained, and the geological structure of the area to be measured is determined; the burial depth range of the area to be measured is determined based on the geological structure, and the burial depth of the target rock layer is measured.
[0019] Obtain the target rock layer a deep within the area A to be measured for porosity measurement.
[0020] Based on the geological structure of area A to be tested, rock layer b in area B is identified as belonging to the same sedimentary stratum as target rock layer a, but at a shallower depth than target rock layer a. Specifically, as follows... Figure 2 As shown, rock core samples were obtained by drilling into rock layer b.
[0021] Based on fractal geometry theory, the relationship between the number of pores and the pore radius of a rock core sample is obtained: ; In the formula: N ( L ≥ R ( ) represents the pore size of the core sample. L Greater than or equal to pore radius R The cumulative number; R max λ represents the maximum value of the pore radius, and λ is the volume coefficient. The volume factor λ is expressed as: ; In the formula: V m To obtain the core sample volume through equal load cyclic loading and unloading experiments, The volume of the core sample obtained through mercury intrusion porosimetry or gas isothermal adsorption experiments; Based on the relationship between the number of pores in the rock core and the pore radius, the formula for calculating the total number of pores inside the rock core sample is as follows: ; In the formula: N t ( L ≥ R min ( ) represents the pore size of the core sample. L Greater than or equal to R min The cumulative number, that is, the total number of pores in the core sample; R min This represents the minimum value of the pore radius; Differentiating the formula for calculating the total quantity, we obtain the pore radius in [...]. R , R + dR The number of pores within the range is: ; In the formula: - dN For pore radius in [ R , R + dR The number of pores within the range, D p It is the fractal dimension; Dividing the number of pores by the total number of pores inside the core sample yields the probability density function of the pore radius distribution in the core sample: ; Therefore, only when R max , R min For the following equation to be satisfied, the probability density function of the pore radius distribution satisfies the normalization principle: ; When the pore radius is distributed in [ R , R + dR When the probability density function within the range satisfies the normalization principle, we get: ; Based on the test results of existing rock pore radius, it can be seen that the distribution of pore radius inside the rock has a very large range, satisfying Rmin / Rmax≪1, and the probability density function of pore radius distribution holds.
[0022] Therefore, the probability density function of pore radius distribution can also serve as an important basis for judging whether the pores inside a rock satisfy fractal theory.
[0023] Then aperture L Less than or equal to R The cumulative pore volume is: ; In the formula: c1 is a constant related to the pore shape. When the pore is spherical, c1 = 4. π / 3; Based on the cumulative pore volume, the pore volume within the core sample is obtained as follows: V p0 for: ; in, R max , R min ,and D p It can be obtained through mercury porosimetry or gas isothermal adsorption experiments.
[0024] Uniaxial compression tests were performed on the obtained rock core samples to obtain the full stress-strain curves of the rock core samples, such as... Figure 3 As shown in the figure, point A is the transition point from the compaction stage to the linear elastoplastic deformation stage of the rock, point B is the yield point of the rock. After this point, microcracks gradually form and expand inside the rock, and the deformation gradually enters the nonlinear elastoplastic stage. Point C is the peak point of the rock's full stress-strain curve, which corresponds to the uniaxial compressive strength of the rock.
[0025] according to Figure 3The full stress-strain curves of the core samples show that before failure, the deformation characteristics of the core samples are not those of a perfectly elastic body. This is mainly because the core samples are composed of pores and matrix, and the deformation of the pore and matrix parts is not coordinated. This phenomenon is common in different lithologies, and it is more pronounced in rocks with higher porosity, such as sandstone. Therefore, based on the deformation of the pore and matrix parts, the full stress-strain curves of the core samples can be divided into four stages: Stage OA is the compaction stage of the core samples, in which the pore and matrix parts of the core samples deform, and the main pores inside the core samples gradually close; Stage AB is the elastic deformation stage of the core samples, in which the deformation of the pore parts of the core samples ends, and only the matrix part undergoes elastic deformation, and this stage is called the elastic stage; Stage BC is the stable development stage of the fractures in the core samples, in which small fractures gradually develop inside, reaching a peak at point C; after point C is the post-peak stage, in which the internal fractures of the core samples expand, develop, and connect, and the bearing capacity decreases.
