CN116663276A - Synchronous inversion method for coal bed gas pressure and permeability - Google Patents
Synchronous inversion method for coal bed gas pressure and permeability Download PDFInfo
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- 239000003245 coal Substances 0.000 title claims abstract description 151
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- 230000001360 synchronised effect Effects 0.000 title claims abstract description 9
- 238000005553 drilling Methods 0.000 claims abstract description 75
- 238000004088 simulation Methods 0.000 claims abstract description 46
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- 238000007789 sealing Methods 0.000 claims abstract description 8
- 239000011159 matrix material Substances 0.000 claims description 24
- 239000011148 porous material Substances 0.000 claims description 20
- 238000001179 sorption measurement Methods 0.000 claims description 11
- 238000009792 diffusion process Methods 0.000 claims description 10
- 230000009977 dual effect Effects 0.000 claims description 9
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 claims description 8
- 238000006073 displacement reaction Methods 0.000 claims description 6
- 238000005065 mining Methods 0.000 claims description 4
- 230000005483 Hooke's law Effects 0.000 claims description 3
- 229910001566 austenite Inorganic materials 0.000 claims description 3
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Abstract
The application provides a synchronous inversion method for coal seam gas pressure and permeability, and belongs to the technical field of coal mine geology and safety. The method solves the problem that the existing test method can not synchronously acquire the gas pressure and the permeability of the coal bed. The technical proposal is as follows: drilling and sealing holes in the underground construction, testing the gas flow of the hole opening, constructing a double-hole/double-seepage gas-solid coupling mathematical model, carrying out numerical calculation according to the geometric model and boundary conditions to obtain a drilling gas flow simulation value, constructing an objective function according to the actual measurement value and the simulation value of the drilling gas flow, and inverting by using an agent optimization algorithm to obtain the original gas pressure and the permeability of the coal bed. The beneficial effects of the application are as follows: the method has the advantages of simple implementation steps, stable and accurate inversion result, capability of reflecting the actual change of the drilling flow more accurately, short test period, saving manpower and material resources, high automation degree, suitability for drilling under the working conditions of layer penetration and layer following, and the like.
Description
Technical Field
The application relates to a synchronous inversion method for coal seam gas pressure and permeability, and belongs to the technical field of coal mine geology and safety.
Background
Coalbed methane pressure and permeability are two key parameters that affect coalbed methane development and the extent of risk of coalbed outburst. The existing underground coal seam gas pressure testing method mainly comprises a direct method for measuring pressure through hole sealing and an indirect method obtained through inverse calculation of parameters such as gas content, gas desorption characteristics or drilling gas flow, and the underground coal seam permeability testing method mainly comprises an indirect method obtained through calculation of parameters such as drilling gas pressure or drilling gas flow.
In the prior art with the publication number of CN110424949A, when drilling holes in a coal bed, the gas emission quantity of the holes is calculated by testing the gas flow and the gas concentration of the holes in real time while drilling, the gas pressure of the coal bed at the drill bit is calculated based on the inversion of the drilling holes and the coal bed permeability parameters, and the gas content of the coal bed is calculated according to the relation between the gas content and the gas pressure. The indirect testing method for the pressure or permeability of the coal bed gas based on the drilling gas flow information takes a gas radial flow equation as a theoretical basis, and ignores the influence of the dynamic evolution of the deformation, the porosity and the permeability of the coal bed on the gas flow, so that the method cannot accurately reflect the actual change of the drilling gas flow, and the test drilling is required to be perpendicular to the coal bed construction, thereby bringing great challenges to the site measuring point selection and the drilling construction. And the test results of the methods are unstable, and the accuracy is difficult to ensure. At present, the permeability test of the coal seam is carried out under the condition of known gas pressure of the coal seam, and the gas pressure and the permeability of the coal seam cannot be synchronously obtained by using the existing method.
Disclosure of Invention
Aiming at the defects of the prior art, the synchronous inversion method for the pressure and the permeability of the coal seam gas is provided, has the advantages of simple steps, high calculation speed and high inversion accuracy, and solves the problem that the existing test method cannot synchronously acquire the pressure and the permeability of the coal seam gas.
