Method for evaluating compressible dessert of deep coal bed gas in complex structure area based on stress demarcation

Abstract

The invention discloses a stress demarcation-based deep coalbed methane compressible dessert evaluation method for a complex structure area. The method comprises the following steps: calculating the maximum horizontal stress, the minimum horizontal stress and the vertical stress of a single well; the single well stress type is judged based on the three-stress magnitude relation, and complex area partitioning is conducted by combining the structural boundary; geological conditions of compressibility of deep coal bed gas in different partitions are defined, and first-level and second-level index systems for compressibility evaluation of different partitions are established; a quantitative characterization method for secondary evaluation index parameters is provided; based on the type of the stress field, determining the weight of the index parameter by adopting a subjective weighting method; establishing a membership function for different evaluation index parameters; calculating the evaluation comprehensive values of the compressible desserts of the coal bed gas single well in different subareas, and carrying out the classification and classification of compressible dessert areas. According to the method, difference setting and unified comparison of compressible dessert evaluation parameter systems of coalbed methane at different depths in the same block are realized, and the existing dessert evaluation index is optimized.

Description

Technical Field

[0001] This invention belongs to the field of exploration and development of medium-deep coalbed methane and pre-compression geological evaluation technology, specifically involving a method for evaluating compressible sweet spots of deep coalbed methane in structurally complex areas based on stress demarcation. Background Technology

[0002] The medium-deep coalbed methane (CBM) resources have enormous potential and are a hot area for CBM exploration and development in my country. With increasing development depth, the in-situ stress of coal reservoirs increases, and the reservoirs are characterized by low porosity and low permeability. Differential-scale fracturing has become an important means of CBM development. Therefore, evaluating the compressible sweet spots of deep coal reservoirs is a crucial technical guarantee for effective fracturing and efficient large-scale development. my country's CBM regions have complex tectonic conditions, influenced by multiple phases of tectonic alteration. In the horizontal plane, this results in complex fault patterns and diverse current in-situ stress types in structurally complex areas. Vertically, the coal seam structure, coal body structure, lithological combinations of the coal seam's top and bottom plates, and water-bearing properties are relatively complex. Achieving a reasonable combination and evaluation of multiple compressible parameters under these complex tectonic conditions is key to determining the appropriate scale of fracturing and differential fracturing of the coal seam.

[0003] With the implementation of differentiated fracturing schemes such as large-scale fracturing, cross-layer fracturing, and roof fracturing in medium-deep coal reservoirs, it has been recognized that the efficient development of deep coalbed methane requires the formulation of differentiated fracturing schemes based on the differentiated sweet spot elements of coal reservoirs. Therefore, the study of compressible sweet spots of coalbed methane in complex structural areas has become an important research direction for the exploration, development, and efficient stimulation of medium-deep coalbed methane. It is necessary to propose a compressible sweet spot evaluation method for deep coalbed methane in structurally complex areas based on stress type delineation. Summary of the Invention

[0004] This invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a method for evaluating the compressible sweet spot of deep coalbed methane in structurally complex areas based on stress delimitation.

[0005] This invention is achieved through the following technical solution:

[0006] A method for evaluating the compressible sweet spot of deep coalbed methane in structurally complex areas based on stress demarcation includes the following steps:

[0007] S1. Calculate the in-situ stress of the target coal seam in a single well for fracturing, and determine the stress type of different single wells based on the in-situ stress;

[0008] The geostress of the target coal seam in the single-well fracturing includes the maximum horizontal stress (σ) of the main coal seam in the single well. h,max ), minimum horizontal stress (σ) of the main coal seam in a single well h,min ) and the vertical stress (σ) of the main coal seam in a single well V );

[0009] The stress types include normal fault type, reverse fault type and strike-slip fault type, and the stress type of a single well is determined based on the relationship between the magnitudes of the three stresses;

[0010] When the vertical stress σ V Maximum horizontal stress σ h,max Minimum horizontal stress σ h,min When the stress type is a normal fault type;

[0011] When the maximum horizontal stress σ h,max Minimum horizontal stress δ h,min Vertical stress σ V When the stress type is a reverse fault type;

[0012] When the maximum horizontal stress σ ……,max Vertical stress σ V Minimum horizontal stress σ h,min When the stress type is a strike-slip fault, the stress type is...

[0013] The geostress of the target coal seam for single-well fracturing is calculated based on well logging and well test data, as well as hydraulic fracturing methods and well logging interpretation.

[0014] S2. Based on the distribution of stress types in single wells and the location of fault boundaries in deep complex structural areas, the deep complex structural areas are zoned and demarcated.

