Geothermal well wall instability evaluation method for coal-geotherm collaborative mining

Abstract

The invention discloses a geothermal well wall instability evaluation method for coal-geothermal collaborative mining, which comprises the following steps: collecting data, and determining physical and mechanical parameters of different rock stratums; establishing a three-dimensional geologic model; assigning the physical and mechanical parameters to the three-dimensional geomechanical model to obtain a three-dimensional geomechanical model; extracting three-dimensional geomechanical model parameters, and constructing a mine rock mass multi-field coupling three-dimensional mechanical model; simplifying the mechanical model into a floor stress model under mining influence, and calculating various stresses in a rock stratum where the geothermal well is located; establishing a geothermal well circumference stress solving model; constructing a geothermal well peripheral stress solving model in a coal-heat co-mining mode to obtain geothermal well peripheral stress; the well periphery stress of the geothermal well is converted into three-direction principal stress, and the maximum principal stress, the middle principal stress and the minimum principal stress are obtained after sorting; and substituting a tensile failure criterion and a strength criterion, and evaluating the well wall stability according to a calculation result. According to the method, geothermal well engineering safety and stability analysis under the coal-heat co-mining environment can be achieved.

Description

Technical Field

[0001] This invention pertains to the collaborative development of mineral resources and new energy sources and the technology of mine rock mechanics, specifically involving a method for evaluating wellbore instability in geothermal wells used in coal-geothermal collaborative mining. Background Technology

[0002] With the transformation of the energy structure and the increasing demand for clean energy, the development of geothermal resources in coal mining areas is receiving increasing attention. Extracting geothermal fluids from coal-bearing strata using drilling technology and coordinating it with coal mining activities (i.e., "co-thermal co-extraction") is an important way to achieve comprehensive resource utilization and increase coal mine revenue and efficiency.

[0003] However, this model faces an extremely complex and severe engineering challenge: wellbore stability in geothermal wells. Traditional stability analysis models for oil and gas wells or geothermal wells are all based on the original geostress field and static geological conditions. However, the coal-thermal co-extraction environment is a highly dynamic and disturbed system, and traditional stability analysis methods for oil and gas wells or geothermal wells are not applicable to this environment. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a method for evaluating the wellbore instability of geothermal wells used in coal-geothermal co-mining, which can realize the safety and stability analysis of geothermal well projects under coal-geothermal co-mining environment.

[0005] Technical solution: This invention provides a method for evaluating wellbore instability in geothermal wells used in coal-geothermal co-mining, comprising:

[0006] Collect borehole data, well logging data, core data, and laboratory test data of the study area, and determine the physical and mechanical parameters of different rock strata through core data and laboratory test data of the study area;

[0007] By using the Kriging interpolation method, borehole data and well logging data are connected from points to form a surface to obtain a three-dimensional geological model; the physical and mechanical parameters of different rock layers are assigned to the three-dimensional geological model to obtain a parameterized three-dimensional geomechanical model.

[0008] Geometric, thermal, and physical-mechanical parameters are extracted from the three-dimensional geomechanical model. These parameters, along with mine production parameters, are combined to construct a multi-field coupled three-dimensional mechanical model of the mine rock mass. This model is then simplified along the strike and dip of the working face into a floor stress model under mining influence. This model reveals the stress distribution patterns in the strata where the geothermal well is located during mining disturbances and geothermal well extraction disturbances. Based on these stress distribution patterns, a floor stress solution model under mining influence is established to calculate the maximum horizontal stress, minimum horizontal stress, vertical stress, and shear stress in the strata where the geothermal well is located.

[0009] A ground stress coordinate system is defined according to the maximum horizontal stress, the minimum horizontal stress and the vertical stress; a wellbore coordinate system is defined, and the components of the maximum horizontal stress, the minimum horizontal stress and the vertical stress in the ground stress coordinate system are converted to be represented in the wellbore coordinate system; the components of each stress in the wellbore coordinate system are converted to be represented in a polar coordinate system, so as to obtain a stress solving model of the geothermal well under static stress;

[0010] The stress solving model of the geothermal well under the coal-heat co-mining mode is constructed by combining the stress solving model of the floor under the mining influence and the stress solving model of the geothermal well, so as to obtain the stress of the geothermal well;

[0011] The stress of the geothermal well is converted into three principal stresses by stress conversion, the three principal stresses are sorted, and the maximum, intermediate and minimum principal stresses are obtained;

[0012] The maximum, intermediate and minimum principal stresses are substituted into the tensile failure criterion and the Mogi-Coulomb strength criterion respectively, and the wellbore stability is evaluated according to the calculation results of and .