In order to achieve the technical aim, the coal seam gas pressure and permeability synchronous inversion method is characterized by constructing a borehole in a well, sealing the borehole, testing the gas flow of the hole, constructing a double-hole/double-permeability gas-solid coupling mathematical model, determining parameters of the double-hole/double-permeability gas-solid coupling mathematical model through a test, generating a geological simulation geometric model, calculating a simulation value corresponding to the borehole gas flow by combining boundary conditions of the geological simulation geometric model, constructing an objective function by utilizing the measured value and the simulation value of the borehole gas flow, and inverting the original gas pressure and the permeability of the coal seam by using an agent optimization algorithm;
the method comprises the following specific steps:
a. sealing the drill holes after the construction of the drill holes is completed, and testing the gas flow values of the drill holes at different times through an orifice flowmeter;
b. constructing a double-pore/double-permeability gas-solid coupling mathematical model, and determining all parameters of the double-pore/double-permeability gas-solid coupling mathematical model based on laboratory and field tests;
c. according to the drilling arrangement and occurrence conditions of coal strata around the drilling, establishing a corresponding geological simulation geometric model of the target coal seam and determining boundary conditions of the geological simulation geometric model;
d. taking the original gas pressure and permeability of the target coal seam as input variables, and calling a solid mechanics and Darcy law module in COMSOL Multiphysics with MATLAB software to simulate and calculate a simulation value corresponding to the drilling gas flow according to the double-pore/double-permeability gas-solid coupling mathematical model, double-pore/double-permeability gas-solid coupling mathematical model parameters, a geological simulation geometric model and geological simulation geometric model boundary conditions;
e. constructing an objective function according to the actual measurement value and the simulation value of the drilling gas flow, giving an optimization parameter constraint condition and an optimization stop condition, and calling a proxy optimization solver in COMSOL Multiphysics with MATLAB software to invert to obtain the original gas pressure and permeability of the coal bed;
f. according to the original gas pressure of the coal bed and the information acquired on site: the porosity of the coal bed, the density of the coal bed, the temperature of the coal bed and the adsorption constant of the coal body are further calculated, and the protruding danger of the coal bed in the range of the geological simulation geometric model is predicted according to the original gas pressure of the coal bed and the gas content of the coal bed.
2. The method for synchronously inverting the pressure and permeability of the coal seam gas according to claim 1, wherein the bedding drilling or the layer penetrating drilling of the target coal seam construction is required to ensure that the coal seam within the range of 50m of the drilling is free of geological structures, aquifers and karst caves during construction, is not influenced by mining, gas drainage and other artificial pressure relief, the bedding drilling length is more than 50m, and an air chamber of 1-2m is reserved at the front end of the drilling after the drilling is sealed.
Further, the dual pore/dual permeability gas-solid coupling mathematical model is:
the coal is regarded as a dual pore medium comprising cracks and matrixes, the deformation of the coal body is small elastic deformation, the coal body obeys the generalized Hooke's law, the gas is ideal gas and is simultaneously stored in the cracks and the matrixes in a free state and an adsorption state, after drilling construction, most of the gas in the cracks and a small part of the gas in the matrixes enter the drilling holes in a seepage mode, and the rest of the gas flows between the cracks and the matrixes in a diffusion mode, and the mathematical expression is as follows:
wherein E is the modulus of elasticity of the coal body and MPa; v is the poisson ratio of the coal body; u (u) i,jj And u j,ji In the form of tensors of displacement of the coal body, wherein the first subscript indicates the displacement component u in the i and j directions, respectively i And u j The second subscripts denote u, respectively i And u j Partial derivative u in the j direction i,j And u j,j The third subscripts denote u respectively i,j And u j,j Partial derivatives in the j and i directions; alpha m And alpha f The specific austenite coefficients, alpha, of the matrix and the fracture, respectively m ＝K/K m -K/K s ，α f ＝1-K/K m ；K、K m And K s Bulk modulus of coal body, coal matrix and coal skeleton, MPa; p is p m,i And p f,i Respectively the matrix gas pressure p m And fracture gas pressure p f Partial derivative in i direction, MPa; e, e L Is the langmuir strain constant; p (P) L Is the Langmuir pressure constant, MPa; f (f) i Is the volumetric force component in the i direction, MPa; phi (phi) m And phi f Porosity of the matrix and the fissure, respectively; ρ ga Is the gas density in standard state, kg/m 3 ；ρ c Is the density of coal mass, kg/m 3 ；V L Is Langmuir volume constant, m 3 /kg; r is an ideal gas constant, J/(mol.K); t is the temperature of the coal body, K; m is M g Is gas molar mass, kg/mol;is Hamiltonian; mu is the dynamic viscosity of the gas, pa.s; k (k) m And k f The permeability, mD, of the matrix and the fissure, respectively; d (D) f0 、D a0 For initial diffusion coefficient, m 2 /s；D r For the residual diffusion coefficient, m 2 /s；λ f And lambda (lambda) a Is the attenuation coefficient s -1 ；L m Is the matrix spacing, m; phi (phi) m0 And phi f0 Initial porosity of the matrix and the fracture, respectively; />Is the average stress, MPa;is the initial average stress, MPa; p is p 0 The original gas pressure of the coal bed is MPa; f (f) 1 And f 2 Is the adsorption coefficient; ζ is a dimensionless coefficient representing a ratio of the initial permeability of the matrix to the initial permeability of the coal seam; k (k) 0 Is the original permeability of the coal bed, mD.
