A method for predicting current ground stress in coal-bearing strata

Abstract

The present invention provides a method for predicting the current in-situ stress of coal-bearing strata, which is applicable to the field of coal-bed (stratum) gas geology. First, drilling cores of different lithologies at different depths, conventional logging, imaging logging, injection / pressure drop test data and reservoir pressure data are collected, and rock mechanics parameters and current in-situ stress of different lithologies are obtained by experiments and logging, and natural fracture parameters are quantitatively characterized. Dynamic rock mechanics profile S0, static rock mechanics profile S1 and rock mechanics profile S2 under fracture conditions are sequentially constructed; then, the combined spring model is used to infer the maximum and minimum horizontal strain coefficients, and on this basis, a mathematical relationship between them and the burial depth and static elastic modulus is established; finally, a current in-situ stress calculation model is constructed to achieve quantitative prediction of the current in-situ stress of coal-bearing strata. This method takes into account comprehensive factors, has rigorous ideas and techniques, is highly operational, and has high credibility in prediction results.

Description

Technical Field

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

[0002] Coal-bearing strata include coal seams, tight sandstone layers, mudstone layers, etc. With the deepening of exploration and development and technological development, 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 the announcement number CN109113742B proposes a method for predicting the current geostress of coal reservoirs. Based on the experimental test of coal rock parameters and the 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 the announcement number CN117647342B proposes a method for measuring geostress of coal reservoirs based on acoustic emission wavelet analysis. The spatial rectangular coordinate system is established based on the wave velocity measurement of the full-diameter drilling core. The full stress-strain curve and acoustic emission signal of the coal sample are obtained through the coal rock compression acoustic emission experiment, and then the stress-strain curve of the coal sample and the acoustic emission signal are used. Wavelet analysis is used to identify the apparent Kaiser point and obtain the stress value in the corresponding direction; Chinese patent application with publication number CN106289581A proposes a coal seam ground stress measurement method and device, which obtains the coal seam ground stress by establishing the minimum horizontal ground stress calculation formula and the maximum horizontal ground stress calculation formula corresponding to the coal seam within a preset range, combined with rock mechanics parameter analysis; Chinese patent application with publication number CN113868976A discloses a method for determining the current ground stress magnitude in a well, which calculates the current three-dimensional ground stress magnitude of a single well through hydraulic fracturing, and confirms the current ground stress by combining acoustic emission Kaiser point analysis.

[0004] However, coal seams generally have characteristics such as low elastic modulus, high Poisson's ratio, low present-day ground stress, 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-day ground stress prediction model in coal-bearing strata, resulting in limited accuracy in the prediction of present-day ground stress in coal-bearing strata, which affects the efficient development of coal-bearing gas. Summary of the Invention

[0005] In response to the above-mentioned technical deficiencies, the purpose of the present invention is to provide a method for predicting the current ground stress of coal-bearing strata, which integrates burial depth, lithology and fracture-weakening rock mechanical property models, combines experimental testing, logging interpretation and theoretical analysis, and constructs a unified current ground stress calculation model for 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 seam, the specific steps of which are as follows:

[0008] The following steps are involved:

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

[0010] Step 2: Conducting experiments using the drill core obtained in step 1 to obtain the static elastic modulus and Poisson's ratio of rocks of different lithology and depth in the coal seam and its roof and floor, as well as the triaxial in-situ stress of rocks of different lithology and depth 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 curves collected in step 1, calculate 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 moduli and Poisson's ratio obtained in Steps 2 and 4, a dynamic and static rock mechanical parameter conversion model is fitted to convert the dynamic rock mechanical parameter profile in Step 4 into a static rock mechanical parameter profile S1;

[0014] Step 6: Using the imaging logging data collected in step 1, natural fractures are identified and their parameter characteristics are quantitatively characterized. 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 elastic modulus and Poisson's ratio of intact rock, 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. 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 groups of cracks, K s1 and K s2 are the shear stiffnesses of the second and first groups of cracks, respectively;

[0019] Step 7: Integrate the reservoir pressure data collected in step 1, the coal seam roof and floor in-situ stress obtained experimentally in step 2, the coal seam current in-situ stress obtained in step 3, and the rock mechanics parameter profile S2 constructed in step 6, and use the combined spring model calculated by current in-situ stress logging to reversely infer the horizontal maximum and horizontal minimum strain coefficients;

[0020] Step 8: Using deep learning methods, the relationships between the maximum horizontal strain coefficient and the minimum horizontal strain coefficient obtained in step 7 and the burial depth and static elastic modulus are established, thereby constructing a current ground stress calculation model to achieve quantitative prediction of the current ground stress of the coal-bearing seam.

[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 transit time 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 in-situ stress calculation formula based on the differential strain in-situ stress 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, in step 3, the current in-situ stress is calculated using the injection / pressure drop data according to the following formula:

[0029]

[0030] Where: S Hmax and S hmin are the maximum and minimum horizontal principal stresses, respectively, and P c is the closing pressure, P r The pressure for crack reopening.

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

[0032]

[0033]

[0034] Where: μ d is the Poisson's ratio of dynamic rock, E d is the dynamic rock elastic modulus, Δt s and Δt p are the time differences of shear and longitudinal waves of the stratum, 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 the Eaton 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 effect of the present invention is that it is difficult to construct a unified current ground stress prediction model for coal-bearing strata. The patent of the present invention integrates the burial depth, lithology, logging data, field test data, etc. of the coal-bearing strata to construct a quantitative evaluation model of the mechanical properties of rocks weakened by natural fractures, and quantitatively characterizes the weakening intensity of the development and distribution of natural fractures on the mechanical properties of rocks. On this basis, it combines rock mechanics experiments, differential strain ground stress experimental tests, logging interpretation, theoretical analysis and deep learning methods to construct a unified current ground stress calculation model for coal-bearing strata, realize the quantitative prediction of current ground stress, and assist in 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 following briefly introduces the drawings required for use in the embodiments or the description of the prior art. 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 any creative work.

