Method for predicting current crustal stress of coal-bearing seam system

Abstract

The invention provides a method for predicting the current ground stress of a coal seam-containing system, and is suitable for the field of coal seam (system) gas geology. The method comprises the following steps: firstly, collecting drilling cores with different depths and different lithologies, conventional logging, imaging logging, injection / pressure drop test data and reservoir pressure data, acquiring mechanical parameters of rocks with different lithologies and current crustal stress by utilizing experiments and logging, and quantitatively representing natural fracture parameters; sequentially constructing a dynamic rock mechanical profile S0, a static rock mechanical profile S1 and a rock mechanical profile S2 under a crack-containing condition; a combined spring model is used for reversely deducing the horizontal maximum strain coefficient and the horizontal minimum strain coefficient, and mathematical relations between the horizontal maximum strain coefficient and the horizontal minimum strain coefficient and the buried depth and the static elastic modulus are established on the basis; and finally, a current crustal stress calculation model is constructed, and quantitative prediction of the current crustal stress of the coal seam-containing system is realized. The method is comprehensive in consideration factors, rigorous in thought technology, high in operability and high in prediction result credibility.

Description

Technical Field

[0001] The invention relates to the field of coal bed (system) gas geology, and in particular to a method for predicting the current ground stress of a coal bed system. Background Art

[0002] Coal-bearing strata include coal seams, tight sandstone layers, mudstone layers, etc. With the deepening of exploration and development and the development of technology, the "three gases" of coal measures (coalbed methane, shale gas and tight sandstone gas) have shown great potential.

[0003] There are corresponding current geostress testing and prediction methods for coal seams, tight sandstone layers and shale layers. The Chinese patent with announcement number CN109113742B proposes a method for predicting the current geostress of coal reservoirs. On the basis of experimental testing of coal rock parameters and calculation of well logging curves, the unknown parameters in the current geostress model of coal reservoirs based on Maxwell are obtained by inversion, and the geostress profile of coal reservoirs is established; the Chinese patent with announcement number CN117647342B proposes a method for measuring geostress of coal reservoirs based on acoustic emission wavelet analysis. A spatial rectangular coordinate system is established based on the wave velocity measurement of full-diameter drilling cores, and the full stress-strain curve and acoustic emission signal of coal samples are obtained through coal rock compression acoustic emission experiments, and then the stress-strain curve of coal samples and acoustic emission signals are used. Wavelet analysis is used to identify the apparent Kaiser point and obtain the stress value in the corresponding direction; the Chinese patent application with announcement number CN106289581A proposes a method and device for measuring coal seam ground stress, by establishing a minimum horizontal ground stress calculation formula and a maximum horizontal ground stress calculation formula corresponding to the coal seam within a preset range, combined with rock mechanics parameter analysis, to obtain the coal seam ground stress; the Chinese patent application with publication number CN113868976A discloses a method for determining the current ground stress size in a well, by calculating the current three-dimensional ground stress size of a single well through hydraulic fracturing, and confirming the current ground stress by combining acoustic emission Kaiser point analysis.

[0004] However, coal seams generally have the characteristics of low elastic modulus, high Poisson's ratio, low present geostress, and high dynamic filtration. There are significant differences in physical properties, rock mechanical properties, etc. between coal seams and their top and bottom sandstone and mudstone layers. The current research method cannot construct a unified present geostress prediction model in coal-bearing strata, resulting in limited accuracy in the prediction of present geostress in coal-bearing strata, which affects the efficient development of coal-bearing gas. Summary of the invention

[0005] In view of the above-mentioned technical deficiencies, the purpose of the present invention is to provide a method for predicting the present geostress of coal-bearing strata, which integrates burial depth, lithology and fracture-weakened rock mechanical property model, combines experimental testing, logging interpretation and theoretical analysis, and constructs a unified calculation model for the present geostress of coal-bearing strata to achieve quantitative prediction.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] The present invention provides a method for predicting the current ground stress of a coal-bearing stratum, and the specific steps are as follows:

[0008] The following steps are involved:

[0009] Step 1, collecting drilling cores, conventional logging, imaging logging, injection / pressure drop test data and reservoir pressure data of different depths and lithologies of coal seams and their roof and floor plates;

[0010] Step 2, using the drilling core obtained in step 1 to conduct experiments, to obtain the static elastic modulus and Poisson's ratio of rocks of different lithology and depths in the coal seam and its roof and floor, as well as the triaxial geostress of rocks of different lithology and depths in the coal seam roof and floor;

