CN109446602B - Numerical test method for extracting gas from ultra-thick coal seam through ground vertical drilling - Google Patents
Numerical test method for extracting gas from ultra-thick coal seam through ground vertical drilling Download PDFInfo
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Abstract
The invention discloses a numerical test method for extracting gas from an extra-thick coal seam by using a ground vertical borehole, which utilizes gas coal rock fracture process analysis software and comprises the steps of establishing engineering documents, establishing a numerical calculation model and grid division, determining key parameters, boundary conditions and control conditions, calculating numerical values and analyzing a numerical result graph. The invention can make up the defects of high cost and difficult operation of large-scale physical experiments and field experiments, ensures the effectiveness and reliability of field ground drilling construction, is favorable for ensuring the safe production of mines, and has very important theoretical significance and practical value for improving the economic benefit and the social benefit of coal mines.
Description
Technical Field
The invention relates to the technical field of coal mine safety production, in particular to a numerical test method for extracting gas from an extra-thick coal seam by using a ground vertical drill hole.
Background
The coal mine gas disaster accident is called as a first killer of coal mine safety production and has strong destructiveness, and gas extraction is a fundamental measure for preventing the coal mine gas accident. As can be seen from the new classification method of gas extraction proposed by professor Shu Qixiang, the ground drilling extraction is suitable for different coal seam mining stages and different gas source spaces. In recent years, the gas extraction ground drilling technology is widely applied to coal mine sites.
In order to solve the problem of low content and high content of gas of certain ultra-thick coal seams and gush out under the high-strength mining condition, some domestic coal mines (like coal group tower mountain coal mines) have been tested by extracting the gas of the coal seams through ground vertical drilling, and the extraction effect is obvious.
However, large-scale physical experiments and field experiments are high in cost and difficult to operate, and a method for performing numerical tests on gas of the extra-thick coal seam pre-extracted by the ground vertical drilling is needed to ensure the effectiveness and reliability of field ground drilling construction and provide basic data and experience for ground drilling of other mines or extra-thick coal seams.
In the field of numerical test calculation, a Real Fracture Process Analysis (RFPA) is a material fracture Process Analysis numerical calculation method based on finite element stress Analysis and statistical damage theory, and is a numerical test tool capable of simulating the whole Process from gradual fracture to instability of a material. Especially RFPA 2D The gas analysis version, namely gas coal rock fracture process analysis software, is mainly used for numerical simulation test research in a gas-containing coal rock fracture process, and numerical calculation results can visually and vividly display rock body stress field and gas flow field evolution and acoustic emission space-time distribution characteristics in the gas-containing coal rock fracture process. It has certain application in the fields of mine rock mechanics and gas prevention and control,
however, no RFPA-based method is available at present 2D The gas analysis board carries out numerical test of extracting gas from the ultra-thick coal seam by vertical drilling on the ground, so far, no people basically do the research on the aspect, and the literature is few. How to build numerical calculation models of different drilling positions and different drilling intervals, set parameter assignment, boundary conditions and control conditions, and analyze … … according to the calculation results of the software by using RFPA 2D The gas analysis board carries out numerical test of extracting gas from the ultra-thick coal seam by vertical drilling on the ground.
Disclosure of Invention
The invention aims to solve the technical problem that the existing ground vertical drilling coal seam gas extraction test is difficult to operate with high cost 2D The gas analysis board provides a numerical test method for pre-pumping gas of an extra-thick coal seam by using a ground vertical drilling hole.
The technical scheme adopted by the invention for solving the technical problems is as follows:
a numerical test method for extracting gas from an extra-thick coal seam by using a ground vertical borehole comprises the following steps:
collecting geological data of an actual mine field, and establishing a geological profile engineering document of a ground vertical drilling extraction extra-thick coal seam gas engineering; the contents of the engineering documents comprise the position of a working face, the up-down relation of a well, the coal bed condition, the gas condition, the coal bed top and bottom plate condition, the geological structure condition and the hydrogeological condition;
the coal bed conditions comprise coal bed thickness, coal bed structure, coal bed inclination angle, coal bed hardness, mined coal bed, coal type, stability and mining index;
the gas condition comprises the gas content of the coal bed, the gas pressure and the gas emission quantity;
the coal seam top and bottom plate conditions comprise rock names, thickness and lithology characteristics;
the geological structure comprises faults and folds.