Further, defining the numerical calculation of the drilling gas flow as a packaging function, wherein in the function, the coal bed gas pressure and the permeability are input variables, the drilling gas flow is output variables, and the time point defined by the output variables is consistent with the time point of on-site observation data;
the geological simulation geometric model is a two-dimensional geological geometric model or a three-dimensional geological geometric model, the calculated amount of the two-dimensional geological geometric model is obviously smaller than that of the three-dimensional geological geometric model, and the calculation efficiency is effectively improved by selecting the two-dimensional geological geometric model under the condition that the calculation results are similar; the calculation formula of the drilling gas flow is as follows:
borehole gas flow simulation value q when constructing a two-dimensional geologic geometric model cal Line integral calculation using borehole boundary darcy's velocity:
borehole gas flow simulation value q when constructing a three-dimensional geological geometry model cal Surface integral calculation using borehole boundary darcy's velocity:
wherein L is the length of the air chamber, and m; omega is the borehole boundary.
Further, the objective function is expressed by the mean absolute percentage error MAPE of the measured and simulated values of the borehole gas flow:
wherein N is the number of drilling gas flow measurement points;is the measured value of the gas flow of the drill hole at the ith moment, m 3 /s。
Further, the optimization parameter constraint condition is a value range of initial gas pressure and permeability of the coal seam, and the value range is determined according to historical observation data of the coal seam or the adjacent coal seam where the test site is located.
Further, the optimization stopping condition is the set maximum function calculation number, when the function calculation number meets the stopping condition, the optimization is stopped, the optimal solution is output, and otherwise, the parameter value is changed to continue the optimization.
Further, the calculation formula of the coal seam gas content m is as follows:
the beneficial effects of the application are as follows: according to the application, the drilling gas flow is tested on site, numerical calculation is carried out on the basis of a double-pore/double-permeability gas-solid coupling mathematical model, a geological geometric model and boundary conditions to obtain a drilling gas flow simulation value, an objective function is constructed according to the actual measurement value and the simulation value of the drilling gas flow, and the original gas pressure and permeability of the coal bed are obtained by inversion of a proxy optimization algorithm, so that the problem that the coal bed gas pressure and permeability cannot be synchronously obtained by the existing test method is solved. Compared with the traditional testing method, the method has the advantages of being reliable in theory, stable and accurate in result, capable of reflecting actual change of drilling flow more accurately, short in testing period, saving in manpower and material resources, high in automation degree, suitable for drilling under most working conditions, and the like, can be widely applied to coal seam gas parameter testing, coal seam outburst risk prediction, gas extraction, outburst prevention effect inspection and the like, and has a certain reference significance for inverse analysis research based on complex numerical models.
Drawings
FIG. 1 is a schematic flow chart of a synchronous inversion method for coal seam gas pressure and permeability.
FIG. 2 shows parameters of a dual pore/dual permeability gas-solid coupled mathematical model according to an embodiment of the present application.
FIG. 3 is a schematic diagram of a geologic simulation geometric model and boundary conditions according to an embodiment of the application.
Fig. 4 is a schematic diagram of the change of the objective function value in the agent optimization process according to the embodiment of the present application.