[0013] Further, the stress solving model of the floor under the mining influence is established by combining the stress distribution law, so as to calculate the maximum horizontal stress, the minimum horizontal stress, the vertical stress and the shear stress in the rock layer where the geothermal well is located, including:

[0014] It is assumed that there is a hydraulic gradient in the process of mining the confined water, and the change of the pore pressure in the geothermal reservoir due to mining is equivalent to a linear load acting on the three-dimensional mechanical model of the mine rock mass under multi-field coupling;

[0015] The linear load and the confined water pressure in the floor stress model under the mining influence are integrated to establish the stress solving model of the floor under the mining influence;

[0016] Based on the stress solving model of the floor under the mining influence, the stress state of any point in the rock layer where the geothermal well is located is derived by the integral method, so as to obtain the maximum horizontal stress, the minimum horizontal stress, the vertical stress and the shear stress in the rock layer where the geothermal well is located.

[0017] Further, based on the stress solving model of the floor under the mining influence, the stress state of any point in the rock layer where the geothermal well is located is derived by the integral method, so as to obtain the maximum horizontal stress, the minimum horizontal stress, the vertical stress and the shear stress in the rock layer where the geothermal well is located, including:

[0018] According to the stress solving model of the floor under the mining influence, a mechanical equation is established, which is used to study the stress of any point and stress distribution state at the point;

[0019] a microelement at a distance of from the coordinate origin , the concentrated stress at this point is represented as , wherein, , the specific value is determined according to the position of any point of the floor at a distance of from the coordinate origin; The micro stress component generated is as follows:

[0020] ;

[0021] In the formula, represents the vertical stress; represents the horizontal stress; represents the shear stress; represents the load borne by the floor; and respectively represent the horizontal and vertical distances between and or

[0022] In to , the above formula is integrated to obtain the vertical stress, the horizontal stress, and the shear stress under different regions;

[0023] According to the vertical stress, the horizontal stress, and the shear stress under different regions, the stress of the floor along the strike direction and the tendency direction of the working face under the coal-heat co-mining mode is calculated.

[0024] Further, the vertical stress, the horizontal stress, and the shear stress under different regions are expressed as follows:

[0025] .

[0026] Further, the stress of the floor along the strike direction and the tendency direction of the working face under the coal-heat co-mining mode is expressed as follows:

[0027] ;

[0028] ;

[0029] In the formula, is the vertical stress under the influence of mining in the strike direction; is the maximum horizontal stress under the influence of mining in the strike direction; is the shear stress under the influence of mining in the strike direction; ​The vertical stress under the mining influence of the tendency direction; The minimum horizontal stress under the mining influence of the tendency direction; The shear stress under the mining influence of the tendency direction.

[0030] Further, the expression of the stress solving model of the geothermal well surrounding stress under the coal-geothermal co-mining mode is as follows: ;

[0031] In the formula, 、 、 The radial, hoop and axial stress components of the well wall surrounding rock; The liquid column pressure in the wellbore; The shear stress component on the plane; The shear stress component on the plane, The shear stress component on the plane, The shear stress component on the plane; The pore elastic coefficient of the hole; The formation pore pressure; wherein,

[0032] ;

[0033] ;

[0034] ;

[0035] ;

[0036] ;

[0037] ;

[0038] ;

[0039] ;

[0040] .

[0041] Further, the geothermal well surrounding stress is converted into three principal stresses by stress transformation, the three principal stresses are sorted to obtain the maximum, intermediate and minimum principal stresses, which comprises:

[0042] The shear stress and normal stress on the plane of the wellbore axis angle are expressed by the well surrounding stress components; ;

[0043] The normal stress​​​ The angle between the wellbore axis and any plane The first derivative of the condition that 0 is 0, the normal stress is solved, and the result is:

[0044] ;

[0045] The remaining two principal stresses , are solved by the above method, and the three principal stresses are represented as:

[0046] ;

[0047] The three principal stresses are sorted by size, as follows:

[0048] ;

[0049] In the formula, is the maximum principal stress; is the intermediate principal stress; is the minimum principal stress.

[0050] Further, the angle between the wellbore axis and the plane The shear stress and the normal stress of the plane are expressed as follows:

[0051] ;

[0052] In the formula, is the angle between the wellbore axis and any plane; , are the normal stress and shear stress on the plane with the wellbore axis angle .

[0053] Further, the tensile failure criterion and the Mogi-Coulomb strength criterion are expressed as follows:

[0054] ;

[0055] ;

[0056] In the formula, is the Biot coefficient; is the maximum principal stress; is the minimum principal stress; is the formation pore pressure; is the tensile strength of rock; is the internal friction angle; is the cohesion; The angle between the wellbore axis and any plane The shear stress in the plane.

[0057] Further, the maximum, intermediate and minimum principal stresses are substituted into the tensile failure criterion and the Mogi-Coulomb strength criterion , the calculation results of and are used to evaluate the wellbore stability, including:

[0058] When or , it indicates that the surrounding rock of the well is subjected to tensile failure or shear failure; when , none of them is satisfied, it indicates that the surrounding rock of the well does not fail.