[0026] This invention obtains core samples from a shallower rock stratum within the same sedimentary stratum as the target stratum. Instead of directly sampling the deep target stratum, it reduces reliance on deep in-situ sampling, overcoming its challenges. This process enables low-cost and rapid sampling, eliminating the need for in-situ rock sample collection and reducing investment in large equipment, thus achieving economic efficiency. It also simplifies complex processes and enhances overall efficiency.
[0027] To better describe the deformation of the core sample, in step S2, when the burial depth of the target rock layer is greater than or equal to a preset burial depth threshold, the volumetric deformation of the pores when the triaxial stress is equal is used as the volumetric deformation of the target rock layer; when the burial depth of the target rock layer is less than the preset burial depth threshold, the volumetric deformation of the pores when the triaxial stress is unequal is used as the volumetric deformation of the target rock layer, specifically including: The elastic modulus and Poisson's ratio of the matrix portion of the rock core sample were obtained by conducting an equal load cyclic loading and unloading test on the rock core sample.
[0028] Generally, rock structure includes pores and rock matrix. In order to construct a porosity calculation model, this invention considers these two parts separately, making the following assumptions: (1) The rock has only pores and no cracks inside.
[0029] (2) The pores inside the rock are spherical.
[0030] (3) Pore structures with different diameters produce the same degree of deformation under a certain stress.
[0031] (4) The pore size distribution inside the rock conforms to the fractal geometry theory.
[0032] (5) When the core sample is not under pressure, the pores are in an uncompressed state.
[0033] (6) The rock matrix satisfies Hooke's Law in the elastic stage.
[0034] Due to the incoordination between the deformation of the pore portion and the matrix portion in the rock, the full stress-strain curve of the core sample cannot characterize the elastic modulus of the rock matrix portion.
[0035] To obtain the elastic modulus of the matrix portion of the rock core sample, an equal-load cyclic loading and unloading test was performed on the rock core sample, such as... Figure 4 As shown, the horizontal axis e For constant load strain, the vertical axis σ1 represents constant load stress. By conducting multiple repeated loading and unloading experiments, and with the maximum load applied each time being the same as the maximum load applied the first time, each loading and unloading curve forms a plastic hysteresis loop. Here, HH' is the hysteresis loop in which the core sample does not undergo plastic deformation in the loading-unloading cycle during the core sample loading and unloading experiment, and H and H' are the starting point and ending point of its stress path, respectively. The plastic hysteresis loops become narrower and closer to each other with the increase of loading and unloading cycles, indicating that the core sample is getting closer to elastic deformation. Until a certain plastic hysteresis loop shows no plastic deformation and only the stress-strain relationship of the elastic deformation stage, the constant load cyclic loading and unloading test is stopped. The elastic modulus of the rock in the elastic deformation stage (HH' segment) at this time is taken as the elastic modulus of the matrix part of the rock core sample, and the Poisson's ratio of the rock in the elastic deformation stage (HH' segment) at this time is taken as the Poisson's ratio of the matrix part of the rock core sample.
[0036] According to the thin-wall theory of hollow spheres, the circumferential stress of a hollow sphere is: ; In the formula: σ θ and For the circumferential stress of a hollow sphere, p The external pressure on the hollow sphere, t The wall thickness of the hollow sphere. r Let be the radius of the hollow sphere; Based on the elastic modulus, Poisson's ratio, and circumferential stress of the hollow sphere in the matrix of the core sample, the circumferential strain of the hollow sphere is obtained according to the generalized Hooke's law. e θ for: ; In the formula: s r The radial stress of the hollow sphere;E ν represents the elastic modulus of the matrix portion of the core sample; ν represents the Poisson's ratio of the matrix portion of the core sample. When the hollow sphere has a thin wall, radial stress s r Set to 0 to obtain radial displacement. u With circumferential strain e θ Relationship: ; By applying equal triaxial stress to the pores of the core sample, the deformation Δ of the volume of the hollow sphere under equal triaxial stress is determined. V 1 is the integral of the surface area of the hollow sphere over the radial displacement: ; In the formula: Δ p This indicates the magnitude of the external pressure exerted on a hollow sphere under stress.
[0037] Specifically, Δ p The value is the difference between the external pressure under stress and the external pressure when no stress is applied, when the triaxial stress is equal.