FIG. 5 is a graph showing inversion results of gas pressure and permeability of a coal seam according to an embodiment of the present application.
FIG. 6 is a graph showing the comparison of the results of numerical simulation of the gas flow rate in the borehole and the field observation data according to the embodiment of the present application.
Detailed Description
The application is described in further detail below with reference to the accompanying drawings.
As shown in FIG. 1, the method for synchronously inverting the pressure and the permeability of coal seam gas comprises the steps of constructing a borehole in a well, sealing the hole, testing the gas flow of the hole, constructing a double-hole/double-permeability gas-solid coupling mathematical model, carrying out numerical calculation according to a geological simulation geometric model and boundary conditions to obtain the simulation value of the gas flow of the borehole, constructing an objective function by utilizing the actual measurement value and the simulation value of the gas flow of the borehole, and inverting the original gas pressure and the permeability of the coal seam by using an agent optimization algorithm.
The method specifically comprises the following steps:
a. sealing the drill holes after the construction of the drill holes is completed, and testing the gas flow values of the drill holes at different times through an orifice flowmeter;
b. constructing a double-pore/double-permeability gas-solid coupling mathematical model, and determining all parameters of the double-pore/double-permeability gas-solid coupling mathematical model based on laboratory and field tests;
c. according to the arrangement of the drill holes and occurrence conditions of coal strata around the drill holes, establishing a corresponding geological geometric model and determining boundary conditions of the geological geometric model; the method comprises the steps of carrying out a first treatment on the surface of the
d. Taking the original gas pressure and permeability of the coal bed as input variables, and calling a solid mechanics and Darcy law module in COMSOL Multiphysics with MATLAB software to calculate the simulation value of the drilling gas flow according to the double-pore/double-permeability gas-solid coupling mathematical model, the double-pore/double-permeability gas-solid coupling mathematical model parameters, the geological geometric model and the boundary conditions of the geometric model;
e. constructing an objective function according to the actual measurement value and the simulation value of the drilling gas flow, giving an optimization parameter constraint condition and an optimization stop condition, and calling a proxy optimization solver in COMSOL Multiphysics with MATLAB software to invert to obtain the original gas pressure and permeability of the coal bed;
f. and further calculating the gas content of the coal bed according to the gas pressure of the coal bed, the porosity of the coal bed, the density of the coal bed, the temperature of the coal bed and the adsorption constant of the coal body, and predicting the outburst risk of the coal bed in the range of the geometric model according to the gas pressure of the coal bed and the gas content of the coal bed.
In the step a, when drilling construction is carried out, the coal seam within the 50m range of the drilling is ensured to have no geological structure, water-bearing layer and karst cave, and is not influenced by mining, gas drainage and other artificial pressure relief, the length of the bedding drilling is more than 50m, and an air chamber of 1-2m is reserved at the front end of the drilling after the drilling is sealed.
Wherein, the dual pore/dual permeability gas-solid coupling mathematical model is specifically:
the coal is regarded as a dual pore medium comprising cracks and matrixes, the deformation of the coal body is small elastic deformation, the coal body obeys the generalized Hooke's law, the gas is ideal gas and is simultaneously stored in the cracks and the matrixes in a free state and an adsorption state, after drilling construction, most of the gas in the cracks and a small part of the gas in the matrixes enter the drilling holes in a seepage mode, and the rest of the gas flows between the cracks and the matrixes in a diffusion mode, and the mathematical expression is as follows:
wherein E is the modulus of elasticity of the coal body and MPa; v is the poisson ratio of the coal body; u (u) i,jj And u j,ji In the form of tensors of displacement of the coal body, wherein the first subscript indicates the displacement component u in the i and j directions, respectively i And u j The second subscripts denote u, respectively i And u j Partial derivative u in the j direction i,j And u j,j The third subscripts denote u respectively i,j And u j,j Partial derivatives in the j and i directions; alpha m And alpha f The specific austenite coefficients, alpha, of the matrix and the fracture, respectively m ＝K/K m -K/K s ，α f ＝1-K/K m ；K、K m And K s Bulk modulus of coal body, coal matrix and coal skeleton, MPa; p is p m,i And p f,i Respectively the matrix gas pressure p m And fracture gas pressure p f Partial derivative in i direction, MPa; e, e L Is the langmuir strain constant; p (P) L Is the Langmuir pressure constant, MPa; f (f) i Is the volumetric force component in the i direction, MPa; phi (phi) m And phi f Porosity of the matrix and the fissure, respectively; ρ ga Is the gas density in standard state, kg/m 3 ；ρ c Is the density of coal mass, kg/m 3 ；V L Is Langmuir volume constant, m 3 /kg; r is an ideal gas constant, J/(mol.K); t is the temperature of the coal body, K; m is M g Is gas molar mass, kg/mol;is Hamiltonian; mu is the dynamic viscosity of the gas, pa.s; k (k) m And k f The permeability, mD, of the matrix and the fissure, respectively; d (D) f0 、D a0 For initial diffusion coefficient, m 2 /s；D r For the residual diffusion coefficient, m 2 /s；λ f And lambda (lambda) a Is the attenuation coefficient s -1 ；L m Is the matrix spacing, m; phi (phi) m0 And phi f0 Initial porosity of the matrix and the fracture, respectively; />Is the average stress, MPa; />Is the initial average stress, MPa; p is p 0 The original gas pressure of the coal bed is MPa; f (f) 1 And f 2 Is the adsorption coefficient; ζ is a dimensionless coefficient representing a ratio of the initial permeability of the matrix to the initial permeability of the coal seam; k (k) 0 Is the original permeability of the coal bed, mD.
In the step d, the numerical calculation of the drilling gas flow is defined as a packaging function, in the function, the coal bed gas pressure and the permeability are input variables, the drilling gas flow is output variables, and the time point defined by the output variables is consistent with the time point of on-site observation data; the calculation formula of the drilling gas flow is as follows:
borehole gas flow simulation value q when constructing a two-dimensional geologic geometric model cal Line integral calculation using borehole boundary darcy's velocity:
borehole gas flow simulation value q when constructing a three-dimensional geological geometry model cal Surface integral calculation using borehole boundary darcy's velocity:
wherein L is the length of the air chamber, and m; omega is the borehole boundary.
The implementation process of the encapsulation function definition algorithm is as follows:
function out＝a18(x,y)
import com.comsol.model.*
import com.comsol.model.util.*
model＝ModelUtil.create('Model')；
model. Hist. Disable; % disabled model history
model.label('a18_dp.mph')；
pstr= [ num2str (x (1)), '[ MPa ]' ]; % input variable coal seam gas pressure
pstr2= [ num2str (y (1)), '[ mD ]' ]; % input variable coal seam permeability
model. Param. Set ('p 0', pstr); % pass input variable
model. Param. Set ('k 0', pstr 2); % pass input variable
… (intermediate code is automatically generated from COMSOL to m-file, and is not modified and is omitted here)
a=mphglobal (model, 'qt', 'dataset', 'dset2','t', [1:1:20 ]); % output variable drilling gas flow
out＝a'；
Wherein the objective function is represented by the Mean Absolute Percentage Error (MAPE) of the measured and simulated values of the borehole gas flow:
wherein N is the number of drilling gas flow measurement points;is the measured value of the gas flow of the drill hole at the ith moment, m 3 /s。
The implementation process of the objective function construction algorithm is as follows:
function R2＝fitness_a18(a)
p0=a (1); % coal seam gas pressure
k0 =a (2); % coal bed permeability
xcatul= [2.15E-05 1.30E-05 1.10E-05.1.03e-05.9.83e-06.50E-06.9.17E-06.9.00E-06.83E-06.8.83E-06.83E-06.67E-06.8.50E-06.50E-06.8.33E-06.8.17E-06.17E-06 ]; % borehole gas flow field test value
xmoni=a18 (p 0, k 0); % borehole gas flow simulation
R2=sum (((xmoni-xcatul)/xcatul) & lt 2 & gt)/20; % objective function
end
In the step e, the constraint condition of the optimization parameter is a value range of initial gas pressure and permeability of the coal seam, and the value range can be determined according to historical observation data of the coal seam or the adjacent coal seam where the test site is located.
In the step e, the optimization stopping condition is the maximum function calculation number, when the function calculation number meets the stopping condition, the optimization is stopped, the optimal solution is output, and otherwise, the parameter value is changed to continue the optimization.