(a) establishing a numerical calculation model for extracting gas from the ultra-thick coal seam by using a ground vertical drilling hole:
specifically determining a construction process, an aperture, a pumping negative pressure and a plugging mode for pumping coal bed gas from a ground vertical borehole according to the actual condition of a construction site; RFPA (radio frequency power amplifier) adopting large finite element analysis and calculation software 2D Firstly, respectively establishing numerical calculation models when the positions of the drilling final holes are positioned at the top of the coal seam, the middle of the coal seam and the bottom of the coal seam, and obtaining an optimal drilling final hole position model through numerical calculation analysis; then respectively establishing numerical calculation models of different drilling hole intervals when the final hole positions of the drilling holes are optimal on the basis; the selection range of the different drilling hole distances comprises 30m, 40m, 50m, 60m, 70m and 80 m;
(b) grid division of a numerical calculation model for extracting gas from an ultra-thick coal seam through a ground vertical drilling hole:
using RFPA 2D The gas analysis version carries out grid division while carrying out numerical test modeling, and the two works are in the same interfaceSetting, namely setting the number of the unit meshes of the calculation model and the actual size of the calculation model in a setting interface according to actual needs; the actual needs comprise the model size, the actual condition proportion, the memory size of the computer server and the computing capacity;
in fact, the RFPA is utilized in step 2 2D The establishment and the grid division of the calculation model performed by the gas analysis version software are performed simultaneously, repeated operation is not needed, the setting steps are simple and clear, and the method is also an RFPA 2D The superiority of gas analysis version software.
according to engineering field test data and physical experiment data, determining key parameters of the numerical calculation model, including homogeneity, elastic modulus, compressive strength, Poisson's ratio, porosity, internal friction angle, compression-tension ratio, air permeability coefficient, gas content coefficient, gas pressure coefficient and permeability coupling coefficient.
in order to enable the established numerical calculation model for extracting the gas of the ultra-thick coal seam from the ground vertical borehole to perform numerical tests at different borehole final hole positions and different borehole intervals under various geological conditions, certain simplified setting is performed on boundary conditions and control conditions of the calculation model, and the method specifically comprises the following steps:
(1) assuming that the rock mechanics parameters conform to Weibull distribution;
(2) judging the rock fracture by adopting a molar-coulomb intensity criterion;
(3) displacement constraint is adopted on two boundaries of the model, rigid constraint is adopted on the bottom boundary, and no matter the displacement constraint or the rigid constraint refers to no displacement deformation;
(4) adding a bedding weak surface with small elastic modulus and small tensile deformation between rock stratums, namely replacing the weak surface between the layers with a linear material;
(5) determining rock components of the old top, the direct bottom and the old bottom of the coal seam, and correspondingly simplifying the rock stratum part above the old top during modeling, namely replacing the pressure of an overlying rock stratum with 0.25 MPa/m;
(6) the loading mode adopts dead weight loading in the vertical direction, namely, the loading in the vertical direction is carried out from the gas-containing coal bed to the old top according to the volume weight of the rock, and the overlying strata part is replaced by vertically downward uniformly distributed force;
(7) the pressure of gas in the coal body is controlled by a water head, namely 1MPa is equal to the height of the water head of 100m, and the gas pressure of the gas in the goaf is set to be 0;
(8) although gas in a coal seam is extracted, the solution type is still regarded as a plane strain problem, a total calculation step is set, the self weight in the Y direction is considered, the unit calculation is a cavity unit method, and the influence of seepage time on model calculation is ignored.
And step 5, numerical calculation:
using RFPA 2D The gas analysis board performs numerical calculation, and the RFPA 2D The numerical calculation process of the gas analysis plate comprises stress calculation analysis, phase change analysis and element phase change analysis.
And 6, analyzing the numerical test result of the gas of the extra-thick coal seam extracted by the ground vertical drilling to obtain the most reasonable final hole position of the drilled hole and the most reasonable spacing between the drilled holes:
firstly, according to the numerical calculation models of the positions of the final holes of the drilled holes at the top of the coal seam, the middle of the coal seam and the bottom of the coal seam, through RFPA 2D And (3) respectively obtaining a coal bed gas extraction stress failure diagram, a coal body stress curve diagram, an acoustic emission diagram, a gas vector cloud diagram and a gas flow curve diagram of the three calculation models through numerical calculation of the gas analysis version, and comparing to obtain the most reasonable final hole position of the drill hole when the coal bed gas is extracted through the ground vertical drill hole.
Secondly, after the best reasonable final hole position of the drill hole is determined in the last step, the model is calculated according to the preliminarily set numerical values of different drill hole distances by combining the actual conditions of the engineering site and through the RFPA 2D Respectively obtaining coal bed gas extraction stress failure diagram, coal body stress curve diagram, acoustic emission diagram, gas vector cloud diagram and gas flow of different drilling hole distance numerical calculation models through numerical calculation of a gas analysis versionA graph for comparative analysis; meanwhile, the drilling input cost and the extraction effect are comprehensively considered, so that the most reasonable drilling distance is obtained when the coal seam gas is extracted by the ground vertical drilling; the selection range of different drill hole spacing comprises 30m, 40m, 50m, 60m, 70m and 80 m.