[0038] When the triaxial stresses are not equal, the mean value of the triaxial stresses in different directions of the hollow sphere is obtained as follows: ; In the formula: s m This represents the average of the triaxial stresses in different directions of the hollow sphere. s i for i Stress in the direction; s j for j Stress in the direction; s k for k Stress in the direction; The mean values of the triaxial stresses of the hollow sphere under its original state and under stress were obtained when the triaxial stresses were unequal. and ; The change in average stress caused by external pressure on a hollow sphere is obtained as follows: ; When the magnitude of the external pressure Δ p Equivalent to the change in mean stress Δ s m , that is, Δ p =Δ s m When the triaxial stresses are unequal, the volume deformation Δ of the hollow sphere is obtained. V2 is: ; In the formula: Δ s m It represents the change in average stress caused by the influence of external pressure.
[0039] In step S3, the volume change rate of the target rock layer is determined based on its volume deformation, specifically including: The volume of the hollow sphere is obtained as follows: ; Based on the volumetric deformation of the target rock strata and the volume of the hollow sphere V The ratio of the two values yields the volume change rate of the target rock layer: ; when i When = 1, it indicates that when the burial depth of the target rock layer is greater than or equal to the preset burial depth threshold, the volumetric deformation of the target rock layer under stress is the volumetric deformation Δ of a hollow sphere when the triaxial stress is equal. V 1; when i When = 2, it indicates that when the burial depth of the target rock layer is less than the preset burial depth threshold, the volumetric deformation of the target rock layer under stress is the volumetric deformation Δ of a hollow sphere when the triaxial stresses are unequal. V 2.
[0040] In step S4, based on the pore volume of the core sample obtained in step S1 and the volume change rate of the target rock layer obtained in step S3, and based on fractal geometry theory, the volume of the target rock layer after deformation of the pore portion under stress is determined, specifically including: Based on the pore volume within the core sample, obtain the volume V of a specific pore within the core sample. b Integrating the rate of change of volume of the target rock layer, we obtain: ; Assuming that the wall thickness of pores with different diameters inside the core sample is proportional to the radius of the hollow sphere, that is: ; In the formula: k It is a constant; Based on the integration result, we obtain V b and radius r The relation is: ; according to V b and radius r The relationship between the number of pores and the distribution law of pore radius, in [ R ,R + dR Within the specified range, based on fractal geometry theory, the porosity calculation model is constructed as follows: ; Based on the porosity calculation model, the following is obtained under stress: R , R + dR The total volume of rock mass pores within the range after overall deformation V p about R The function is: ; Based on total volume V p about R The function is used to obtain the volume of the pore portion of the core sample after deformation under stress. V p1 for: .
[0041] In step S5, the volume of the matrix portion of the core sample under stress is obtained, specifically including: The volume of the core sample obtained through the constant load cyclic loading and unloading experiment is V m : ; In the formula: V g0 This indicates the volume of the matrix portion of the rock core sample; When the matrix portion of the core sample satisfies the generalized Hooke's law, the strains obtained in different directions are as follows: ; ; ; Based on the strain in different directions, the volume of the matrix portion of the core sample under stress is obtained as follows: .
[0042] Based on the volume of the pore portion of the target rock layer after deformation under stress and the volume of the matrix portion of the core sample, the porosity of the target rock layer under stress is determined as follows: .
[0043] This invention obtains core samples from rock layer b, which is located in the same sedimentary stratum as the target rock layer a but is shallower. The same sedimentary stratum implies similarities in sedimentary environment and material origin. Shallow rock layers are more similar to the target rock layer in initial characteristics such as mineral composition and grain structure. This sampling method provides a reasonable basis for subsequent inferences about the properties of the target rock layer based on shallow rock samples, effectively enhancing the reliability of the prediction results and avoiding prediction bias caused by excessive sample differences.
[0044] This invention determines the burial depth of the target rock strata based on the geological structure of the area to be tested, and uses this as a standard to classify different stress conditions. When the burial depth of the target rock strata is greater than or equal to a preset threshold, it is considered to be in a state of equal triaxial stress, and the volumetric deformation is calculated according to the volumetric deformation of a hollow sphere in this state; when the burial depth is less than the threshold, it is calculated according to the case of unequal triaxial stress. This detailed distinction and accurate simulation of stress states at different burial depths fully considers the complexity of the stress on rocks in the actual geological environment, enabling the prediction method to more realistically reflect the mechanical response of the target rock strata at different depths, thereby improving the accuracy of porosity prediction.