The agent optimization algorithm is realized by the following steps:
clc
clear
close all
lb= [0.2,0.001]; % parameter constraint lower bound
ub= [2,0.1]; % parameter constraint upper bound
Maxn=300; % maximum function count
Calculation of% parameters (SG)
rng default％For reproducibility
options＝optimoptions('surrogateopt','MaxFunctionEvaluations',MaxN,'PlotFcn',@surrogateopt plot,'Display','iter')；
fun= @ (a) fitness_a18 (a); % transfer of independent variables for optimization function
[ a, fval, exitflag, output ] = surrogateopt (fun, lb, ub, [ ], [ ], [ ], [ ], options); invocation of a% proxy optimization solver
In the step f, the calculation formula of the coal seam gas content m is as follows:
the concrete example of the synchronous inversion method for the pressure and the permeability of the coal seam gas is as follows:
two penetrating holes are constructed on a coal and gas outburst coal bed in a mine bottom drainage roadway, geological structures, water-bearing layers and karst cave are avoided within the 50m range of the holes, and the effects of mining, gas drainage and other artificial pressure relief are avoided. After the drilling construction is finished, the holes are sealed, air chambers about 1m are reserved, and the gas flow value of the holes in the natural emission 20d is tested through an orifice flowmeter. Based on laboratory and field tests, the parameters required for the models of elastic modulus, poisson's ratio, bulk modulus, langmuir constant, initial porosity, adsorption coefficient, diffusion coefficient, and attenuation coefficient were determined as shown in fig. 2. According to the two groups of drilling arrangements and occurrence conditions of the coal rock stratum, a two-dimensional geometric model with the size of 50m multiplied by 50m is established, as shown in figure 3, a load boundary with the size of 4.5MPa is applied to the top of the model, and the overlying rock stratum is simulatedThe pressure, the two sides of the model and the bottom apply a roller supporting boundary and a fixed boundary respectively. Pressure boundaries of 1atm (absolute pressure) are applied around the borehole, and the remaining boundaries are zero flow boundaries. Constraint conditions of gas pressure and permeability of the coal bed are respectively set to be [0.2MPa,2MPa ]]And [0.001mD,0.1mD]The maximum function operand is set to 300. And automatically inverting the gas pressure and the permeability of the coal seam of the two groups of drilling holes according to the programmed program, wherein the change of the objective function value in the inversion process is shown in figure 4. The parameter inversion results are compared with the coal seam gas pressure actual measurement values, as shown in fig. 5. According to the data comparison result, the relative error between the inversion result and the measured value of the coal seam gas pressure is respectively 0.28% and 4.77%, and is smaller than 5%, so that the requirements of on-site practical application can be met. Substituting the parameter inversion result into the double-hole/double-seepage gas-solid model for numerical calculation to obtain a simulation result of the drilling gas flow, and comparing the simulation result with on-site observation data of the drilling gas flow, as shown in fig. 6, fig. 6 is a diagram of the numerical simulation result of the drilling gas flow of the 1# test drilling and the 2# test drilling and the on-site observation data. According to the data comparison result, the simulation result of the drilling gas flow is basically consistent with the field observation data, so that the constructed double-hole/double-seepage gas-solid model can effectively reflect the actual change rule of the drilling gas flow. According to the adsorption constant and environmental parameters of the coal body, further calculating that the gas content of the coal seam of the two groups of holes is 7.65m respectively 3 T and 7.59m 3 And (t) predicting that the coal bed within the range of the two sets of drilling geometric models has outstanding danger according to the coal bed gas pressure and the coal bed gas content.
The technical features of the present application that are not described in the present application may be implemented by or using the prior art, and are not described in detail herein, but the above description is not intended to limit the present application, and the present application is not limited to the above examples, but is also intended to be within the scope of the present application by those skilled in the art.