[0015] The deep structurally complex region is generally divided into 1 to 3 structural zones;

[0016] S3. Establish a compressibility evaluation index system for different structural zones;

[0017] Based on coal and rock experimental tests and well logging data, parameters such as coal seam gas content, coal seam thickness, permeability, Langevin volume, coal body structure, and aquifer distribution in the roof and floor are obtained. Then, the parameters are combined, calculated, and classified to establish a compressibility evaluation index system for different structural zones.

[0018] The compressibility evaluation index system includes primary evaluation indicators and secondary evaluation indicators;

[0019] The primary index parameters include development potential parameters, coal and rock pressure suitability parameters, and fracturing and fracture control parameters. The development potential parameters include resource abundance, reservoir pressure gradient, and desorption index. The coal and rock pressure suitability parameters include coal body pressure suitability index, structural pressure suitability index, and aquifer pressure avoidance index. The fracturing and fracture control parameters include coal and rock fracture control index, intra-layer fracture control index, inter-layer stress difference (stress difference between coal seam and roof and floor layers), and horizontal stress difference (horizontal stress difference of coal seam).

[0020] The resource abundance, reservoir pressure gradient, desorption index, coal body pressure suitability index, structural pressure suitability index, aquifer pressure avoidance index, coal-rock fracture control index, intra-layer fracture control index, stress difference between coal seam and roof and floor, and horizontal stress difference of coal seam are used as secondary index parameters.

[0021] S4. Quantitatively characterize the compressibility evaluation index system;

[0022] The quantitative characterization of the compressibility evaluation index system is to quantitatively characterize the secondary index parameters, specifically including: resource abundance characterization, reservoir pressure gradient characterization, desorption index characterization, coal body compressibility index characterization, structural compressibility index characterization, aquifer pressure avoidance index characterization, coal and rock fracture control index characterization, intra-layer fracture control index characterization, inter-layer stress difference characterization, and horizontal stress difference characterization.

[0023] The computational model for the resource abundance characterization is as follows:

[0024]

[0025] In the formula: R represents resource abundance, in units of 10. 8 m 3 / km 2 H 煤 D is the coal seam thickness in meters; D is the coal and rock density in cubic meters per second. 3 / t;Q g This refers to the gas content of the coal seam, expressed in cubic meters (m³). 2 / t; D, Q g Determined comprehensively by drilling logging interpretation and gas content experimental testing and calibration;

[0026] The reservoir pressure gradient characterization is determined based on well test data;

[0027] The calculation model for the desorption index is as follows:

[0028]

[0029] In the formula: GL is the desorption exponent, in units of m. 3 t -1 MPa -1 V L The pressure is measured in meters (m). 3 / t;P L V is the Langmuir volume, in MPa. L and P L The value of P is determined by a combination of single-well isothermal adsorption experiments and logging curve interpretation; P is the reservoir pressure in MPa; P is determined by multiplying the reservoir pressure gradient by the burial depth.

[0030] The calculation model for the coal body compressibility index is as follows:

[0031]

[0032] In the formula: MF is the coal seam compressibility index, which is dimensionless; MG is the coal seam structure index, which is the ratio of the cumulative thickness of primary structural coal and fractured coal to the total coal seam thickness, which is dimensionless. The primary structural coal and fractured coal are quantitatively characterized by coal core information and well logging interpretation; MD is the coal seam roof lithology index, which is the proportion of sandstone within 2m of the coal seam roof, which is dimensionless. The thickness of sandstone is quantitatively determined by cuttings logging and well logging interpretation.

[0033] The calculation model for constructing the appropriate pressure index is as follows:

[0034] GF=|k|±a

[0035] In the formula: GF is the structural pressure index, which is dimensionless and indicates the influence of fault complexity and fold deformation degree on fracturing by comprehensively representing the distance from the fault and the curvature; k and a are constant values, which are different for different stress types. k and a are determined by using thin plate theory to quantify the curvature in a plane and to comprehensively represent the production plane calibration of a single well.

[0036] The calculation model for the aquifer pressure avoidance index is as follows:

[0037]

[0038] In the formula: WF is the aquifer pressure avoidance index, in meters; H w The thickness of the aquifer is expressed in meters (m). The porosity of the aquifer is expressed as a percentage (S). w The aquifer water saturation percentage is represented by the aquifer pressure avoidance index, which essentially characterizes the aquifer lithology and aquifer thickness (H) within a 20m radius of the coal seam roof based on well logging interpretation. w ), Aquifer water saturation (S) w ) to perform quantitative characterization;

[0039] The calculation model for the coal and rock fracture control index is as follows:

[0040]

[0041] In the formula: MK is the coal and rock fracture control index, in units of 10. -3 μm 2 H L Thickness of bright to semi-bright coal, in meters; H m K represents the total coal seam thickness in meters. m Permeability, in units of 10 -3 μm 2 H L K m Core testing and well test calibration were used respectively, and quantitative calculations were performed based on comprehensive interpretation of well logging curves.