[0045] Based on the determined volumetric deformation, the volume change rate of the target rock layer is further calculated. This step provides a key parameter for subsequent porosity calculation. The volume change rate can intuitively reflect the degree of volume change of the target rock layer under stress. Accurate calculation of this parameter helps to accurately grasp the deformation of the pore portion under stress, laying the foundation for constructing a reasonable porosity calculation model.
[0046] The method proposed in this invention focuses more on the direct analysis and calculation of actual samples, reducing reliance on complex models and constraints. This allows it to more directly reflect the true condition of the target rock strata and improves the certainty of porosity prediction. Due to the complexity and variability of underground geological conditions, establishing an accurate inversion model that reflects the actual situation is not easy, and appropriate constraints are needed to ensure the reliability of the inversion results. Furthermore, the information obtained through various geophysical techniques includes comprehensive data such as the radioactivity of underground rock strata, seismic source signals, and acoustic characteristics. This information is not specifically targeted at the porosity parameter. Extracting porosity-related content from a large amount of complex information for inversion analysis is challenging and may be subject to information interference.
[0047] The method proposed in this invention focuses more on the direct analysis and calculation of actual samples, reducing the reliance on complex models and constraints, and can more directly reflect the true situation of the target rock strata, thus improving the certainty of porosity prediction.
[0048] Compared with existing technologies, this invention directly obtains core samples from shallow rock strata related to the target rock layer. It measures and calculates parameters closely related to porosity, such as pore size and volume of the core samples. This more targeted data acquisition enables more accurate porosity prediction and reduces unnecessary data processing and analysis steps. The method proposed in this invention overcomes the technical limitations of geophysical inversion methods, which primarily rely on inversion models and constraints.
[0049] This invention utilizes the pore volume of a stress-free rock core sample and the rate of change of the target rock layer volume to construct a porosity calculation model based on fractal geometry theory. Fractal geometry theory can effectively describe the complexity and self-similarity of rock pore structures. The model constructed using this theory can more accurately characterize the deformation of pores under stress, considering the influence of the microscopic features of the pore structure on porosity, making the model more scientific and reasonable, and applicable to porosity prediction of rocks of different types and structures.
[0050] This invention, based on a constructed porosity calculation model, determines the volume of the pore portion of a target rock stratum after deformation under stress. This result provides crucial data for the final porosity calculation. The pore deformation volume calculated through a scientifically sound model more accurately reflects the state of pores under actual geological conditions, avoiding errors caused by simplified assumptions or empirical formulas, and improving the accuracy of the prediction results.
[0051] The present invention also includes obtaining constants through fitting methods. k Repeat steps S2 to S5 to obtain the average value of the elastic modulus and Poisson's ratio of the matrix portion of the obtained core sample.
[0052] Elastic modulus and Poisson's ratio are fundamental parameters for calculating the deformation and porosity changes of rocks under stress. More accurate values for elastic modulus and Poisson's ratio allow for more precise simulations of the mechanical behavior and porosity evolution of rocks by constructing pore volume as a function of radius based on theories such as hollow sphere thin-walled structures and fractal geometry. This results in more reliable results when calculating the pore volume function of radius, determining the volume of deformed pore portions, and ultimately calculating the porosity of target rock layers, thus improving the accuracy of porosity prediction.
[0053] In practical rock mechanics and porosity analysis, the physical properties and mechanical behavior of rocks are often influenced by a variety of complex factors, making them difficult to accurately describe using simple theoretical models. Constants are obtained through fitting techniques. k The values can be adjusted and optimized based on actual measurement data, making the acquired key parameters better reflect the actual situation. This helps to more accurately characterize the deformation and porosity changes of rocks under different stress states, providing a more reliable basis for subsequent porosity calculations.
[0054] The method proposed in this invention integrates multiple factors such as the pore volume in the core sample, the elastic modulus and Poisson's ratio of the matrix, stress state, and fractal geometry theory for calculation. It comprehensively considers various factors affecting porosity, making the final porosity result closer to the actual situation and improving the accuracy of the test results.
[0055] Compared to traditional methods, the rock porosity prediction method proposed in this invention achieves lower cost and faster sampling due to the shorter sampling location. It eliminates the need for in-situ rock sample collection, reducing investment in large equipment and thus offering economic efficiency. It also simplifies complex processes and enhances efficiency. This invention considers the influence of in-situ stress, improving the reliability and accuracy of the obtained rock porosity data. By converting the porosity of target rock layer a at a deep depth in study area A into the porosity of rock layer b at a shallow depth or even the surface in area B, this invention avoids on-site sampling of the target rock layer. Taking CO2 geological sequestration as an example, it minimizes disturbance to the target strata, ensuring the safety of subsequent engineering projects.