[0059] Beneficial effects: Compared with the prior art, the significant technical effects of the present application are as follows: the coal-geothermal cooperative mining geothermal well wall instability evaluation method provided by the present application realizes the dynamic prediction of the stress around the geothermal well under the mining disturbance condition by constructing a stress solving model around the geothermal well in the coal-geothermal cooperative mining mode, and overcomes the traditional static evaluation method; the method constitutes a complete technical system from "coal seam floor stress calculation" to "wellbore stress solving" to "wellbore stress solving in the coal-geothermal cooperative mining process", which significantly improves the safety and reliability of the cooperative mining process, and provides a powerful guarantee for the long-term stable operation of the geothermal well. BRIEF DESCRIPTION OF DRAWINGS

[0060] Figure 1 is the flowchart of the present application;

[0061] Figure 2 is the schematic diagram of the three-dimensional geological model in the present application;

[0062] Figure 3 is the schematic diagram of the floor stress model under the influence of mining in the present application;

[0063] Figure 4 is the schematic diagram of the floor stress solving model under the influence of mining in the present application;

[0064] Figure 5 is the stress diagram of the geothermal well in the stratum in the present application;

[0065] Figure 6 is the wellbore stress distribution model diagram in the present application;

[0066] Figure 7 is the mechanical model decomposition diagram of the wellbore surrounding rock in the present application;

[0067] Figure 8 is the wellbore stress component schematic diagram in the present application. DETAILED DESCRIPTION

[0068] The technical solutions of the present application will be described in detail below in combination with the specific embodiments and the accompanying drawings.

[0069] As Figure 1 shown, the present application is a geothermal well wall instability evaluation method for coal-geothermal collaborative mining, which comprises the following steps:

[0070] S1, collect drilling data, logging data, core data, and indoor test data of the study area, and determine the physical and mechanical parameters of different rock layers through core data and indoor test data of the study area.

[0071] In this step, the rock samples of the study area are tested in the laboratory to obtain the mechanical parameters (elastic modulus, Poisson's ratio, cohesion, internal friction angle, etc.) and thermal physical parameters of each stratum (especially coal rock stratum, geothermal reservoir, and key water-resisting layer).

[0072] S2, connect the drilling data and logging data from points to surfaces by Kriging interpolation method to obtain a three-dimensional geological model; assign the physical and mechanical parameters of different rock layers to the three-dimensional geological model to obtain a parameterized three-dimensional geomechanical model.

[0073] The specific implementation process of step S2 is as follows:

[0074] S2.1, connect the drilling data and logging data from points to surfaces by Kriging interpolation method to construct a high-precision three-dimensional geological model that can accurately reflect the spatial distribution characteristics of stratum interface, fault, structure, etc., as Figure 2 shown, the model shows the three-dimensional distribution of the geological structure of the study area;

[0075] S2.2, assign the physical and mechanical parameters of different rock layers to the three-dimensional geological model to obtain a parameterized three-dimensional geomechanical model that can be used for mechanical analysis;

[0076] S2.3, import the related data and parameters into the three-dimensional geological modeling software to realize the visualization of the three-dimensional geomechanical model and the online viewing function of the related mechanical parameters.

[0077] S3, extract the geometric parameters, thermophysical parameters, and physical and mechanical parameters in the three-dimensional geomechanical model, and combine the geometric parameters, physical and mechanical parameters, and mine production parameters to construct a mine rock mass multi-field coupled three-dimensional mechanical model; simplify the mine rock mass multi-field coupled three-dimensional mechanical model along the strike and inclination of the working face into a floor stress model under the influence of mining, which is used to reveal the stress distribution law of the rock stratum where the geothermal well is located (i.e. the coal seam floor of the working face) in the process of mining disturbance and geothermal well extraction disturbance; combine the stress distribution law to establish a floor stress solving model under the influence of mining, so as to calculate the maximum horizontal stress, minimum horizontal stress, vertical stress, and shear stress in the rock stratum where the geothermal well is located.

[0078] Step S3 is to obtain the stress distribution law of the geothermal well rock stratum under the action of mining disturbance. This step is based on the three-dimensional geomechanical model established in step S2, and the specific implementation process of step S3 is as follows:

[0079] S3.1, extract the geometric parameters, thermophysical parameters, and physical and mechanical parameters in the three-dimensional geomechanical model, and combine the geometric parameters, physical and mechanical parameters, and mine production parameters to construct a mine rock mass multi-field coupled three-dimensional mechanical model.

[0080] It is worth noting that before extracting the parameters, it is necessary to first investigate the layout, mining sequence, and height of the coal mine working face in the coal-heat co-mining area and other relevant engineering characteristics.

[0081] In this step, based on the three-dimensional geomechanical model established in step S2, a mine rock mass multi-field coupled three-dimensional mechanical model that can reflect the unloading effect of mining is simplified and constructed. In the simplification process, only the overburden rock action, confined water action, rock parameters, and mining conditions are considered, and other factors are not considered.