[0042] The calculation model characterized by the in-layer control fracture index is as follows:

[0043]

[0044] In the formula: CK is the in-layer control fracture coefficient, with the unit of MPa; a is the number of interlayer intervals, with the unit of piece; σ 夹min is the minimum horizontal stress of the interlayer interval, with the unit of MPa; σ 煤min is the minimum horizontal stress of the coal seam, with the unit of MPa; σ 夹min , σ 煤min are both quantitatively characterized by logging interpretation;

[0045] The interlayer stress difference is the minimum horizontal stress difference between the coal seam and the roof and floor, and the method for characterizing the interlayer stress difference is to quantitatively characterize the stress based on logging curves;

[0046] The horizontal stress difference is the difference between the maximum horizontal stress and the minimum horizontal stress of the coal seam, and the method for characterizing the horizontal stress difference is to quantitatively characterize the stress of the coal seam based on logging curves;

[0047] S5. Based on the stress types in the structural partition, use the subjective weighting method to determine the weights of the compressibility evaluation index system;

[0048] Specifically: according to the stress types in each structural partition, respectively determine the weights of the primary index parameters (A i ) and the secondary index parameters (A ij ) according to the subjective weighting method to determine the weights of the primary index parameters (a i ) and the weights of the secondary index parameters (a ij );

[0049] When the stress type of the structural partition is reverse fault type, a1≥a2>a3, and

[0050] When the stress type of the structural partition is normal / strike-slip fault type, a1≤a2<a3, and

[0051] The evaluation index system and corresponding weights of the deep coalbed methane compressibility parameters in the structurally complex area delimited based on stress types are shown in Table 1;

[0052] Table 1: Evaluation index system of deep coalbed methane compressibility parameters in the structurally complex area delimited based on stress types

[0053]

[0054]

[0055] S6. Determine the membership function of different evaluation index parameters;

[0056] The membership function is a piecewise linear function composed of three levels: good, medium, and poor. The membership function is determined based on the correlation between production output and indicator parameters, as well as expert experience.

[0057] The expression for the membership function is:

[0058]

[0059] In the formula: A xy Weights are assigned to different evaluation index parameters at each level, dimensionless, x = 1, 2, 3…; y = 1, 2, 3…; A represents different evaluation index parameters at each level; A0 and A1 are boundary parameters. The correlation between secondary index parameters of different stress types and the maximum daily gas production of the vertical well is comprehensively and quantitatively characterized, where A0 corresponds to a maximum daily gas production of 2000 m³ / s of the vertical well. 3 The index parameter value corresponding to / d, A1 corresponds to a maximum daily gas production of 5000 m³ for a direct-dwelling well. 3 / d corresponds to the index parameter value; K and B are the constant parameters of the piecewise function, which are quantitatively determined according to the boundary parameters and the type of piecewise function (linear, exponential, etc.);

[0060] S7. Based on the membership function and weight of each index parameter, calculate the comprehensive evaluation value of the compressible sweet spot of single-well coalbed methane in different structural zones, draw plane contour maps, and classify and grade the compressible sweet spots in structurally complex areas.

[0061] The beneficial effects of this invention are:

[0062] This invention provides a method for evaluating the compressibility sweet spot of deep coalbed methane in structurally complex areas based on stress demarcation. First, the method calculates the geostress of a single well in a structurally complex area to determine the stress type of the well, and then divides the structurally complex area into zones based on the planar distribution of the well and the structural boundaries. Next, it analyzes the factors influencing coal and rock compressibility in different zones, identifying the key factors affecting coal seam compressibility evaluation in each zone, and establishing a geological index evaluation system for coalbed methane compressibility in different zones. Further, based on production data such as coal and rock core tests and well tests, the system quantifies each index parameter through well logging interpretation. Finally, it determines the weight of each index parameter in different stress field types using a subjective weighting method, establishes a secondary index parameter membership function, and classifies the evaluation level of the compressibility sweet spot in the entire complex area.

[0063] Compared to existing technologies that fail to consider the differences in coal seam parameters caused by structural complexity in the evaluation of compressible sweet spots in deep coalbed methane in structurally complex areas (i.e., the main controlling factors of deep and shallow coalbed methane sweet spots are different), and use the same evaluation index parameter weights for the entire area, this invention emphasizes that the stress variation with depth is the basis for controlling the fracturing quality of medium and deep coal seams. First, the structurally complex area is divided into zones based on stress type. Then, a differentiated evaluation index parameter weighting system is formulated for different stress types. The evaluation scheme realizes the differentiated setting and unified comparison of evaluation parameter systems for compressible sweet spots at different depths in the same block, which helps in the selection and evaluation of compressible sweet spots in structurally complex areas.