[0011] Step 3, using the injection / pressure drop test data collected in step 1, calculate and obtain the current ground stress of the coal seam;

[0012] Step 4, using the conventional logging curve collected in step 1, calculate and obtain the dynamic elastic modulus and Poisson's ratio of the rock, and establish the dynamic rock mechanics parameter profile S0;

[0013] Step 5, based on the rock static and dynamic elastic modulus and Poisson's ratio obtained in Step 2 and Step 4, a dynamic and static rock mechanics parameter conversion model is fitted to thereby convert the dynamic rock mechanics parameter profile of Step 4 into a static rock mechanics parameter profile S1;

[0014] Step 6, using the imaging logging data collected in step 1, natural fractures are identified, their parameter characteristics are quantitatively characterized, and based on the model of fracture-weakening rock mechanical properties, the static rock mechanical parameter profile obtained in step 5 is converted into a rock mechanical profile S2 of actual underground fracture conditions;

[0015] The mechanical property model of fracture-weakened rock is as follows:

[0016]

[0017]

[0018] Where: E eq and μ eq are the equivalent rock elastic modulus, E and μ are the intact rock elastic modulus and Poisson's ratio, α and β are the acute angles between the first group of cracks and the horizontal plane, and between the second group and the first group of cracks, S1 and S2 are the spacings between the second group and the first group of cracks, respectively. K n1 and K n2 are the normal stiffness of the second and first group of cracks, respectively, K s1 and K s2 are the shear stiffness of the second and first group of cracks, respectively;

[0019] Step 7, combining the reservoir pressure data collected in step 1, the top and bottom geostress of the coal seam obtained experimentally in step 2, the current geostress of the coal seam obtained in step 3, and the rock mechanics parameter profile S2 constructed in step 6, and using the combined spring model calculated by the current geostress logging, inversely infer the horizontal maximum and horizontal minimum strain coefficients;

[0020] Step 8, using deep learning methods to establish the relationship between the maximum horizontal strain coefficient, the minimum horizontal strain coefficient, the burial depth, and the static elastic modulus obtained in step 7, and then construct a current ground stress calculation model to achieve quantitative prediction of the current ground stress of the coal-bearing strata;

[0021] The relationship between the maximum horizontal strain coefficient, the minimum horizontal strain coefficient, the burial depth and the elastic modulus is as follows:

[0022]

[0023] Where: ε H and ε h are the maximum and minimum horizontal strain coefficients, d is the burial depth, E eq is the equivalent elastic modulus.

[0024] Preferably, the conventional logging curves in step 1 include gamma GR, density DEN and acoustic time difference AC, and the injection / pressure drop test has at least 3 cycles, based on which the fracture pressure, closure pressure and fracture reopening pressure are interpreted.

[0025] Preferably, the triaxial geostress calculation formula based on the differential strain geostress experiment in step 2 is as follows:

[0026] (σ1-P):(σ2-P):(σ3-P)=[μ(ε y +ε z )+(1-μ)ε x ]:[μ(ε z +ε x )+(1-μ)ε y ]:[μ(ε x +ε y )+(1-μ)ε z ]

[0027] Where: σ1, σ2 and σ3 are the maximum, intermediate and minimum principal stresses respectively, ε x , ε y and ε z are the strains in the x, y and z directions respectively, μ is the Poisson’s ratio, and P is the reservoir pressure.

[0028] Preferably, the current in-situ stress is calculated using the injection / pressure drop data in step 3 according to the following formula:

[0029]

[0030] Where: S Hmax and S hmin are the maximum and minimum horizontal principal stresses, respectively, c is the closing pressure, P r To reopen the cracks.

[0031] Preferably, the formula for calculating the dynamic elastic modulus and dynamic Poisson's ratio of rock based on conventional logging curves in step 4 is as follows:

[0032]

[0033]

[0034] Where: μ d is the dynamic rock Poisson's ratio, E d is the dynamic rock elastic modulus, Δt s and Δt p are the transverse wave and longitudinal wave time differences of the stratum respectively, and ρ is the rock density.