[0082] S3.2, simplify the mine rock mass multi-field coupled three-dimensional mechanical model along the strike and inclination of the working face into a floor stress model under the influence of mining. The floor stress model under the influence of mining reveals the stress distribution law of the rock stratum where the geothermal well is located (i.e. the coal seam floor of the working face) in the process of mining disturbance and geothermal well extraction disturbance.

[0083] In this step, for the convenience of theoretical derivation and analysis, the mine rock mass multi-field coupled three-dimensional mechanical model is reasonably simplified into a floor stress model under the influence of mining (along the strike and inclination of the working face, respectively). The overburden load under the influence of mining is simplified into a linear distribution equivalent load based on the theory of elasticity. The floor stress model under the influence of mining clearly reveals the distribution law of the pressure relief zone , stress concentration zone and undisturbed zone around the goaf, as shown in Figure 3 and Figure 4 . Figure 3 Figure (a) in the above two figures is a floor stress diagram under the coal-heat co-mining mode.Figure 3 (b) of FIG. is a stress diagram of the floor trend under the coal-geothermal co-mining mode.

[0084] S3.3, a stress solving model of the floor under the mining influence is established in combination with the stress distribution law, so that the maximum horizontal stress, the minimum horizontal stress, the vertical stress and the shear stress in the rock layer where the geothermal well is located are calculated. Step S3.3 specifically includes the following steps:

[0085] S3.3.1, equivalent treatment of the water pressure of the confined water in the geothermal reservoir in the process of the geothermal reservoir extraction and pressure reduction:

[0086] It is assumed that there is a hydraulic gradient in the process of the confined water extraction, and the change (pressure reduction) of the pore pressure of the geothermal reservoir due to the extraction is equivalent to a linear load acting on the three-dimensional mechanical model of the multi-field coupling of the mine rock mass, as shown in FIG. Figure 3 .

[0087] S3.3.2, the water pressure of the confined water in the floor stress model under the mining influence is integrated , a stress solving model of the floor under the mining influence is established, as shown in FIG. Figure 4 . The model is the basis for analyzing the stress state of the floor. Figure 4 (a) of FIG. is a two-dimensional calculation model diagram of the floor tendency stress under the coal-geothermal co-mining mode; Figure 4 (b) of FIG. is a two-dimensional calculation model diagram of the floor tendency stress under the coal-geothermal co-mining mode.

[0088] S3.3.3, quantitative characterization of the stress state of the floor:

[0089] Based on the stress solving model of the floor under the mining influence, the stress state of any point in the rock layer where the geothermal well is located is derived by the integral method, so that the maximum horizontal stress, the minimum horizontal stress, the vertical stress and the shear stress in the rock layer where the geothermal well is located are obtained.

[0090] First, the micro stress components generated by the micro load are calculated, and then the integral is performed in the corresponding integral interval, and finally the total stress components (vertical stress , horizontal stress , shear stress ) of any point in the entire model region are obtained. These stress components are the far-field stress input boundary conditions for the subsequent stress analysis around the well. The specific implementation process of step S3.3.3 is as follows:

[0091] S3.3.3.1, according to the stress solving model of the floor under the mining influence shown in FIG. Figure 4 , a mechanical equation (1) is established to study the stress distribution state of any point and in the floor of the coal seam under the coal-geothermal co-mining mode. Specifically, formula (1):

[0092] (1)

[0093] wherein, is the average density of the floor rock, ; is the buried depth of the coal seam, in units of m; ; is the water pressure of the confined water, in units of MPa; ; is the compaction rate of the goaf, in units of %; ; is the pressure-bearing coefficient of the coal wall along the strike; is the pressure-bearing coefficient of the inclined coal wall on both sides; , , and are the lengths of the plastic zones on both sides of the working face, ; is the length of the working face, in units of m; ; is the length of the unloading zone of the geothermal reservoir, in units of m; ; , and are the lengths of the plastic zone, the elastic zone, and the stress release zone along the strike of the working face, in units of m. .

[0094] S3.3.3.2, the micro stress component at an arbitrary position of the floor is calculated, specifically as follows:

[0095] A microelement is taken at a distance of from the coordinate origin, and the concentrated stress at this position is represented as , wherein, , the specific value is determined according to the position of the arbitrary point of the floor at a distance of from the coordinate origin; The micro stress component is as shown in formula (2):

[0096] (2)

[0097] wherein, represents the vertical stress; represents the horizontal stress; represents the shear stress; represents the load borne by the floor; and respectively represent and or horizontal and vertical distance between them, .

[0098] S3.3.3.3、in to Integrate formula (2) to get the vertical stress, horizontal stress, shear stress in different regions, the specific calculation formula is as follows:

[0099] (3)

[0100] S3.3.3.4, according to the vertical stress, horizontal stress, shear stress in different regions, the stress of the floor along the working face direction and the tendency direction under the coal and heat mining mode is calculated.