[0064] Compared to existing similar technologies that evaluate sweet spots related to coalbed methane by using single geological parameters (e.g., coal seam thickness, coal structure, gas content, permeability), this invention takes into account the existing fracturing technology system, which includes cross-layer fracturing, tectonic coal roof fracturing, and deep coal seam large-scale fracture network fracturing. It innovatively proposes a combination of six indices: desorption index, coal body compressibility index, tectonic compressibility index, aquifer pressure avoidance index, coal-rock fracture control index, and intra-layer fracture control index. These indices greatly optimize the existing sweet spot evaluation indices from three dimensions: resource availability, coal-rock compressibility, and coal-rock fracture network modification effect. The evaluation parameter system has strong innovation and comprehensiveness. Attached Figure Description

[0065] Figure 1 This is a flowchart of the method of the present invention;

[0066] Figure 2 This is a schematic diagram of structural complexity zoning based on single-well stress type delineation in Embodiment 1 of the present invention (the left side is a schematic diagram of single-well stress type determination, and the right side is a schematic diagram of planar zoning based on different stress types).

[0067] For ordinary technicians in this field, other relevant drawings can be obtained based on the above drawings without any creative work. Detailed Implementation

[0068] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0069] Example 1

[0070] like Figure 1 As shown, a method for evaluating the compressible sweet spot of deep coalbed methane in structurally complex areas based on stress demarcation includes the following steps:

[0071] S1. Calculate the in-situ stress of the target coal seam in a single well for fracturing, and determine the stress type of different single wells based on the in-situ stress;

[0072] The geostress of the target coal seam in single-well fracturing includes the maximum horizontal stress σ of the main coal seam in the single well. h,max Minimum horizontal stress σ of the main coal seam in a single well h,min and the vertical stress σ of the main coal seam in a single well V ;

[0073] The stress types include normal fault type, reverse fault type and strike-slip fault type, and the stress type of a single well is determined based on the relationship between the magnitudes of the three stresses;

[0074] When the vertical stress σ V Maximum horizontal stress σ h,max Minimum horizontal stress σ h,min When the stress type is a normal fault type;

[0075] When the maximum horizontal stress σ h,max Minimum horizontal stress δ h,min Vertical stress σ V When the stress type is a reverse fault type;

[0076] When the maximum horizontal stress σ h,max Vertical stress σ V Minimum horizontal stress σ h,min When the stress type is a strike-slip fault, the stress type is...

[0077] The geostress of the target coal seam for single-well fracturing is calculated based on well logging and well test data, as well as hydraulic fracturing methods and well logging interpretation.

[0078] In this embodiment, the in-situ stress of the target coal seam in the single-well fracturing is calculated based on well test data;

[0079] The maximum horizontal stress σ h,max The calculation formula is:

[0080] σ h,max =3×σ h,min -P f -P0+S t

[0081] In the formula: σ h,max σ represents the maximum horizontal stress, measured in MPa. h,min P represents the minimum horizontal stress, measured in MPa. f P0 is the rupture pressure, in MPa; P0 is the pore pressure, in MPa; S0 is the pore pressure. t Tensile strength, unit is MPa;

[0082] The minimum horizontal stress σ h,min The calculation formula is:

[0083] σ h,min =P c

[0084] In the formula: σ h,min P represents the minimum horizontal stress, measured in MPa. c The closing pressure is expressed in MPa.

[0085] The vertical stress σ V The formula for calculating (overlying static pressure) is:

[0086] σ V =ρ×g×h

[0087] In the formula: σ V ρ is the vertical stress, in MPa; ρ is the density, in g / cm³. 3 g is the acceleration due to gravity, with units of m / s². 2 h represents depth, in meters (m).

[0088] S2. Based on the distribution of stress types in single wells and the location of fault boundaries within the deep complex structural area, the deep complex structural area is divided and demarcated.

[0089] The deep structurally complex region is generally divided into 1 to 3 structural zones;

[0090] In this embodiment, the deep structurally complex area is divided into two structural zones based on stress type, one of which is a normal fault type (vertical stress σ). V Maximum horizontal stress σ h,max Minimum horizontal stress σ h,min Another structural partition is the reverse fault and strike-slip fault type (maximum horizontal stress σ). h,max Minimum horizontal stress δ h,min Vertical stress σ V Or the maximum horizontal stress σ h,max Vertical stress σ V Minimum horizontal stress σ h,min );

[0091] S3. Establish a compressibility evaluation index system for different structural zones;

[0092] Based on actual production transformation experience and correlation analysis between output and relevant parameters, the influence of various levels of index parameters in different stress type zones on the evaluation of compressible sweet spots of coalbed methane is clarified, and a compressible evaluation index system for different structural zones is established.