[0056] In summary, the method proposed in this invention determines the porosity of the target rock layer under stress by obtaining the volume of the matrix portion of the rock core sample under stress and combining it with the volume of the pore portion of the target rock layer after deformation. This method comprehensively considers the changes of both the rock pores and the matrix under stress, fully reflecting the influence of the overall structural characteristics of the rock on the porosity, making the prediction results more accurate and comprehensive, and providing reliable rock porosity data support for geological exploration, resource development, engineering construction and other fields.
[0057] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.
[0058] Furthermore, unless otherwise stated, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. All references to this specification are incorporated by way of citation to disclose and describe methods relating to those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.
Claims
1. A method for predicting rock porosity, characterized in that, Includes the following steps: Obtain a rock stratum that is in the same sedimentary stratum as the target rock stratum but is shallower than the target rock stratum. Take a sample from this shallow rock stratum to obtain a core sample and test the pore volume within the core sample. Based on the geological structure of the area to be tested, the burial depth of the target rock layer is determined; the volumetric deformation of the pores in the rock core sample is detected by loading and unloading experiments; when the burial depth of the target rock layer is greater than or equal to the preset burial depth threshold, the volumetric deformation of the pores when the triaxial stress is equal is taken as the volumetric deformation of the target rock layer. When the burial depth of the target rock layer is less than the preset burial depth threshold, the volumetric deformation of the pores when the triaxial stress is unequal is taken as the volumetric deformation of the target rock layer. Determine the rate of change of volume of the target rock stratum based on its volumetric deformation. Based on the pore volume and the volume change rate of the target rock layer, and using fractal geometry theory, the volume of the pore portion of the target rock layer after deformation under stress is determined. Obtain the volume of the matrix portion of the core sample under stress; The porosity of the target rock layer under stress is determined based on the volume of the pore portion of the target rock layer after deformation under stress and the volume of the matrix portion of the core sample.
2. The rock porosity prediction method according to claim 1, characterized in that, The determination of the burial depth of the target rock strata based on the geological structure of the area to be measured specifically includes: By obtaining geological survey reports, mineral exploration reports, and hydrogeological reports of the area to be measured, the geological structure of the area to be measured is determined; based on the geological structure, the burial depth range of the area to be measured is determined, and the burial depth of the target rock strata is measured.
3. The rock porosity prediction method according to claim 2, characterized in that, When the burial depth of the target rock stratum is greater than or equal to a preset burial depth threshold, the volumetric deformation of the pores when the triaxial stress is equal is used as the volumetric deformation of the target rock stratum, specifically including: The elastic modulus and Poisson's ratio of the matrix portion of the rock core sample were obtained by conducting an equal load cyclic loading and unloading test on the rock core sample. When the burial depth of the target rock layer is greater than or equal to a preset burial depth threshold, based on the thin-walled theory of hollow spheres, equal triaxial stress is applied to the pores of the rock core sample. The volumetric deformation of the hollow sphere when the triaxial stress is equal is taken as the volumetric deformation Δ of the target rock layer under stress. V 1: ； In the formula: r Let be the radius of the hollow sphere; t The thickness of the hollow sphere; E This represents the elastic modulus of the matrix portion of the rock core sample. v Δ is the Poisson's ratio of the matrix portion of the core sample; p This indicates the magnitude of the external pressure exerted on a hollow sphere under stress.