In step 6, whether the numerical test result has reached the expected reasonable result is judged according to the following criteria:
a coal seam gas extraction stress failure diagram: the gas content and the gas pressure of the coal bed can be reduced by pumping the coal bed gas, and the coal bed can be damaged under the action of the overlying strata pressure. Whether the set position of the final hole of the drill hole and the distance between the drill holes are reasonable or not can be judged by observing the size of the stress failure range in the coal seam gas extraction stress failure diagram. The reasonable final hole position and the borehole interval have the largest stress failure range in the coal seam gas extraction stress failure cloud picture, the whole gas-containing coal seam can be swept, the gas-containing coal seam is not concentrated around the boreholes, namely, the coal seam between the two boreholes is subjected to through failure, and the aim of reducing the gas content and the gas pressure of the coal seam to the maximum extent is fulfilled.
Coal body stress curve diagram: the stress curve numerical value reflects the magnitude of stress values of different positions of a coal body containing gas, the stress values of the coal body between two drill holes at the reasonable final hole position and the drill hole spacing are uniform in the initial extraction stage of the stress curve, and the stress peak value is expanded towards the middle of the two drill holes along with the increase of extraction time; in a gas extraction medium-term stress curve, the stress value is maximum; and in the later period of extraction, the stress peak disappears, the stress values of the gas-containing coal bodies are all lower than a certain value and approximate to a horizontal straight line, which shows that the gas content in the gas-containing coal layer tends to be stable and does not change rapidly. And indicating that the coal body crack between the two drill holes is subjected to through damage. The drilling arrangement mode can effectively reduce the gas content in the coal seam, and a good gas extraction effect is achieved.
Acoustic emission graph: the acoustic emission graph reflects the sweep range of acoustic waves in the coal seam containing gas, namely the expansion range of the cracks, and acoustic emissions of reasonable final hole positions and drilling hole distances are uniformly distributed in the whole coal seam containing gas, so that the cracks can be expanded to the thickness of the whole coal seam after gas extraction; rather than being concentrated around the borehole or sporadically distributed at a location in the coal seam.
Gas vector cloud picture: the gas vector diagram is the trend of gas flow, if the gas content in the gas-containing coal layer is large, the gas vector is in a dense and aggregation state, and the gas content is gradually reduced along with the increase of the extraction time, so that the gas vector diagram is in a sparse state. The reasonable gas vectors of the final hole position and the drill hole spacing are changed from concentrated distribution in the early extraction stage to sparse distribution in the later extraction stage, which shows that the final hole position of the drill hole can play a role in reducing the gas content of the coal seam and does not present a serious gathering or sparse state.
Gas flow curve chart: the reasonable gas flow curve of the final hole position and the drill hole interval shows more gentle stages, namely, the stable extraction stage is longer, and the time is 10 to 14 days, namely, the drill hole arrangement mode can more effectively and more fully extract the coal seam gas; otherwise, it is not reasonable, and the drilling arrangement is not preferable.
The invention has the following beneficial effects: the numerical test method for extracting the gas of the ultra-thick coal seam by the ground vertical drilling is realized, the defects of high cost and difficult operation of large-scale physical experiments and field experiments are overcome, the effectiveness and the reliability of field ground drilling construction are ensured, the safety production of a mine is favorably ensured, and the numerical test method has very important theoretical significance and practical value for improving the economic benefit and the social benefit of a coal mine.
Drawings
FIG. 1 is a flow chart of a numerical test method for extracting gas from an ultra-thick coal seam by a ground vertical drilling hole;
FIG. 2 is a schematic diagram of different final hole positions of a ground vertical drilling extraction super-thick coal seam;
FIG. 3 is a schematic diagram of different borehole intervals of a ground vertical borehole extraction super-thick coal seam;
FIG. 4 shows an embodiment using RFPA 2D Setting a schematic diagram of a boundary condition;
FIG. 5 is a graph of the numerical calculations for an example borehole termination location at the top of a coal seam;
FIG. 6 is a graph of the results of numerical calculations for an example borehole termination location at the middle of a coal seam;
FIG. 7 is a graph of the numerical calculations for an example borehole termination location at the bottom of a coal seam;
FIG. 8 is a comparison graph of extracted gas flow when the final hole position of the drilled hole is located at different positions of a coal seam according to the embodiment;
FIG. 9 is a numerical calculation result at a borehole spacing of 50m according to the embodiment;
FIG. 10 is a graph of the stress of the coal seam at a borehole spacing of 50m according to an example.
Detailed Description
The technical solution of the present invention is further illustrated below with reference to the following embodiments and examples.