[0093] The compressibility evaluation index system includes primary evaluation indicators and secondary evaluation indicators;

[0094] The primary index parameters include development potential parameters, coal and rock pressure suitability parameters, and fracturing and fracture control parameters. The development potential parameters include resource abundance, reservoir pressure gradient, and desorption index. The coal and rock pressure suitability parameters include coal body pressure suitability index, structural pressure suitability index, and aquifer pressure avoidance index. The fracturing and fracture control parameters include coal and rock fracture control index, intra-layer fracture control index, inter-layer stress difference (stress difference between coal seam and roof and floor layers), and horizontal stress difference (horizontal stress difference of coal seam).

[0095] The resource abundance, reservoir pressure gradient, desorption index, coal body pressure suitability index, structural pressure suitability index, aquifer pressure avoidance index, coal-rock fracture control index, intra-layer fracture control index, stress difference between coal seam and roof and floor, and horizontal stress difference of coal seam are used as secondary index parameters.

[0096] In this embodiment, based on two types of structural zones—reverse fault-strike-slip fault zone and normal fault zone—the influence of 10 parameters in 3 categories on resource availability and fracturing effect is analyzed, and a primary index parameter system and a secondary index parameter system are defined, as shown in Table 2 below:

[0097] Table 2

[0098]

[0099] S4. Quantitatively characterize the compressibility evaluation index system;

[0100] The quantitative characterization of the compressibility evaluation index system is to quantitatively characterize the secondary index parameters, specifically including: resource abundance characterization, reservoir pressure gradient characterization, desorption index characterization, coal body compressibility index characterization, structural compressibility index characterization, aquifer pressure avoidance index characterization, coal and rock fracture control index characterization, intra-layer fracture control index characterization, inter-layer stress difference characterization, and horizontal stress difference characterization.

[0101] The computational model for the resource abundance characterization is as follows:

[0102]

[0103] In the formula: R represents resource abundance, in units of 10. 8 m 3 / km 2 H 煤 D is the coal seam thickness in meters; D is the coal and rock density in cubic meters per second. 3 / t;Q g Gas content, unit: m³ 2 / t; D, Q g Determined comprehensively by drilling logging interpretation and gas content experimental testing and calibration;

[0104] The reservoir pressure gradient characterization is determined based on well test data;

[0105] The calculation model for the desorption index is as follows:

[0106]

[0107] In the formula: GL is the desorption exponent, in units of m. 3 t -1 MPa -1 V L The pressure is measured in meters (m). 3 / t;P L V is the Langmuir volume, in MPa. L and P L The value is determined by a combination of single-well isothermal adsorption experiments and well logging curve interpretation; P represents reservoir pressure in MPa; P is determined by multiplying the reservoir pressure gradient by the burial depth; the calculation model for the coal body pressure suitability index is as follows:

[0108]

[0109] In the formula: MF is the coal seam compressibility index, in dimensionless units; MG is the coal seam structure index, which is the ratio of the cumulative thickness of primary structural coal and fractured coal to the total coal seam thickness, in dimensionless units. The primary structural coal and fractured coal are quantitatively characterized by coal core information and well logging interpretation; MD is the coal seam roof lithology index, which is the proportion of sandstone within 2m of the coal seam roof, in dimensionless units. The sandstone thickness is quantitatively determined by cuttings logging and well logging interpretation.

[0110] The calculation model for constructing the appropriate pressure index is as follows:

[0111] GF=|k|±a

[0112] In the formula: GF is the structural compressibility index, which is dimensionless and indicates the influence of fault complexity and fold deformation on hydraulic fracturing by comprehensively representing the distance from the fault and the curvature.

[0113] Curvature is quantified in a plane using thin plate theory. Based on the principle of being far from the fault distribution area (K=50) and the high-yield well distribution calibration area, |k|=15 and a=5 are determined.

[0114] The calculation model for the aquifer pressure avoidance index is as follows:

[0115]

[0116] In the formula: WF is the aquifer pressure avoidance index, in meters; H w The thickness of the aquifer is expressed in meters (m). S represents the porosity of the aquifer, expressed as a percentage (%). wThe aquifer water saturation index is expressed as a percentage (%). The aquifer pressure avoidance index essentially characterizes the lithology and thickness (H) of the aquifer within a 20m radius of the coal seam roof as interpreted by well logging. w ), Aquifer water saturation (S) w ) to perform quantitative characterization;

[0117] The calculation model for the coal and rock fracture control index is as follows:

[0118]

[0119] In the formula: MK is the coal and rock fracture control index, in units of 10. -3 μm 2 H L Thickness of bright to semi-bright coal, in meters; H m K represents the total coal seam thickness in meters. m Permeability, in units of 10 -3 μm 2 Well test permeability (mD; H) was used L K m Core testing and well test calibration were used respectively, and quantitative calculations were performed based on comprehensive interpretation of well logging curves.