[0048] Figure 1 The present invention provides a schematic flow chart of a method for predicting the current ground stress of a coal-bearing seam. DETAILED DESCRIPTION

[0049] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative efforts 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 in-situ stress of coal-bearing strata. The method first collects drilling cores, conventional logging, imaging logging, injection / pressure drop test data and reservoir pressure data of different lithologies at different depths, obtains the mechanical parameters and current in-situ stress of rocks of different lithologies through experiments and logging, quantitatively characterizes the parameters of natural fractures, and sequentially constructs a dynamic rock mechanics profile S0, a static rock mechanics profile S1 and a rock mechanics profile S2 under fracture conditions. The combined spring model is used to infer the maximum and minimum horizontal strain coefficients. On this basis, a mathematical relationship between the maximum and minimum strain coefficients and the burial depth and static elastic modulus is established, and then a current in-situ stress calculation model is constructed to achieve quantitative prediction of the current in-situ stress of coal-bearing strata.

[0051] The specific steps are as follows:

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

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

[0054] Step 2: Using the plunger sample and fan-shaped body obtained in step 1, 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 depth in the coal seam and its roof and floor, as well as the triaxial in-situ stress of rocks of different lithology and depth in the coal seam roof and floor;

[0055] The calculation formula of triaxial geostress based on 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 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, and P c is the closing pressure, P r The pressure for crack reopening.

[0062] Step 4: Using the conventional logging curves collected in step 1, calculate 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 rock based on conventional logging curves is:

[0064]

[0065]

[0066] Where: μ d is the Poisson's ratio of dynamic rock, 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 formation respectively, and 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-static rock mechanical parameter conversion model is fitted to convert the dynamic rock mechanical parameter profile of Step 4 into a static rock mechanical parameter profile S1;

[0068] Step 6: Using the imaging logging data collected in step 1, natural fractures are identified and their parameter characteristics are quantitatively characterized. 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 elastic modulus and Poisson's ratio of intact rock, 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. 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 groups of cracks, K s1 and K s2 are the shear stiffnesses of the second and first groups of cracks, respectively.

[0073] Step 7: Integrate the reservoir pressure data collected in step 1, the coal seam roof and floor in-situ stress obtained experimentally in step 2, the coal seam current in-situ stress obtained in step 3, and the rock mechanics parameter profile S2 constructed in step 6, and use the combined spring model calculated by current in-situ stress logging to reversely 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 the Eaton 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 between the normal compaction curve and the well logging, 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] In step 8, the deep learning method is used 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 a current ground stress calculation model is constructed to achieve quantitative prediction of the current ground stress of the coal-bearing seam.

[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 may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A method for predicting the current ground stress of a coal-bearing seam, characterized in that: The following steps are involved: Step 1: Collect drilling cores, conventional logging, imaging logging, injection / pressure drop test data and reservoir pressure data of coal seams and their roof and floor plates at different depths and lithologies; Step 2: Conducting experiments using the drill core obtained in step 1 to obtain the static elastic modulus and Poisson's ratio of rocks of different lithology and depth in the coal seam and its roof and floor, as well as the triaxial in-situ stress of rocks of different lithology and depth 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 curves collected in step 1, calculate 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 moduli and Poisson's ratio obtained in Steps 2 and 4, a dynamic and static rock mechanical parameter conversion model is fitted to convert the dynamic rock mechanical parameter profile in Step 4 into a static rock mechanical parameter profile S1; Step 6: Using the imaging logging data collected in step 1, natural fractures are identified and their parameter characteristics are quantitatively characterized. 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 elastic modulus and Poisson's ratio of intact rock, 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. 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 groups of cracks, K s1 and K s2 are the shear stiffnesses of the second and first groups of cracks, respectively; Step 7: Integrate the reservoir pressure data collected in step 1, the coal seam roof and floor in-situ stress obtained experimentally in step 2, the coal seam current in-situ stress obtained in step 3, and the rock mechanics parameter profile S2 constructed in step 6, and use the combined spring model calculated by current in-situ stress logging to reversely infer the horizontal maximum and horizontal minimum strain coefficients; Step 8: Using deep learning methods, the relationships between the maximum horizontal strain coefficient and the minimum horizontal strain coefficient obtained in step 7 and the burial depth and static elastic modulus are established, thereby constructing a current ground stress calculation model to achieve quantitative prediction of the current ground stress of the coal-bearing seam. 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, characterized in that In step 1, conventional logging curves include gamma ray (GR), density (DEN), and acoustic time difference (AC). The injection / pressure drop test has at least three cycles, and the fracture pressure, closure pressure, and fracture reopening pressure are interpreted based on them.

3. The method according to claim 1, characterized in that The triaxial geostress calculation formula based on the differential strain geostress experiment in step 2 is as follows: (σ1-P):(σ2-P):(σ3-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: 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.

4. The method according to claim 3, characterized in that In step 3, the current in-situ stress is calculated using the injection / pressure drop data according to the following formula: Where: S Hmax and S hmin are the maximum and minimum horizontal principal stresses, respectively, and P c is the closing pressure, P r The pressure for crack reopening.

5. The method according to claim 4, 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 Poisson's ratio of dynamic rock, E d is the dynamic rock elastic modulus, Δt s and Δt p are the time differences of shear and longitudinal waves of the stratum, and ρ is the rock density.

6. The method according to claim 5, 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 the Eaton 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, characterized in that The deep learning method in step 8 is not limited to convolutional neural networks.

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