The specific implementation mode is as follows: as shown in fig. 1, the embodiment is a numerical test method for extracting gas from an ultra-thick coal seam by using a ground vertical borehole, and the method comprises the following steps:
collecting geological data of an actual mine field, and establishing a geological profile engineering document of a ground vertical drilling extraction extra-thick coal seam gas engineering; the contents of the engineering documents comprise the position of a working face, the up-down relation of a well, the coal bed condition, the gas condition, the coal bed top and bottom plate condition, the geological structure condition and the hydrogeological condition;
the coal bed conditions comprise coal bed thickness, coal bed structure, coal bed inclination angle, coal bed hardness, mined coal bed, coal type, stability degree and mining index;
the gas condition comprises the gas content of the coal bed, the gas pressure and the gas emission quantity;
the coal seam top and bottom plate conditions comprise rock names, thickness and lithology characteristics;
the geological structure comprises faults and folds.
(a) establishing a numerical calculation model for extracting gas from the ultra-thick coal seam by using a ground vertical drilling hole:
specifically determining the construction process, the pore diameter, the extraction negative pressure and the plugging mode of extracting coal seam gas from the ground vertical borehole according to the actual condition of a construction site; RFPA (radio frequency power amplifier) adopting large finite element analysis and calculation software 2D Firstly, respectively establishing numerical calculation models when the positions of the drilling final holes are positioned at the top of the coal seam, the middle of the coal seam and the bottom of the coal seam, and obtaining an optimal drilling final hole position model through numerical calculation analysis; then respectively establishing numerical calculation models of different drilling intervals when the final hole positions of the drill holes are optimal on the basis; the selection range of the different drilling hole distances comprises 30m, 40m, 50m, 60m, 70m and 80 m;
(b) grid division of a numerical calculation model for extracting gas from an ultra-thick coal seam through a ground vertical drilling hole:
using RFPA 2D The gas analysis version carries out grid division while carrying out numerical test modeling, the two works are set in the same interface, and the number of unit grid divisions of the calculation model and the actual size of the calculation model are set in the setting interface according to actual needs; the actual requirements comprise the model size, the actual condition proportion, the memory size of the computer server and the computing power;
in fact, RFPA is utilized in step 3 2D The establishment and the grid division of the calculation model performed by the gas analysis version software are performed simultaneously, repeated operation is not needed, the setting steps are simple and clear, and the method is also an RFPA 2D The superiority of gas analysis software.
according to engineering field test data and physical experiment data, determining key parameters of the numerical calculation model, including homogeneity, elastic modulus, compressive strength, Poisson's ratio, porosity, internal friction angle, compression-tension ratio, air permeability coefficient, gas content coefficient, gas pressure coefficient and permeability coupling coefficient.
in order to enable the established numerical calculation model for extracting the gas of the ultra-thick coal seam from the ground vertical borehole to perform numerical tests at different borehole final hole positions and different borehole intervals under various geological conditions, certain simplified setting is performed on boundary conditions and control conditions of the calculation model, and the method specifically comprises the following steps:
(1) assuming that the rock mechanics parameters conform to Weibull distribution;
(2) rock fracture is judged by a molar-coulomb strength criterion;
(3) displacement constraint is adopted on two boundaries of the model, rigid constraint is adopted on the bottom boundary, and no matter the displacement constraint or the rigid constraint refers to displacement-free deformation;
(4) adding a bedding weak surface with small elastic modulus and small tensile deformation between rock stratums, namely replacing the weak surface between the layers with a linear material;
(5) determining rock components of an old top, an immediate bottom and an old bottom of a coal seam, and correspondingly simplifying the rock stratum part above the old top during modeling, namely replacing the pressure of an overlying rock stratum with 0.25 MPa/m;
(6) the loading mode adopts dead weight loading in the vertical direction, namely, the loading in the vertical direction is carried out from the gas-containing coal bed to the old top according to the volume weight of the rock, and the overlying rock layer part is replaced by vertically downward uniformly distributed force;
(7) the pressure of gas in the coal body is controlled by a water head, namely 1MPa is equal to the water head height of 100m, and the gas pressure of the goaf is set to be 0;
(8) although gas in a coal seam is extracted, the solving type is still regarded as a plane strain problem, the total calculation step is set, the deadweight in the Y direction is considered, the unit calculation is a cavity unit method, and the influence of seepage time on model calculation is ignored.
using RFPA 2D The software performs numerical calculation, the RFPA 2D The numerical calculation process of the gas analysis plate comprises stress calculation analysis, phase change analysis and elementary phase change analysis.