[0120] The calculation model for the intralayer controlled seam index is as follows:

[0121]

[0122] In the formula: CK is the intra-layer joint control coefficient, in MPa; a is the number of interlayers, in units; σ 夹min σ represents the minimum horizontal stress in the interlayer, expressed in MPa. 煤min σ represents the minimum horizontal stress in a coal seam, expressed in MPa. 夹min σ 煤min All were quantitatively characterized using well logging interpretation; interlayers were determined by well logging interpretation, and stress differences were quantitatively characterized using a spring model based on well logging interpretation.

[0123] The interlayer stress difference is the minimum horizontal stress difference between the coal seam and the roof and floor. The interlayer stress difference is characterized by quantitative stress characterization based on well logging curves.

[0124] The horizontal stress difference is the difference between the maximum and minimum horizontal stress in the coal seam. The method for characterizing the horizontal stress difference is to quantitatively characterize the coal seam stress based on well logging curves.

[0125] Specifically, based on the spring model, well logging interpretation is used to quantitatively characterize stress;

[0126] The calculation model for the maximum horizontal stress is as follows:

[0127]

[0128] In the formula: σ h,max P represents the maximum principal stress, expressed in MPa. p σ is the pore pressure, in MPa; μ is Poisson's ratio, dimensionless; v α is the vertical stress, in MPa; α is the BIOT coefficient, a dimensionless regional parameter, which is 0.6 in this embodiment; EH1 and EH2 are tectonic stress coefficients, dimensionless, obtained by stress parameter calibration based on well test interpretation of different stress zones in step S1; E is Young's modulus, in GPa.

[0129] The calculation model for the minimum horizontal stress is as follows:

[0130]

[0131] In the formula: σ h,min The minimum principal stress is expressed in MPa; P p σ is the pore pressure, in MPa; μ is Poisson's ratio, dimensionless; v σ represents vertical stress, measured in MPa. v BIOT coefficient, dimensionless, regional parameter, valued at 0.6 in this embodiment; EH1 and EH2 are tectonic stress coefficients, dimensionless, obtained by stress parameter calibration based on well test interpretation of different stress zones in step S1; E is Young's modulus, in GPa.

[0132] The formula for calculating Poisson's ratio is:

[0133]

[0134] Where: Δt s Δt represents the transverse wave time difference of the target segment, in μs / ft; p The target segment's P-wave time difference is expressed in μs / ft.

[0135] The formula for calculating Young's modulus is:

[0136]

[0137] Where: Δt s Δt represents the transverse wave time difference of the target segment, in μs / ft; p P-wave transit time of the target section, μs / ft; ρb: rock skeleton bulk density, g / cm³ 3 ;

[0138] Since the stress algorithm in step S1 mainly requires parameters obtained from well test data, which is relatively scarce, and the stress test results are the stress of the entire target section, this stress is mainly used for stress type zoning. The calculation formula for the maximum and minimum horizontal stress in step S4 is based on parameters obtained from single-well logging data, which is relatively abundant and has high stress resolution. It can interpret and obtain the stress of interlayers and coal seams in the entire section, and is mainly used for the stress distribution of each single well in the area, for the fine prediction of plane parameters.

[0139] The difference between the S4 algorithm and the S1 algorithm is as follows: The stress interpretation model of the S4 algorithm needs to be calibrated using the stress results calculated by the actual measurements in the S1 algorithm. Only after the model is calibrated can stress parameters be predicted using well logging data for wells without S1 test data. Therefore, the calculation method here is a supplement to the S1 method. Compared with the S1 method, this algorithm can be extended to wells without well test data, and this method can calculate the stress within the formation, with higher interpretation accuracy.

[0140] S5. Based on the stress type of structural partitions, the weights of the compressibility evaluation index system are determined using a subjective weighting method.

[0141] Based on the correlation analysis between geological parameters and production data of a small number of production wells, and following a subjective weighting method (expert scoring method), the weights of primary index parameters (a) were determined for reverse fault type structural zones and normal fault / strike-slip fault type structural zones, respectively. i The weights of the secondary indicator parameters (a) and the secondary indicator parameters (a) ij See Table 3 for details;

[0142] Table 3

[0143]

[0144]

[0145] S6. Determine the membership function for different evaluation index parameters;

[0146] The membership function is a piecewise linear function composed of three levels: good, medium, and poor. The membership function is determined based on the correlation between production output and indicator parameters, as well as expert experience.

[0147] Specifically, taking the membership function of the resource abundance index as an example:

[0148]

[0149] Among them, the boundary value range is determined according to the correlation analysis between the average daily gas production of a typical single well and the resource abundance, with the range set at a daily gas production of <500m³. 3 / d、>1000m 3 / d corresponds to resource abundances of 0.2 and 0.7, respectively.