[0035] Preferably, the formula for calculating the current ground stress based on the combined spring model in step 7 is as follows:

[0036]

[0037]

[0038] Where: S Hmax and S hmin are the maximum and minimum horizontal principal stresses, E eq and μ eq are the equivalent rock elastic modulus, S v is the vertical principal stress, δ represents the Biot coefficient, P is the reservoir pressure, ε H and ε h represent the maximum and minimum horizontal strain coefficients, respectively;

[0039] The reservoir pressure is calculated using Eaton's method:

[0040]

[0041] Where: S v is the vertical principal stress, P is the reservoir pressure, P i is the hydrostatic pressure, Δtn and Δt are the acoustic time differences between the normal compaction curve and the well logging, respectively, and n is the Eaton coefficient;

[0042] The Biot coefficient is calculated using the Krief empirical formula:

[0043]

[0044] Where: δ represents the Biot coefficient, is the porosity.

[0045] Preferably, the deep learning method in step 8 is not limited to convolutional neural networks.

[0046] The beneficial effects of the present invention are as follows: in view of the difficulty in constructing a unified current geostress prediction model for coal-bearing strata, the present invention patent comprehensively considers the burial depth, lithology, logging data, field test data, etc. of coal-bearing strata to construct a quantitative evaluation model of the mechanical properties of rocks weakened by natural fractures, and quantitatively characterizes the weakening strength of the mechanical properties of rocks caused by the development and distribution of natural fractures. On this basis, a unified current geostress calculation model for coal-bearing strata is constructed by combining rock mechanics experiments, differential strain geostress experimental tests, logging interpretation, theoretical analysis, and deep learning methods, thereby realizing the quantitative prediction of current geostress and facilitating the efficient development of coal-bearing gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0048] Figure 1 A schematic flow chart of a method for predicting current ground stress in a coal-bearing stratum provided by the present invention. DETAILED DESCRIPTION

[0049] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0050] like Figure 1As shown, the present invention provides a method for predicting the current geostress of coal-bearing strata. The method first collects drilling cores of different depths and lithologies, conventional logging, imaging logging, injection / pressure drop test data and reservoir pressure data, obtains the mechanical parameters and current geostress of rocks of different lithologies by experiments and logging, quantitatively characterizes the parameters of natural fractures, sequentially constructs a dynamic rock mechanics profile S0, a static rock mechanics profile S1 and a rock mechanics profile S2 under fracture conditions, and uses a combined spring model to infer the maximum and minimum horizontal strain coefficients. On this basis, a mathematical relationship between the maximum and minimum horizontal strain coefficients and the burial depth and static elastic modulus is established, and then a current geostress calculation model is constructed to achieve quantitative prediction of the current geostress of coal-bearing strata.

[0051] The specific steps are as follows:

[0052] Step 1: Collect drilling cores of different lithologies and depths of the coal seam and its roof and floor, and process them into plunger samples with a diameter of 25 mm × height of 50 mm; collect full-size drilling cores of different lithologies and depths of the coal seam roof and floor, and process them into fan-shaped bodies with a radius of 50 mm × height of 50 mm × 50 mm and an angle of 90°; collect conventional logging, imaging logging, injection / pressure drop test data and reservoir pressure data;

[0053] Conventional logging curves include gamma GR, density DEN and acoustic time difference AC. The injection / pressure drop test has at least 3 cycles, which can interpret the fracture pressure, closure pressure and fracture reopening pressure.

[0054] Step 2: using the plunger sample and the fan-shaped body obtained in step 1, respectively, triaxial rock mechanics experiments and differential strain in-situ stress experiments are carried out to obtain the static elastic modulus and Poisson's ratio of rocks of different lithology and different depths in the coal seam and its roof and floor plates, as well as the triaxial in-situ stress of rocks of different lithology and different depths in the coal seam roof and floor plates;

[0055] The triaxial geostress calculation formula based on the differential strain geostress experiment is as follows:

[0056] (σ1-P):(σ2-P):(σ3-P)=[μ(ε y +ε z )+(1-μ)εx]:[μ(ε z +ε x )+(1-μ)ε y ]:[μ(ε x +ε y )+(1-μ)ε z ]

[0057] Where: σ1, σ2 and σ3 are the maximum, intermediate and minimum principal stresses respectively, ε x , ε y and ε zare the strains in the x, y and z directions respectively, μ is the Poisson’s ratio, and P is the reservoir pressure.

[0058] Step 3, using the injection / pressure drop test data collected in step 1, calculate and obtain the current ground stress of the coal seam;

[0059] The current in-situ stress can be calculated using the injection / pressure drop data according to the following formula:

[0060]

[0061] Where: S Hmax and S hmin are the maximum and minimum horizontal principal stresses, respectively, c is the closing pressure, P r To reopen the cracks.