Claims (8)
1. The synchronous inversion method for the gas pressure and the permeability of the coal seam is characterized by constructing a borehole in a well, sealing the hole, testing the gas flow of the hole, constructing a double-hole/double-permeability gas-solid coupling mathematical model, determining parameters of the double-hole/double-permeability gas-solid coupling mathematical model through a test, generating a geological simulation geometric model, combining boundary conditions of the geological simulation geometric model to calculate a simulation value of the gas flow of the corresponding borehole, constructing an objective function by utilizing the actual measurement value and the simulation value of the gas flow of the borehole, and inverting the original gas pressure and the permeability of the coal seam through a proxy optimization algorithm;
the method comprises the following specific steps:
a. sealing the drill holes after the construction of the drill holes is completed, and testing the gas flow values of the drill holes at different times through an orifice flowmeter;
b. constructing a double-pore/double-permeability gas-solid coupling mathematical model, and determining all parameters of the double-pore/double-permeability gas-solid coupling mathematical model based on laboratory and field tests;
c. according to the drilling arrangement and occurrence conditions of coal strata around the drilling, establishing a corresponding geological simulation geometric model of the target coal seam and determining boundary conditions of the geological simulation geometric model;
d. taking the original gas pressure and permeability of the target coal seam as input variables, and calling a solid mechanics and Darcy law module in COMSOL Multiphysics with MATLAB software to simulate and calculate a simulation value corresponding to the drilling gas flow according to the double-pore/double-permeability gas-solid coupling mathematical model, double-pore/double-permeability gas-solid coupling mathematical model parameters, a geological simulation geometric model and geological simulation geometric model boundary conditions;
e. constructing an objective function according to the actual measurement value and the simulation value of the drilling gas flow, giving an optimization parameter constraint condition and an optimization stop condition, and calling a proxy optimization solver in COMSOL Multiphysics with MATLAB software to invert to obtain the original gas pressure and permeability of the coal bed;
f. according to the original gas pressure of the coal bed and the information acquired on site: the porosity of the coal bed, the density of the coal bed, the temperature of the coal bed and the adsorption constant of the coal body are further calculated, and the protruding danger of the coal bed in the range of the geological simulation geometric model is predicted according to the original gas pressure of the coal bed and the gas content of the coal bed.
2. The method for synchronously inverting the pressure and permeability of the coal seam gas according to claim 1, wherein the bedding drilling or the layer penetrating drilling of the target coal seam construction is required to ensure that the coal seam within the range of 50m of the drilling is free of geological structures, aquifers and karst caves during construction, is not influenced by mining, gas drainage and other artificial pressure relief, the bedding drilling length is more than 50m, and an air chamber of 1-2m is reserved at the front end of the drilling after the drilling is sealed.
3. The simultaneous inversion method of the pressure and the permeability of the coal-bed gas according to claim 1, wherein the dual-pore/dual-permeability gas-solid coupling mathematical model is as follows:
the coal is regarded as a dual pore medium comprising cracks and matrixes, the deformation of the coal body is small elastic deformation, the coal body obeys the generalized Hooke's law, the gas is ideal gas and is simultaneously stored in the cracks and the matrixes in a free state and an adsorption state, after drilling construction, most of the gas in the cracks and a small part of the gas in the matrixes enter the drilling holes in a seepage mode, and the rest of the gas flows between the cracks and the matrixes in a diffusion mode, and the mathematical expression is as follows:
wherein E is the modulus of elasticity of the coal body and MPa; v is the poisson ratio of the coal body; u (u) i,jj And u j,ji In the form of tensors of displacement of the coal body, wherein the first subscript indicates the displacement component u in the i and j directions, respectively i And u j The second subscripts denote u, respectively i And u j Partial derivative u in the j direction i,j And u j,j The third subscripts denote u respectively i,j And u j,j Partial derivatives in the j and i directions; alpha m And alpha f The specific austenite coefficients, alpha, of the matrix and the fracture, respectively m ＝K/K m -K/K s ，α f ＝1-K/K m ；K、K m And K s Bulk modulus of coal body, coal matrix and coal skeleton, MPa; p is p m,i And p f,i Respectively the matrix gas pressure p m And fracture gas pressure p f Partial derivative in i direction, MPa; e, e L Is the langmuir strain constant; p (P) L Is the Langmuir pressure constant, MPa; f (f) i Is the volumetric force component in the i direction, MPa; phi (phi) m And phi f Porosity of the matrix and the fissure, respectively; ρ ga Is the gas density in standard state, kg/m 3 ；ρ c Is the density of coal mass, kg/m 3 ；V L Is Langmuir volume constant, m 3 /kg; r is an ideal gas constant, J/(mol.K); t is the temperature of the coal body, K; m is M g Is gas molar mass, kg/mol;is Hamiltonian; mu is the dynamic viscosity of the gas, pa.s; k (k) m And k f The permeability, mD, of the matrix and the fissure, respectively; d (D) f0 、D a0 For initial diffusion coefficient, m 2 /s；D r For the residual diffusion coefficient, m 2 /s；λ f And lambda (lambda) a Is the attenuation coefficient s -1 ；L m Is the matrix spacing, m; phi (phi) m0 And phi f0 Initial porosity of the matrix and the fracture, respectively; />Is the average stress, MPa; />Is the initial average stress, MPa; p is p 0 The original gas pressure of the coal bed is MPa; f (f) 1 And f 2 Is the adsorption coefficient; ζ is a dimensionless coefficient representing a ratio of the initial permeability of the matrix to the initial permeability of the coal seam; k (k) 0 Is the original permeability of the coal bed, mD.