[0150] S7. Based on the membership function and weight of each index parameter, calculate the comprehensive evaluation value of the compressible sweet spot area of ​​single-well coalbed methane in different structural zones, draw a plane contour map, and classify and grade the compressible sweet spots in the structurally complex areas, as detailed in Table 4.

[0151] Table 4

[0152]

[0153]

[0154] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for evaluating the compressible sweet spot of deep coalbed methane in structurally complex areas based on stress delimitation, characterized in that: Includes the following steps: S1. Calculate the in-situ stress of the target coal seam in a single well for fracturing, and determine the stress type of different single wells based on the in-situ stress; S2. Based on the distribution of stress types in single wells and the location of fault boundaries within the deep complex structural area, the deep complex structural area is divided and demarcated. S3. Establish a compressibility evaluation index system for different structural zones; S4. Quantitatively characterize the compressibility evaluation index system; S5. Based on the stress type of structural partitions, the weights of the compressibility evaluation index system are determined using a subjective weighting method. S6. Determine the membership function for different evaluation index parameters; S7. Based on the membership function and weight of each index parameter, calculate the comprehensive evaluation value of the compressible sweet spot of single-well coalbed methane in different structural zones, draw plane contour maps, and classify and grade the compressible sweet spots in structurally complex areas.

2. The method for evaluating the compressible sweet spot of deep coalbed methane in structurally complex areas based on stress demarcation according to claim 1, characterized in that: The geostress of the target coal seam in single-well fracturing includes the maximum horizontal stress σ of the main coal seam in the single well. h,max Minimum horizontal stress σ of the main coal seam in a single well h,min and the vertical stress σ of the main coal seam in a single well V The geostress of the target coal seam in the single-well fracturing is calculated based on well logging and well test data, as well as hydraulic fracturing methods and well logging interpretation.

3. The method for evaluating the compressible sweet spot of deep coalbed methane in structurally complex areas based on stress demarcation as described in claim 2, characterized in that: The stress types include normal fault type, reverse fault type and strike-slip fault type, and the stress type of a single well is determined based on the relationship between the magnitudes of the three stresses; When the vertical stress σ V Maximum horizontal stress σ h,max Minimum horizontal stress σ h,min When the stress type is a normal fault type; When the maximum horizontal stress σ h,max Minimum horizontal stress δ h,min Vertical stress σ V When the stress type is a reverse fault type; When the maximum horizontal stress σ h,max Vertical stress σ V Minimum horizontal stress σ h,min At that time, the stress type is a strike-slip fault.

4. The method for evaluating compressible sweet spots of deep coalbed methane in structurally complex areas based on stress demarcation as described in claim 1, characterized in that: The specific steps of step S3, establishing a compressibility evaluation index system for different structural zones, are as follows: based on coal rock experimental tests and well logging data, the gas content, coal seam thickness, permeability, Langmuir volume, coal body structure, and aquifer distribution in the roof and floor are obtained. Then, the parameters are combined, calculated, and classified to establish a compressibility evaluation index system for different structural zones.

5. The method for evaluating compressible sweet spots of deep coalbed methane in structurally complex areas based on stress demarcation as described in claim 4, characterized in that: The compressibility evaluation index system includes primary evaluation indicators and secondary evaluation indicators. The primary indicator parameters include development potential parameters, coal and rock compressibility parameters, and fracturing and fracture control parameters. The development potential parameters include resource abundance, reservoir pressure gradient, and desorption index. The coal and rock compressibility parameters include coal body compressibility index, structural compressibility index, and aquifer pressure avoidance index. The fracturing and fracture control parameters include coal and rock fracture control index, intra-layer fracture control index, inter-layer stress difference, and horizontal stress difference. Resource abundance, reservoir pressure gradient, desorption index, coal body compressibility index, structural compressibility index, aquifer pressure avoidance index, coal and rock fracture control index, intra-layer fracture control index, inter-layer stress difference between coal seam and roof and floor, and horizontal stress difference of coal seam are used as secondary indicator parameters.

6. The method for evaluating compressible sweet spots of deep coalbed methane in structurally complex areas based on stress demarcation according to claim 1, characterized in that: The quantitative characterization of the compressibility evaluation index system refers to the quantitative characterization of secondary index parameters, specifically including: resource abundance characterization, reservoir pressure gradient characterization, desorption index characterization, coal body compressibility index characterization, structural compressibility index characterization, aquifer pressure avoidance index characterization, coal-rock fracture control index characterization, intra-layer fracture control index characterization, inter-layer stress difference characterization, and horizontal stress difference characterization.