[0062] Step 4, using the conventional logging curve collected in step 1, calculate and obtain the dynamic elastic modulus and Poisson's ratio of the rock, and establish the dynamic rock mechanics parameter profile S0;

[0063] The formula for calculating the dynamic elastic modulus and dynamic Poisson's ratio of rocks based on conventional logging curves is:

[0064]

[0065]

[0066] Where: μ d is the dynamic rock Poisson's ratio, E d is the dynamic rock elastic modulus, Δt s and Δt p are the time differences of shear wave and longitudinal wave of the stratum respectively, ρ is the rock density.

[0067] Step 5, based on the static and dynamic elastic moduli and Poisson's ratio of the rock obtained in Steps 2 and 4, a dynamic and static rock mechanics parameter conversion model is fitted to thereby convert the dynamic rock mechanics parameter profile of Step 4 into a static rock mechanics parameter profile S1;

[0068] Step 6, using the imaging logging data collected in step 1, natural fractures are identified, their parameter characteristics are quantitatively characterized, and based on the model of fracture-weakening rock mechanical properties, the static rock mechanical parameter profile obtained in step 5 is converted into a rock mechanical profile S2 of actual underground fracture conditions;

[0069] The mechanical property model of fracture-weakened rock is as follows:

[0070]

[0071]

[0072] Where: E eq and μ eq are the equivalent rock elastic modulus, E and μ are the intact rock elastic modulus and Poisson's ratio, α and β are the acute angles between the first group of cracks and the horizontal plane, and between the second group and the first group of cracks, S1 and S2 are the spacings between the second group and the first group of cracks, respectively. K n1 and K n2 are the normal stiffness of the second and first group of cracks, respectively, K s1 and K s2 are the shear stiffnesses of the second and first groups of cracks, respectively.

[0073] Step 7, combining the reservoir pressure data collected in step 1, the top and bottom geostress of the coal seam obtained experimentally in step 2, the current geostress of the coal seam obtained in step 3, and the rock mechanics parameter profile S2 constructed in step 6, and using the combined spring model calculated by the current geostress logging, inversely infer the horizontal maximum and horizontal minimum strain coefficients;

[0074] The formula for calculating the current ground stress based on the combined spring model is as follows:

[0075]

[0076]

[0077] Where: S Hmax and S hmin are the maximum and minimum horizontal principal stresses, E eq and μ eq are the equivalent rock elastic modulus, S v is the vertical principal stress, δ represents the Biot coefficient, P is the reservoir pressure, ε H and ε h Represent the maximum and minimum horizontal strain coefficients respectively.

[0078] The reservoir pressure is calculated using Eaton's method:

[0079]

[0080] Where: S v is the vertical principal stress, P is the reservoir pressure, P i is the hydrostatic pressure, Δtn and Δt are the acoustic time differences of the normal compaction curve and the logging data, respectively, and n is the Eaton coefficient.

[0081] The Biot coefficient is calculated using the Krief empirical formula:

[0082]

[0083] Where: δ represents the Biot coefficient, is the porosity.

[0084] Step 8: Use deep learning methods to establish the relationship between the horizontal maximum strain coefficient, the horizontal minimum strain coefficient obtained in step 7 and the burial depth and static elastic modulus, and then construct a current ground stress calculation model to achieve quantitative prediction of the current ground stress of the coal-bearing strata.

[0085] The relationship between the maximum horizontal strain coefficient, the minimum horizontal strain coefficient, the burial depth and the elastic modulus is as follows:

[0086]

[0087] Where: ε H and ε h are the maximum and minimum horizontal strain coefficients, d is the burial depth, E eq is the equivalent elastic modulus.

[0088] Among them, deep learning methods are not limited to convolutional neural networks.