And 6, analyzing the numerical test result of the gas of the extra-thick coal seam extracted by the ground vertical drilling to obtain the most reasonable final hole position of the drilled hole and the most reasonable spacing between the drilled holes:
firstly, according to the numerical calculation models of the positions of the final holes of the drilled holes at the top of the coal seam, the middle of the coal seam and the bottom of the coal seam, through RFPA 2D And (3) respectively obtaining a coal bed gas extraction stress failure diagram, a coal body stress curve diagram, an acoustic emission diagram, a gas vector cloud diagram and a gas flow curve diagram of the three calculation models through numerical calculation of the gas analysis version, and comparing to obtain the most reasonable final hole position of the drill hole when the coal bed gas is extracted through the ground vertical drill hole.
Secondly, after the best reasonable drilling final hole position is determined in the previous step, the numerical calculation model of different drilling intervals is preliminarily set according to the practical situation of the engineering site, and the numerical calculation model is processed through RFPA 2D Respectively obtaining a coal bed gas extraction stress failure diagram, a coal body stress curve diagram, an acoustic emission diagram, a gas vector cloud diagram and a gas flow curve diagram of different drilling spacing numerical calculation models through numerical calculation of a gas analysis version, and performing comparative analysis; meanwhile, the investment cost of the drill holes and the extraction effect are comprehensively considered, so that the most reasonable drill hole spacing is obtained when the coal seam gas is extracted through the ground vertical drill holes; the selection range of different drill hole spacing comprises 30m, 40m, 50m, 60m, 70m and 80 m.
In step 6, whether the numerical test result has reached the expected reasonable result is judged according to the following criteria:
a coal bed gas extraction stress failure diagram: the gas content and the gas pressure of the coal bed can be reduced by pumping the coal bed gas, and the coal bed can be damaged under the action of the overlying strata pressure. Whether the set final hole position of the drill holes and the drill hole spacing are reasonable or not can be judged by observing the size of the stress failure range in the coal bed gas extraction stress failure diagram. The reasonable final hole position and the borehole interval have the largest stress failure range in the coal seam gas extraction stress failure cloud picture, the whole gas-containing coal seam can be swept, the gas-containing coal seam is not concentrated around the boreholes, namely, the coal seam between the two boreholes is subjected to through failure, and the aim of reducing the gas content and the gas pressure of the coal seam to the maximum extent is fulfilled.
Coal body stress curve diagram: the stress curve numerical value reflects the magnitude of stress values of different positions of a coal body containing gas, the stress values of the coal body between two drill holes at the reasonable final hole position and the drill hole spacing are uniform in the initial extraction stage of the stress curve, and the stress peak value is expanded towards the middle of the two drill holes along with the increase of extraction time; in a gas extraction medium-term stress curve, the stress value is maximum; and in the later period of extraction, the stress peak disappears, the stress values of the gas-containing coal bodies are all lower than a certain value and approximate to a horizontal straight line, which shows that the gas content in the gas-containing coal layer tends to be stable and does not change rapidly. Indicating that the coal body crack between the two drill holes is subjected to through destruction. The drilling arrangement mode can effectively reduce the gas content in the coal seam, and a good gas extraction effect is achieved.
Acoustic emission graph: the acoustic emission graph reflects the sweep range of acoustic waves in the coal seam containing gas, namely the expansion range of the cracks, and acoustic emissions of reasonable final hole positions and drilling hole distances are uniformly distributed in the whole coal seam containing gas, so that the cracks can be expanded to the thickness of the whole coal seam after gas extraction; rather than being concentrated around the borehole or sporadically distributed at a location in the coal seam.
Gas vector cloud picture: the gas vector diagram is the trend of gas flow, if the gas content in the gas-containing coal layer is large, the gas vector is in a dense and aggregation state, and the gas content is gradually reduced along with the increase of the extraction time, so that the gas vector diagram is in a sparse state. The reasonable gas vectors of the final hole position and the drill hole spacing are changed from concentrated distribution in the early extraction stage to sparse distribution in the later extraction stage, which shows that the final hole position of the drill hole can play a role in reducing the gas content of the coal seam and does not present a serious gathering or sparse state.
Gas flow curve chart: the reasonable gas flow curve of the final hole position and the drill hole interval shows more gentle stages, namely, the stable extraction stage is longer, and the time is 10 to 14 days, namely, the drill hole arrangement mode can more effectively and more fully extract the coal seam gas; otherwise, it is not reasonable, and the drilling arrangement is not preferable.