[0101] In step S3.3.3.4, the total stress components ( , and ) are calculated, and the stress distribution expression in Figure 4 The stress along the working face direction and the tendency direction of the floor under the coal and heat mining mode can be easily obtained, which is respectively the strike stress ( , and ) and the tendency stress ( , and ) obtained in

[0102] The expression is as follows:

[0103] (4)

[0104] (5)

[0105] In the formula, is the vertical stress under the influence of mining in the strike direction; is the maximum horizontal stress under the influence of mining in the strike direction; is the shear stress under the influence of mining in the strike direction; is the vertical stress under the influence of mining in the tendency direction; is the minimum horizontal stress under the influence of mining in the tendency direction; is the shear stress under the influence of mining in the tendency direction.

[0106] S4, define the stress coordinate system according to the maximum horizontal stress, the minimum horizontal stress and the vertical stress; define the wellbore coordinate system, and convert the components of the maximum horizontal stress, the minimum horizontal stress and the vertical stress in the stress coordinate system to the wellbore coordinate system; convert the components of each stress in the wellbore coordinate system to the polar coordinate system to obtain the geothermal well stress solving model under the action of static stress.

[0107] Step S3 has calculated the maximum horizontal stress, the minimum horizontal stress, the vertical stress, and the shear stress in the rock formation where the geothermal well is located. However, to evaluate the instability of the geothermal well, the distribution of the stress around the geothermal well also needs to be considered. In step S4, only the calculation of the stress around the geothermal well under the action of static stress is considered. The specific implementation process of step S4 is as follows:

[0108] S4.1, define the in-situ stress coordinate system according to the maximum horizontal stress, the minimum horizontal stress, and the vertical stress.

[0109] Define the in-situ stress coordinate system (ICS): the maximum horizontal principal stress direction, the minimum horizontal principal stress direction, the vertical stress direction.

[0110] S4.2, define the wellbore coordinate system and convert the components of the maximum horizontal stress, the minimum horizontal stress, and the vertical stress in the in-situ stress coordinate system to the wellbore coordinate system. The specific process is as follows:

[0111] S4.2.1, define the wellbore coordinate system (BCS): take the wellbore axis as the reference, consider the deviation angle (straight well , horizontal well ) and the azimuth angle (the angle between the wellbore direction and the direction).

[0112] S4.2.2, through a strict coordinate transformation matrix, accurately convert the six components of the dynamic in-situ stress affected by mining in step S4.1 to the wellbore coordinate system, as shown in Figure 5 , where Figure 5 (a) in the figure is the far-field stress diagram of the geothermal well in the formation, Figure 5 (b) in the figure is the stress diagram around the geothermal well in the formation,

[0113] The specific solving formula is:

[0114] (6)

[0115] where , , ; is the normal stress in the plane of the wellbore coordinate system; is the normal stress in the plane of the wellbore coordinate system; is the normal stress in the plane of the wellbore coordinate system; is the shear stress in the plane of the wellbore coordinate system ; is the shear stress in the plane of the wellbore coordinate system ; is the shear stress in the plane of the wellbore coordinate system , ; is the inclination angle, and the unit is ; is the azimuth angle, and the unit is .

[0116] S4.3, the components of each stress in the wellbore coordinate system are converted into the polar coordinate system to represent the stress solution model of the geothermal well under the action of static stress. Specifically as follows:

[0117] S4.3.1, the surrounding rock of the well is regarded as a porous elastic medium, and is assumed to be in a plane strain state.

[0118] S4.3.2, the polar coordinate system is established with the wellbore center as the origin .

[0119] S4.3.3, the combined action of the liquid column pressure in the well , the formation pore pressure , and the converted far-field ground stress is considered.

[0120] S4.3.4, as shown in Figures 6-8 , based on the theory of elasticity and the principle of linear superposition, the complex stress state is decomposed into the superposition of multiple simple models (such as only subjected to internal pressure, only subjected to uniaxial ground stress, etc.), which are solved respectively and then superposed.

[0121] Figure 6 the (a) figure in Figure 6 is a schematic diagram of the three-dimensional mechanics model of the well wall confining pressure; Figure 6 the (b) figure in Figure 7 is a schematic diagram of the plane mechanics model of the well wall surrounding rock. According to the linear superposition theory,

[0122] S4.3.5, the complete stress tensor of any point around the geothermal well is finally derived, including the radial stress component of the well wall surrounding rock , the hoop stress component of the well wall surrounding rock , the axial stress component of the well wall surrounding rock , and the shear stress , , . The specific solving formula is:

[0123] (7)

[0124] wherein: , , are the radial, hoop and axial stress components of the wellbore wall surrounding rock, ; are the shear stress components on the plane; are the shear stress components on the plane, are the shear stress components on the plane, ; is the liquid column pressure in the wellbore, ; is the formation pore pressure; is the distance between any position around the well and the wellbore axis, ; is the wellbore radius, m; is the wellbore angle, ; is the Poisson's ratio; is the Biot's coefficient.