7. The method for evaluating the compressible sweet spot of deep coalbed methane in structurally complex areas based on stress demarcation according to claim 6, characterized in that: The computational model for the resource abundance characterization is as follows: In the formula: R represents resource abundance, in units of 10. 8 m 3 / km 2 ; H 煤 D is the coal seam thickness in meters; D is the coal and rock density in cubic meters per second. 3 / t;Q g This refers to the gas content of the coal seam, expressed in cubic meters (m³). 2 / t; D, Q g Determined comprehensively by drilling logging interpretation and gas content experimental testing and calibration; The reservoir pressure gradient characterization is determined based on well test data; The calculation model for the desorption index is as follows: In the formula: GL is the desorption exponent, with units of m. 3 t -1 MPa -1 V L The pressure is measured in meters (m). 3 / t;P L V is the Langmuir volume, in MPa. L and P L The value of P is determined by a combination of single-well isothermal adsorption experiments and logging curve interpretation; P is the reservoir pressure in MPa; P is determined by multiplying the reservoir pressure gradient by the burial depth. The calculation model for the coal body compressibility index is as follows: In the formula: MF is the coal body compressibility index, which is dimensionless; MG is the coal body structure index, which is the ratio of the cumulative thickness of primary structural coal and fractured coal to the total coal seam thickness, which is dimensionless. The primary structural coal and fractured coal are quantitatively characterized by coal core information and well logging interpretation. MD is the lithology index of the coal seam roof, which is the proportion of sandstone within 2m of the coal seam roof. It is dimensionless, and the sandstone thickness is quantitatively interpreted by cuttings logging and well logging. The calculation model for constructing the appropriate pressure index is as follows: GF=|k|±a In the formula: GF is the structural pressure index, which is dimensionless and indicates the influence of fault complexity and fold deformation degree on fracturing by comprehensively representing the distance from the fault and the curvature; k and a are constant values, which are different for different stress types. k and a are determined by using thin plate theory to quantify the curvature in a plane and to comprehensively represent the production plane calibration of a single well. The calculation model for the aquifer pressure avoidance index is as follows: In the formula: WF is the aquifer pressure avoidance index, in meters; H w The thickness of the aquifer is expressed in meters (m). The porosity of the aquifer is expressed as a percentage (S). w The aquifer's water saturation level (%). The calculation model for the coal and rock fracture control index is as follows: In the formula: MK is the coal and rock fracture control index, in units of 10. -3 μm 2 H L Thickness of bright to semi-bright coal, in meters; H m K represents the total coal seam thickness in meters. m Permeability, in units of 10 -3 μm 2 H L K m Core testing and well test calibration were used respectively, and quantitative calculations were performed based on comprehensive interpretation of well logging curves. The calculation model for the intralayer controlled seam index is as follows: In the formula: CK is the intra-layer joint control coefficient, in MPa; a is the number of interlayers, in units; σ 夹min σ represents the minimum horizontal stress in the interlayer, expressed in MPa. 煤min σ represents the minimum horizontal stress in a coal seam, expressed in MPa. 夹min σ 煤min All were quantitatively characterized using well logging interpretation; The interlayer stress difference is the minimum horizontal stress difference between the coal seam and the roof and floor. The interlayer stress difference is characterized by quantitative stress characterization based on well logging curves. The horizontal stress difference is the difference between the maximum and minimum horizontal stress in the coal seam. The method for characterizing the horizontal stress difference is to quantitatively characterize the coal seam stress based on well logging curves.

8. The method for evaluating the compressible sweet spot of deep coalbed methane in structurally complex areas based on stress demarcation according to claim 1, characterized in that: Step S5 specifically involves: adjusting the primary index parameter A according to the stress type of each structural zone. i and secondary indicator parameter A ij The weight 'a' of the primary indicator parameter is determined using a subjective weighting method. i and the weight of the secondary indicator parameter a ij ; When the stress type of the structural partition is reverse fault, a1≥a2>a3, and When the stress type of the structural partition is a normal / strike-slip fault type, a1 ≤ a2 < a3, and 9. The method for evaluating compressible sweet spots of deep coalbed methane in structurally complex areas based on stress demarcation according to claim 1, characterized in that: The membership function is a piecewise linear function composed of three levels: good, medium, and poor.

10. The method for evaluating compressible sweet spots of deep coalbed methane in structurally complex areas based on stress demarcation according to claim 9, characterized in that: The expression for the membership function is: In the formula: A xy Weights are assigned to different evaluation index parameters at each level, dimensionless, x = 1, 2, 3…; y = 1, 2, 3…; A represents different evaluation index parameters at each level; A0 and A1 are boundary parameters. The correlation between secondary index parameters of different stress types and the maximum daily gas production of the vertical well is comprehensively and quantitatively characterized, where A0 corresponds to a maximum daily gas production of 2000 m³ / s of the vertical well. 3 / d corresponds to the index parameter value; A1 corresponds to a maximum daily gas production of 5000m³ for the direct well. 3 / d corresponds to the index parameter value; K and B are the constant parameters of the piecewise function, which are quantitatively determined according to the boundary parameters and the type of piecewise function.

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