Example (b):
in the embodiment, a numerical test of extracting gas from an extra-thick coal seam by a ground vertical drilling is carried out on the working face of the Tashan coal mine 8101.
the working face of the Tashan coal mine 8101 of the Shanxi coal-sharing group is positioned at the east part of a panel, the north part is adjacent to a seven-mountain coal mine, the south part and the 1070 air return lane are used as boundaries and communicated with the 1070 belt lane and an auxiliary transportation lane, the west part is an 8102 goaf, and the east part is a solid coal area; 8101 the working face length is 1445m, the inclined length is 231.4m, the mined coal seam is 3# to 5# coal seam, the average coal seam thickness is 20.08m, which belongs to super thick coal seam, and the mining experience and gas parameter measurement result of the mine in the past year can be known as follows: the gas content of the No. 3 to No. 5 coal seams is 1.78m on average 3 T, the average original gas pressure is 0.2MPa, and the permeability coefficient of the coal seam is 171.71-428.8 m 2 ·MPa -2 ·d -1 (ii) a The working face is adjacent to the 8102 goaf, and the coal pillar is 8 m; the working face is produced in trial at 2016, 11 and 15 days, and by the time of 2016, 11 and 24 days, the head of the working face is pushed by 25m, and the tail of the working face is pushed by 34 m.
8101 the gas emission from the working face mainly includes the sum of the gas emission from the mining layer, the adjacent layer and the surrounding rock. Under normal conditions, the absolute gas emission amount of 8101 working face is 25-35m 3 Min, when the working face meets the fault, coal body breakage and other areas in the advancing process, the gas emission quantity is obviously increased, and the absolute gas emission quantity of 8101 working face is expected to reach 40m 3 More than min; 8101 working face with gas emission of 2.25m 3 T, wherein the producing zone is 1.5m 3 T, upper adjacent layer and surrounding rock is 0.75m 3 T is calculated. 8101 the general idea of gas control on the working face: the method is based on wind rows, mainly adopts the pumping and discharging of vertical holes on the ground as main means, and adopts the pumping and discharging of the buried pipes at the upper corners and the measures of end plugging of the working face, primary water production fracturing and the like as auxiliary means.
specifically determining the construction process, the pore diameter, the extraction negative pressure, the plugging mode and the like of extracting coal seam gas from the ground vertical borehole according to the actual condition of a construction site; RFPA (radio frequency power amplifier) adopting large finite element analysis and calculation software 2D Gas analysis plate, first of allRespectively establishing numerical calculation models when the positions of the drilling final holes are positioned at the top of the coal seam, the middle of the coal seam and the bottom of the coal seam, wherein a black layer is the coal seam as shown in figure 2; through numerical calculation and analysis, the optimal drilling hole final position is located at the bottom of the coal seam, and then numerical calculation models of different drilling hole intervals (40m, 50m and 60m in sequence from small to large) are respectively established on the basis, as shown in fig. 3. The numerical model adopts two-dimensional plane strain analysis, the sizes of three different drilling terminal hole position numerical calculation models are all set to be 50 multiplied by 100m, 100 multiplied by 200 is divided into 20000 units, and each unit represents 0.5 m; the size of each of the three different borehole interval numerical calculation models was set to 210 × 100m, and the division 420 × 200 is 84000 units, each unit representing 0.5 m.
and obtaining key parameters of the numerical calculation model according to engineering field test data or physical experiment data, and the key parameters are shown in a table 1.
TABLE 1
according to the concrete data of a mine field, determining the rock components of the old top, the direct bottom and the old bottom of the coal seam, and correspondingly simplifying the rock stratum part above the old top during modeling, namely replacing the pressure of an overlying rock stratum with 0.25 MPa/m; adding a bedding weak surface with small elastic modulus and small tensile deformation between rock stratums, namely replacing the weak surface between the layers with a linear material; displacement constraint is adopted on two boundaries of the model, rigid constraint is adopted on the bottom boundary, and no matter the displacement constraint or the rigid constraint refers to displacement-free deformation; the loading mode adopts dead weight loading in the vertical direction, namely the loading from the coal layer containing gas to the old top in the vertical direction is carried out according to the volume weight of the rock, and the overlying rock layer part is replaced by vertically downward uniformly distributed force; and (3) supposing that the mechanical parameters of the rock accord with the Weber distribution, and judging the rock fracture by adopting a molar-Coulomb strength criterion.
The upper and lower boundaries of the model are air-tight rock strata, namely the gas flow of the upper and lower rock strata is 0, and the gas pressure of the left and right boundaries is 0.2 MPa; the initial conditions were: and at the moment t is 0, the gas pressure in the coal bed is 0.2 MPa.
According to the engineering field test and the physical experiment data, the gas seepage boundary condition of the calculation model is set, as shown in fig. 4. The scribed region in fig. 4 is a main setting parameter, where Heterogeneity is the degree of homogeneity, and corresponds to parameter m in the Weibull statistical distribution function, which reflects the homogeneity of the rock medium; average Value is the Average Value of unit physical mechanical parameters (such as elastic modulus, strength, Poisson ratio, volume weight and the like); the Waterhead is generally adopted to control and set the gas pressure at two boundaries of the coal seam.