[0125] S5, combining the floor stress solving model under the influence of mining and the wellbore stress solving model of geothermal well to construct a geothermal well wellbore stress solving model under the coal-heat co-mining mode, and obtaining the geothermal well wellbore stress.

[0126] The geothermal well wellbore stress solving under the coal-heat co-mining mode is different from the solving of the conventional geothermal well wellbore stress. The geothermal well wellbore stress under the coal-heat co-mining mode is a dynamic change process under the influence of mining. Based on the stress calculation formula of any point in the floor in the mining process (i.e. the floor stress solving model under the influence of mining) in step S3 and the geothermal well wellbore stress calculation formula (i.e. the geothermal well wellbore stress solving model) in step S4, the geothermal well wellbore stress dynamic evolution formula under the influence of mining can be solved, which is as follows:

[0127] The joint formula (4) (5) (7) is solved and calculated, and the expression of the geothermal well wellbore stress solving model under the coal-heat co-mining mode can be obtained, as formula (8):

[0128] (8)

[0129] wherein, , , are the radial, hoop and axial stress components of the wellbore wall surrounding rock; is the liquid column pressure in the wellbore; are the shear stress components on the plane; are the shear stress components on the the shear stress component on the plane, is the shear stress component on the plane; is the borehole elastic coefficient; is the formation pore pressure; is the vertical stress under the influence of mining, with the unit of ; is the maximum horizontal stress under the influence of mining, with the unit of ; is the minimum horizontal stress under the influence of mining, with the unit of . , , The specific values are obtained from formulas (4) and (5). In formula (8),

[0130] ;

[0131] ;

[0132] ;

[0133] ;

[0134] ;

[0135] ;

[0136] ;

[0137] ;

[0138] .

[0139] S6, through stress transformation, converting the stress around the geothermal well into three principal stresses, sorting the three principal stresses to obtain the maximum, intermediate and minimum principal stresses.

[0140] This step is the evaluation of the instability of the well wall under the mode of coal and heat co-mining. The radial stress, hoop stress, axial stress and shear stress component around the geothermal well under the influence of mining can be solved through formula (8). When evaluating the stability of the geothermal well, the maximum, intermediate and minimum principal stresses need to be considered. Through the way of stress transformation, the stress around the geothermal well is converted into three principal stresses, and then the three principal stresses are sorted to obtain the maximum, intermediate and minimum principal stresses. Then, through the tensile strength criterion and mogi-coulomb strength criterion, it is judged whether the geothermal well is stretched and sheared, and finally the stability of the well wall of the geothermal well is evaluated.

[0141] In step S6, the stress state at the well wall is extracted from the solved stress components around the geothermal well, specifically, by stress transformation, the three principal stresses of the well wall rock are solved (sorted by size). The specific implementation process of step S6 is as follows:

[0142] S6.1, the shear stress and normal stress of the plane with the wellbore axis angle of ;

[0143] In this embodiment, the shear stress and normal stress of the plane with the wellbore axis angle of , and the expressions are as follows:

[0144] (9)

[0145] In the formula, is the angle between the wellbore axis and any plane, and the unit is ; , are the normal stress and shear stress of the plane where the angle between the wellbore axis and any plane is , and the unit is .

[0146] S6.2, by solving the condition that the first derivative of the normal stress with respect to the angle between the wellbore axis and any plane is 0, the normal stress is solved:

[0147] (10)

[0148] Solving formula (10) gives:

[0149] (11)

[0150] S6.3, the remaining two principal stresses , are solved by the above method, and the three principal stresses are represented as:

[0151] (12)

[0152] S6.4, sort the three principal stresses by size, specifically as follows:

[0153] (13)

[0154] In the formula, is the maximum principal stress, and the unit is​​​​​ ; is the intermediate principal stress, unit is MPa ; is the minimum principal stress, unit is MPa .

[0155] S7, the three principal stresses are substituted into the tensile failure criterion and Mogi-Coulomb strength criterion , and the wellbore stability is evaluated according to the calculation results of F1 and F2.

[0156] In this scheme, Mogi-Coulomb strength criterion and tensile failure criterion suitable for rock materials are selected as the instability criterion, specifically:

[0157] Since the wellbore collapse mainly occurs in the minimum principal stress direction with high circumferential stress concentration, the shear stress on the wellbore exceeds the shear resistance of the rock, that is, the shear failure depends on the shear strength of the stratum rock, and the shear failure criterion can be written as: . By substituting formula (13), the critical value of the wellbore collapse pressure of the geothermal well in the coal-geothermal co-mining mode can be solved.