4. The rock porosity prediction method according to claim 3, characterized in that, When the burial depth of the target rock stratum is less than a preset burial depth threshold, the volumetric deformation of the pores when the triaxial stresses are unequal is used as the volumetric deformation of the target rock stratum, specifically including: The mean values of the triaxial stresses of the hollow sphere under its original state and under stress were obtained when the triaxial stresses were unequal. and ; The change in average stress caused by external pressure on a hollow sphere is obtained as follows: ; When the burial depth of the target rock stratum is less than the preset burial depth threshold, t The magnitude Δ of the external pressure exerted on the hollow sphere under stress at any given time p Equivalent to Δ σ m ; The volumetric deformation of a hollow sphere under unequal triaxial stresses is taken as the volumetric deformation Δ of the target rock layer under stress. V 2: 。 5. The rock porosity prediction method according to claim 4, characterized in that, The determination of the volume change rate of the target rock stratum based on its volume deformation specifically includes: Based on the volumetric deformation of the target rock strata under stress and the volume of the hollow sphere V The ratio of the two values yields the volume change rate of the target rock layer: ； ； in, i The value can be 1 or 2; when i When Δ = 1, it indicates that when the burial depth of the target rock layer is greater than or equal to the preset burial depth threshold, the volumetric deformation Δ of the target rock layer under stress is... V 1; when i When Δ = 2, it indicates that when the burial depth of the target rock layer is less than the preset burial depth threshold, the volumetric deformation Δ of the target rock layer under stress is... V 2.
6. The rock porosity prediction method according to claim 1, characterized in that, The process of obtaining the pore volume within the core sample specifically includes: The relationship between the number of pores and the pore radius in a rock core sample is obtained as follows: ； In the formula: N ( L ≥ R ( ) represents the pore size of the core sample. L Greater than or equal to pore radius R The cumulative number; R max λ represents the maximum value of the pore radius, and λ is the volume coefficient. The volume factor λ is: ； In the formula: V m To obtain the core sample volume through equal load cyclic loading and unloading experiments, The volume of the core sample obtained through mercury intrusion porosimetry or gas isothermal adsorption experiments; Based on the relationship between the number of pores and the pore radius of a rock core sample, the formula for calculating the total number of pores inside the rock core sample is as follows: ； In the formula: N t ( L ≥ R min ( ) represents the pore size of the core sample. L Greater than or equal to R min The cumulative number, that is, the total number of pores in the core sample; R min This represents the minimum value of the pore radius; Differentiating the formula for calculating the total quantity, we obtain the pore radius in [...]. R , R + dR The number of pores within the range is: ； In the formula: - dN For the pore radius in [ R , R + dR The number of pores within the range, D p It is the fractal dimension; Dividing the number of pores by the total number of pores inside the core sample yields the probability density function of the pore radius distribution in the core sample: ； when R max , R min When the following equation is satisfied, the probability density function of the pore radius distribution satisfies the normalization principle: ； When the pore radius is distributed in [ R , R + dR When the probability density function within the range satisfies the normalization principle, we get: ； Then aperture L Less than or equal to R The cumulative pore volume is: ； In the formula: c 1 is a constant related to the pore shape; The pore volume within the core sample is obtained based on the cumulative pore volume. V p0 for: 。 7. The rock porosity prediction method according to claim 1, characterized in that, The determination of the volume of the target rock layer after deformation of the pore portion under stress, based on the pore volume and the volume change rate of the target rock layer and fractal geometry theory, specifically includes: Based on the pore volume within the core sample V p0 To obtain the volume of a specific pore within a rock core sample. V b Integrating the rate of change of volume of the target rock layer, we obtain: ； Assuming that the wall thickness of pores with different diameters inside the core sample is proportional to the radius of the hollow sphere, that is: ； In the formula: k It is a constant; Based on the integration result, we obtain V b and radius r The relation is: ； according to V b and radius r The relationship between the number of pores and the distribution law of pore radius, in [ R , R + dR Within the specified range, based on fractal geometry theory, the porosity calculation model is constructed as follows: ； Based on the porosity calculation model, the following is obtained under stress: R , R + dR The total volume of rock mass pores within the range after overall deformation V p about R The function is: ； Based on total volume V p about R The function is used to obtain the volume of the pore portion of the core sample after deformation under stress. V p1 for: 。 8. The rock porosity prediction method according to claim 7, characterized in that, The acquisition of the volume of the matrix portion of the core sample under stress specifically includes: The volume of the core sample obtained through the constant load cyclic loading and unloading experiment is: V m : ； In the formula: V g0 This indicates the volume of the matrix portion of the rock core sample; Based on the volume of the matrix portion of the core sample and using the generalized Hooke's law, the volume of the matrix portion of the core sample under stress is obtained as follows: in, σ m This represents the average of the triaxial stresses in different directions of the hollow sphere.
9. The rock porosity prediction method according to claim 8, characterized in that, The porosity of the target rock layer under stress is determined based on the volume of the pore portion after deformation under stress and the volume of the matrix portion of the core sample. 。