The boundary conditions and the control conditions are adopted in the established numerical calculation models of different drill hole final hole positions and different drill hole intervals for extracting the gas of the ultra-thick coal seam through the vertical drill hole on the ground.
And step 5, numerical calculation:
in the established numerical calculation model of different drill hole final hole positions and different drill hole intervals for extracting the gas of the ultra-thick coal seam through the ground vertical drill hole, the total calculation steps are set to be 200 steps and are all seepage problems. Each calculation model adopts continuous calculation, the destruction criterion adopts the molar coulomb criterion in rock mechanics, and the Weibull statistical distribution function is adopted to describe the destruction phenomenon of gas-solid coupling.
(1) analyzing numerical calculation results of different drilling final hole positions
The results of numerical calculations when the final hole positions of the ground vertical drilled holes are located at different positions of the gas-containing coal seam are shown in fig. 5 to 7, wherein 1, 4 and 7 refer to acoustic emission diagrams, 2, 5 and 8 refer to gas vector diagrams, and 3, 6 and 9 refer to stress failure diagrams. The extracted gas flow rate comparison curve is shown in fig. 8.
By comprehensively comparing fig. 5, 6 and 7, it can be found that the stress failure range is the largest when the final hole position of the drill hole is arranged at the bottom of the gas-containing coal layer, and the whole gas-containing coal layer can be reached; and stress failure ranges at the top and the middle of the coal seam are only concentrated around the drill hole and at the middle upper part of the coal seam, so that the influence on extraction at the bottom of the coal seam containing gas is small. In addition, the acoustic emission in fig. 7 is distributed in the whole gas-containing coal seam, which illustrates that the cracks can be expanded to the whole coal seam thickness after gas extraction; whereas in fig. 5 the acoustic emission is concentrated only at the periphery of the borehole, in fig. 6 the acoustic emission is distributed in the gas-bearing coal seam, but only sporadically at the bottom of the coal seam. As can be seen from the gas vector diagram, the gas vectors in fig. 5 are always in sparse horizontal distribution, which shows that the gas extracted from the drilling terminal position is less and the extraction effect is not ideal; in fig. 6, the gas vectors are distributed densely all the time, and arrows appear densely, which indicates that the gas extracted at the terminal position of the drill hole is large all the time, and has a certain effect of reducing the gas content in the coal seam, but is not obvious, because the gas vectors of the coal seam still appear in an accumulated manner, the gas content in the coal seam is still large; in fig. 7, the gas vectors are changed from the centralized distribution in the initial stage of extraction to the sparse distribution in the later stage of extraction, which shows that the final hole position of the drill hole can play a role in reducing the gas content in the coal seam, so that the gas content in the coal seam is reduced from large to small, and the extraction effect is obvious.
By comprehensively comparing the three gas flow curves in the graph 8, it can be found that when the final hole position of the drill hole is positioned at the top of the coal seam, the gas extraction amount cannot meet the calculation requirement, and the flow curve is relatively smooth; when the final hole is positioned in the middle of the coal seam, although the maximum value meets the calculation requirement, the stable stage of the flow curve is short, which indicates that the extraction effect is not ideal; when the final hole is positioned at the bottom of the coal seam, the gas flow curve shows that the number of gentle stages is large, namely the stable extraction stage is long, and the drilling hole arrangement mode can more effectively and more fully extract the coal seam gas.
In conclusion, when the position of the final hole of the drill hole is located at different positions of a coal seam containing gas, the coal seam is damaged in different degrees, wherein the position of the final hole of the drill hole is located at the bottom and is damaged most seriously, the acoustic emission in the coal seam can almost reach the whole coal seam, the gas vector and the change trend of the extracted gas flow curve are reasonable, the coal seam gas can be extracted to the maximum extent, and the extraction effect is good. Therefore, it is reasonable to determine the final hole location for the surface vertical borehole to be placed at the bottom of the gas bearing coal seam.
(2) Analyzing numerical calculation results of different borehole spacing
Using RFPA 2D And (3) performing numerical calculation analysis on stress failure, acoustic emission and gas vectors under three different drilling intervals (40m, 50m and 60m) when the final hole position of the ground drilling is positioned at the bottom of the coal seam by using gas analysis version software. Through comparative analysis, when the distance between the drill holes is 40m and 50m, the coal body fractures between the two drill holes can be subjected to destructive communication, and the extraction effect is obvious; when the distance between the drill holes is 60m, the coal body fracture between the two drill holes can be communicated, but cannot be destructively communicated, and the extraction effect is not very obvious. And when the drilling interval is 40m, the whole coal mine drilling arrangement is too dense, and the construction cost is greatly increased. Therefore, on the basis of comprehensively considering the drilling input cost and the extraction effect, the final determination that the drilling distance is 50m is more reasonable. The comprehensive graph of stress failure, acoustic emission and gas vector when the borehole spacing is 50m is shown in fig. 9, wherein 10 is an acoustic emission graph, 11 is a gas vector graph, 12 is a stress failure graph, and the coal seam stress curves of different gas extraction stages are shown in fig. 10.