[0158] In this embodiment, the tensile failure criterion and Mogi-Coulomb strength criterion , whose expressions are as follows:

[0159] (14)

[0160] (15)

[0161] In the formula, is the Biot coefficient; is the maximum principal stress; is the minimum principal stress; is the formation pore pressure; is the tensile strength of rock; is the internal friction angle, is the cohesion; is the angle between the wellbore axis and the plane on which the shear stress is located.

[0162] When or , it indicates that the surrounding rock of the well occurs tensile failure or shear failure; when , , neither of them is satisfied, it indicates that the surrounding rock of the well does not occur failure.

[0163] By calculating the stress around the well in different mining stages (corresponding to different mining influence degree and extraction pressure relief degree) and substituting into the failure criterion, the dynamic and real-time evaluation of the wellbore stability risk of the geothermal well can be realized.

[0164] The output result can guide the selection of the optimal drilling fluid density window, predict the possible unstable well section and timing, and provide accurate decision support for taking corresponding wellbore strengthening measures (such as plugging and casing).

[0165] The present application is suitable for safety and stability analysis of geothermal well engineering in the process of collaborative exploitation of geothermal energy and coal resources in coal mine area. Especially, it is a comprehensive method for dynamically evaluating the wellbore instability risk of geothermal well in the coal-heat co-mining mode, which integrates geological modeling, multi-field coupled mechanics theory and numerical analysis.

Claims

1. A method for evaluating the instability of a geothermal well wall for coal-geothermal co-extraction, characterized in that, The application relates to a method for calculating the stress of a geothermal well in a coal-geothermal co-mining mode. The method comprises the following steps: Collecting drilling data, logging data, core data and indoor test data of a research area, determining the physical and mechanical parameters of different rock layers through the core data and the indoor test data of the research area; Connecting the drilling data and the logging data from points to surfaces through Kriging interpolation to obtain a three-dimensional geological model; assigning the physical and mechanical parameters of different rock layers to the three-dimensional geological model to obtain a parameterized three-dimensional geomechanical model; Extracting geometric parameters, thermal physical parameters and physical and mechanical parameters from the three-dimensional geomechanical model, and combining the geometric parameters, the thermal physical parameters, the physical and mechanical parameters and mine production parameters to construct a mine rock mass multi-field coupling three-dimensional mechanical model; simplifying the mine rock mass multi-field coupling three-dimensional mechanical model into a floor stress model under the influence of mining along the working face trend and the inclination to reveal the stress distribution law of the rock layer where the geothermal well is located in the process of mining disturbance and geothermal well extraction disturbance; Combining the stress distribution law to establish a floor stress solving model under the influence of mining, so as to calculate the maximum horizontal stress, the minimum horizontal stress, the vertical stress and the shear stress in the rock layer where the geothermal well is located; Defining a geostress coordinate system according to the maximum horizontal stress, the minimum horizontal stress and the vertical stress; defining a wellbore coordinate system and converting the components of the maximum horizontal stress, the minimum horizontal stress and the vertical stress in the geostress coordinate system into the wellbore coordinate system; converting the components of each stress in the wellbore coordinate system into the polar coordinate system to obtain a geothermal well wellbore stress solving model under the action of static stress; Combining the floor stress solving model under the influence of mining and the geothermal well wellbore stress solving model to construct a geothermal well wellbore stress solving model in the coal-geothermal co-mining mode, and obtaining the geothermal well wellbore stress; Converting the geothermal well wellbore stress into three principal stresses through stress transformation, sorting the three principal stresses to obtain the maximum, intermediate and minimum principal stresses; 2. The method for evaluating the instability of the geothermal well wall for coal-geothermal co-extraction according to claim 1, characterized in that, Substituting the maximum, intermediate and minimum principal stresses into tensile failure criterion F1 and Mogi-Coulomb strength criterion F2 respectively, and evaluating the wellbore stability according to the calculation results of F1 and F2. The method for calculating the stress of the geothermal well in the coal-geothermal co-mining mode comprises the following steps: Assuming that there is a hydraulic gradient in the process of mining confined water, and equivalent the pore pressure change of the geothermal reservoir caused by extraction to linear load acting on the mine rock mass multi-field coupling three-dimensional mechanical model; Based on the stress solution model of floor under mining influence, the stress state of any point in the rock layer where the geothermal well is located is derived by integral method, so as to obtain the maximum horizontal stress, minimum horizontal stress, vertical stress and shear stress in the rock layer where the geothermal well is located. Based on the stress solution model of floor under mining influence, the stress state of any point in the rock layer where the geothermal well is located is derived by integral method, so as to obtain the maximum horizontal stress, minimum horizontal stress, vertical stress and shear stress in the rock layer where the geothermal well is located.