[0089] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for predicting the current ground stress of coal-bearing strata, It is characterized in that The following steps are involved: Step 1, collecting drilling cores, conventional logging, imaging logging, injection / pressure drop test data and reservoir pressure data of different depths and lithologies of coal seams and their roof and floor plates; Step 2, using the drilling core obtained in step 1 to conduct experiments, to obtain the static elastic modulus and Poisson's ratio of rocks of different lithology and depths in the coal seam and its roof and floor, as well as the triaxial geostress of rocks of different lithology and depths in the coal seam roof and floor; Step 3, using the injection / pressure drop test data collected in step 1, calculate and obtain the current ground stress of the coal seam; Step 4, using the conventional logging curve collected in step 1, calculate and obtain the dynamic elastic modulus and Poisson's ratio of the rock, and establish the dynamic rock mechanics parameter profile S0; Step 5, based on the rock static and dynamic elastic modulus and Poisson's ratio obtained in Step 2 and Step 4, a dynamic and static rock mechanics parameter conversion model is fitted to thereby convert the dynamic rock mechanics parameter profile of Step 4 into a static rock mechanics parameter profile S1; Step 6, using the imaging logging data collected in step 1, natural fractures are identified, their parameter characteristics are quantitatively characterized, and based on the model of fracture-weakening rock mechanical properties, the static rock mechanical parameter profile obtained in step 5 is converted into a rock mechanical profile S2 of actual underground fracture conditions; The mechanical property model of fracture-weakened rock is as follows: Where: E eq and μ eq are the equivalent rock elastic modulus, E and μ are the intact rock elastic modulus and Poisson’s ratio, α and β are the acute angles between the first group of cracks and the horizontal plane, and between the second group and the first group of cracks, respectively. 1 and S 2 are the crack spacings of the second and first groups, respectively, and K n1 and K n2 are the normal stiffness of the second and first group of cracks, respectively, K s1 and K s2 are the shear stiffness of the second and first group of cracks, respectively; Step 7, combining the reservoir pressure data collected in step 1, the top and bottom geostress of the coal seam obtained experimentally in step 2, the current geostress of the coal seam obtained in step 3, and the rock mechanics parameter profile S2 constructed in step 6, and using the combined spring model calculated by the current geostress logging, inversely infer the horizontal maximum and horizontal minimum strain coefficients; Step 8, using deep learning methods to establish the relationship between the maximum horizontal strain coefficient, the minimum horizontal strain coefficient, the burial depth, and the static elastic modulus obtained in step 7, and then construct a current ground stress calculation model to achieve quantitative prediction of the current ground stress of the coal-bearing strata; The relationship between the maximum horizontal strain coefficient, the minimum horizontal strain coefficient, the burial depth and the elastic modulus is as follows: Where: ε H and ε h are the maximum and minimum horizontal strain coefficients, d is the burial depth, E eq is the equivalent elastic modulus.

2. The method according to claim 1, It is characterized in that In step 1, conventional logging curves include gamma GR, density DEN and acoustic time difference AC. The injection / pressure drop test has at least 3 cycles, from which the fracture pressure, closure pressure and fracture reopening pressure are interpreted.

3. The method according to claim 1, It is characterized in that The triaxial geostress calculation formula based on the differential strain geostress experiment in step 2 is as follows: (s 1 -P):(s 2 -P):(s 3 -P)= [with y +e z )+(1-μ)e x ]:[with z +e x )+(1-μ)e y ]:[with x +e y )+(1-μ)e z ] Where: 1 , σ 2 and σ 3 are the maximum, intermediate and minimum principal stresses, respectively, and ε x , ε y and ε z are the strains in the x, y and z directions respectively, μ is the Poisson’s ratio, and P is the reservoir pressure.

4. The method according to claim 3, It is characterized in that In step 3, the injection / pressure drop data is used to calculate the current ground stress according to the following formula: Where: S Hmax and S hmin are the maximum and minimum horizontal principal stresses, respectively, c is the closing pressure, P r To reopen the cracks.

5. The method according to claim 4, It is characterized in that In step 4, the formula for calculating the dynamic elastic modulus and dynamic Poisson's ratio of rock based on conventional logging curves is as follows: Where: μ d is the dynamic rock Poisson's ratio, E d is the dynamic rock elastic modulus, Δt s and Δt p are the transverse wave and longitudinal wave time differences of the stratum respectively, and ρ is the rock density.

6. The method according to claim 5, It is characterized in that The formula for calculating the current ground stress based on the combined spring model in step 7 is as follows: Where: S Hmax and S hmin are the maximum and minimum horizontal principal stresses, E eq and μ eq are the equivalent rock elastic modulus, S v is the vertical principal stress, δ represents the Biot coefficient, P is the reservoir pressure, ε H and ε h represent the maximum and minimum horizontal strain coefficients, respectively; The reservoir pressure is calculated using Eaton's method: Where: S v is the vertical principal stress, P is the reservoir pressure, P i is the hydrostatic pressure, Δtn and Δt are the acoustic time differences between the normal compaction curve and the well logging, respectively, and n is the Eaton coefficient; The Biot coefficient is calculated using the Krief empirical formula: Where: δ represents the Biot coefficient, is the porosity.

7. The method according to claim 1, It is characterized in that The deep learning method in step 8 is not limited to convolutional neural networks.

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