4. The method for simultaneous inversion of coal-bed gas pressure and permeability according to claim 1, wherein the numerical calculation of the borehole gas flow rate is defined as a packaging function in which the coal-bed gas pressure and permeability are input variables, the borehole gas flow rate is output variables, and the time point defined by the output variables is consistent with the time point of on-site observation data;
the geological simulation geometric model is a two-dimensional geological geometric model or a three-dimensional geological geometric model, the calculated amount of the two-dimensional geological geometric model is obviously smaller than that of the three-dimensional geological geometric model, and the calculation efficiency is effectively improved by selecting the two-dimensional geological geometric model under the condition that the calculation results are similar; the calculation formula of the drilling gas flow is as follows:
borehole gas flow simulation value q when constructing a two-dimensional geologic geometric model cal Line integral calculation using borehole boundary darcy's velocity:
borehole gas flow simulation value q when constructing a three-dimensional geological geometry model cal Surface integral calculation using borehole boundary darcy's velocity:
wherein L is the length of the air chamber, and m; omega is the borehole boundary.
5. The simultaneous inversion method of coalbed methane pressure and permeability according to claim 1, wherein the objective function is represented by an average absolute percentage error MAPE of the measured and simulated values of the borehole methane flow:
wherein N is the number of drilling gas flow measurement pointsA number;is the measured value of the gas flow of the drill hole at the ith moment, m 3 /s。
6. The method for simultaneous inversion of gas pressure and permeability of a coal seam according to claim 1, wherein the constraint condition of the optimization parameter is a range of values of initial gas pressure and permeability of the coal seam, which is determined according to historical observation data of the coal seam or the adjacent coal seam where the test site is located.
7. The method for simultaneous inversion of gas pressure and permeability in coal seam according to claim 1, wherein the optimal stopping condition is a set maximum function calculation number, when the function calculation number meets the stopping condition, the optimization is stopped, the optimal solution is output, and otherwise, the parameter value is changed to continue the optimization.
8. The simultaneous inversion method of coal-bed gas pressure and permeability according to claim 1, wherein the calculation formula of the coal-bed gas content m is as follows:
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| CN103334739A (en) * | 2013-06-28 | 2013-10-02 | 山东科技大学 | Method and device for determining gas pressure of coal seam |
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| US20210262341A1 (en) * | 2019-06-24 | 2021-08-26 | China University Of Mining And Technology | Inversion calculation method of coal-bed gas parameters of fast test while-drilling |
| CN115983097A (en) * | 2022-12-05 | 2023-04-18 | 陕煤集团神木柠条塔矿业有限公司 | Coal seam gas extraction characteristic parameter rapid inversion method based on borehole extraction data |
| CN116861813A (en) * | 2023-07-05 | 2023-10-10 | 西安科技大学 | Coal bed gas basic parameter dynamic visualization method based on monitoring data calculation |
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| CN103334739A (en) * | 2013-06-28 | 2013-10-02 | 山东科技大学 | Method and device for determining gas pressure of coal seam |
| CN107330220A (en) * | 2017-07-20 | 2017-11-07 | 中国矿业大学(北京) | Consider this coal seam concordant gas drilling design method of permeability anisotropy |
| CN108732076A (en) * | 2018-05-18 | 2018-11-02 | 西安科技大学 | A kind of coal seam hydraulic fracture Permeability Prediction method |
| US20210262341A1 (en) * | 2019-06-24 | 2021-08-26 | China University Of Mining And Technology | Inversion calculation method of coal-bed gas parameters of fast test while-drilling |
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