3. The method for evaluating the instability of the geothermal well wall for coal-geothermal co-extraction according to claim 2, characterized in that, The stress solving model based on mining influence on floor stress deduces stress state of any point of rock layer where the geothermal well is located through integral method, so as to obtain maximum horizontal stress, minimum horizontal stress, vertical stress and shear stress in the rock layer where the geothermal well is located. The stress solving model based on mining influence on floor stress deduces stress state of any point of rock layer where the geothermal well is located through integral method, so as to obtain maximum horizontal stress, minimum horizontal stress, vertical stress and shear stress in the rock layer where the geothermal well is located. According to the stress solution model of floor under mining influence, the mechanical equation is established to study the stress distribution state of any point of coal seam floor under the mode of coal and heat mining and ​ At a distance of coordinate origin Take a micro-element , the concentration stress at this point is expressed as , wherein , the specific The value is determined according to the position of any point of the bottom plate at a distance of coordinate origin ; The micro stress component As follows: ； wherein represents the vertical stress; represents the horizontal stress; represents the shear stress; represents the load on the floor; and respectively represent and or the horizontal and vertical distance between In To The above formula is integrated to obtain the vertical stress, horizontal stress, and shear stress in different regions. Combining the linear load and the confined water pressure in the floor stress model under the influence of mining to establish the floor stress solving model under the influence of mining; 4. The method for evaluating the instability of the geothermal well wall for coal-geothermal co-extraction according to claim 3, characterized in that, According to the vertical stress, the horizontal stress and the shear stress in different regions, the stress of the floor along the working face trend and the inclination in the coal-geothermal co-mining mode is calculated. 。 5. The method for evaluating the instability of the geothermal well wall for coal-geothermal co-extraction according to claim 3, characterized in that, The expressions of the vertical stress, the horizontal stress and the shear stress in different regions are as follows: ； ； wherein is the vertical stress under the influence of mining in the strike direction; is the maximum horizontal stress under the influence of mining in the strike direction; is the shear stress under the influence of mining in the strike direction; is the vertical stress under the influence of mining in the dip direction; is the minimum horizontal stress under the influence of mining in the dip direction; is the shear stress under the influence of mining in the dip direction.

6. The method for evaluating the instability of the geothermal well wall for coal-geothermal co-extraction according to claim 1, characterized in that, The expressions of the stress of the floor along the working face trend and the inclination in the coal-geothermal co-mining mode are as follows: The expression of the geothermal well wellbore stress solving model in the coal-geothermal co-mining mode is as follows: ； wherein, , , is a radial, hoop, axial stress component of the wellbore surrounding rock; is a fluid column pressure in the wellbore; is a shear stress component on the plane; is a shear stress component on the plane; is a shear stress component on the plane, is a shear stress component on the plane, is a shear stress component on the plane; is a shear stress component on the plane; is a borehole elastic coefficient; is a formation pore pressure; is a vertical stress under the influence of mining; is a maximum horizontal stress under the influence of mining; is a minimum horizontal stress under the influence of mining; wherein, ； ； ； ； ； ； ； ； 。 7. The method for evaluating the instability of the geothermal well wall for coal-geothermal co-extraction according to claim 1, characterized in that, The stress transformation is used to convert the stress around the geothermal well into three principal stresses, the three principal stresses are sorted to obtain the maximum, intermediate and minimum principal stresses, and the method comprises the following steps: The shear stress on the plane with the wellbore axis angle and normal stress and normal stress ; By normal stress The angle between the wellbore axis and any plane The first derivative of the condition that the angle between the wellbore axis and any plane The first derivative of the condition that the angle between the wellbore axis and any plane ； The other two principal stresses are obtained by the above method , The three principal stresses are represented as ； The three principal stresses are sorted according to the size, and the specific process is as follows: ； wherein is the maximum principal stress; is the intermediate principal stress; is the minimum principal stress.

8. The method for evaluating the instability of the geothermal well wall for coal-geothermal co-exploration according to claim 7, characterized in that, The wellbore axis included angle is Shear stress And normal stress The expression is as follows: ； wherein is the angle between the wellbore axis and any plane; , are the normal and shear stresses on the plane with the wellbore axis angle face.

9. The method for evaluating the instability of the geothermal well wall for coal-geothermal co-exploration according to claim 1, characterized in that, The tensile failure criterion F1 and the Mogi-Coulomb strength criterion F2 are expressed as follows: ； ； wherein, is the bio coefficient; is the maximum principal stress; is the minimum principal stress; is the formation pore pressure; is the rock tensile strength; is the internal friction angle; is the cohesion; is the angle between the wellbore axis and any plane is the shear stress on the plane.

10. The method for evaluating the instability of the geothermal well wall for coal-geothermal co-exploration according to claim 1, characterized in that, said maximum, intermediate, and minimum principal stresses are substituted into the tensile failure criterion and the Mogi-Coulomb strength criterion , the wellbore stability is evaluated based on the calculation results of and , including: When or , it means that the surrounding rock of the well occurs tensile failure or shear failure; when , , none of them is satisfied, it means that the surrounding rock of the well does not occur failure.

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