The invention is further explained by combining the practical application effect of extracting gas from the surface borehole of the Tashan mine 8101.
1, number of boreholes and borehole spacing
According to the numerical test optimization analysis result, the relevant theory of the O-shaped ring and the actual situation of the Tashan mine are combined, the influence of factors such as the first time pressure and the coal stopping and not discharging is considered, 31 drill holes are designed and arranged in the 8101 working face ground vertical drill hole drainage mode, the arrangement distance is 50m along the direction of the working face, and the arrangement distance is adjusted reasonably according to the gas control situation of the working face in the later period. And during normal recovery, opening 3-5 drill holes and simultaneously performing drainage. Stopping drainage when the drill hole enters the goaf for 100-120 m, and stopping drainage when the drill hole is positionedAnd starting the pumping when the pressure relief belt is 30-40 m away from the working surface. The drainage pump selects and utilizes a 2BEC87 type water ring vacuum pump in a ground area gas pump station, and the rated drainage capacity of a single pump is 735m 3 And/min. The main pumping and discharging pipeline is a DN600 pipeline, and the branch pumping and discharging pipeline is a DN300 pipeline.
2, drilling structure
The effective aperture of the vertical gas drainage drill hole on the ground of the working surface of the Tashan mine 8101 is 311mm, wherein the aperture of the drill hole of 0-120 m is 425mm, the diameter of the drill hole is 355 x 10mm, and the end hole point of 120m (2 m or 5m away from a 3# to 5# coal seam) is a naked hole.
3,8101 analysis of gas extraction effect of ground borehole of working face
According to the field data of the Tashan mine, the 8101 working face begins to perform drainage by using ground No. 0, No. 1 and No. 2 vertical drilling in 2016, 11 and 15 years, the concentration of gas in a total drainage pipeline is between 0.122 and 2.154 percent by the time of 2016, 11 and 25 days, and the absolute gas emission quantity of the working face is 5.656m 3 /min～10.23m 3 Min, wherein the pure amount of gas exhausted by the buried pipe at the upper corner is 0.59m 3 /min～0.64m 3 The volume of the solution is 6.21 to 9.01 percent per minute; the daily gas drainage purity of the vertical drilling hole on the ground is 2.09m 3 /min～6.18m 3 Min, accounting for 42.94-60.22% of the total amount; the pure quantity of the system air exhaust gas is 2.81m 3 /min～3.43m 3 The concentration is 33.53 to 48.05 percent of the total amount per minute. The daily cumulant of gas drainage of the opened three ground drill holes is 3013m 3 ～8892m 3 Average daily drawing and releasing amount of each hole is 1004m 3 ～2964m 3 Three-hole cumulative total pumping amount 51165.6m 3 。
The analysis shows that the gas extraction effect of the vertical drilling on the ground of the working surface of the Tashan mine 8101 is good, and the gas extraction method can be continuously popularized and applied to the gas extraction work of a coal mine.
Finally, it should be noted that: the above embodiments and examples are only intended to illustrate the technical solution of the present invention, and not to limit the same; although the present invention has been described in detail with reference to the foregoing embodiments or examples, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments and examples can be modified, or some of the technical features can be equivalently replaced; such modifications or substitutions do not depart from the spirit and scope of the embodiments or examples of the present invention.
Claims (6)
1. A numerical test method for extracting gas from an extra-thick coal seam by using a ground vertical borehole is characterized by comprising the following steps:
step 1, establishing an engineering document of extracting gas of an extra-thick coal seam by a ground vertical drilling hole;
step 2, establishing a numerical calculation model and grid division of the gas of the ground vertical drilling extracted extra-thick coal seam;
step 3, determining key parameters of a numerical calculation model for extracting gas from the ultra-thick coal seam through the ground vertical drilling;
step 4, setting boundary conditions and control conditions for extracting gas of the ultra-thick coal seam by using a ground vertical drilling hole;
step 5, calculating numerical values;
step 6, analyzing the numerical test result of the gas of the extra-thick coal seam extracted by the ground vertical drilling to obtain the most reasonable final hole position of the drilled hole and the most reasonable spacing between the drilled holes;
wherein, the step 1 specifically comprises:
collecting geological data of an actual mine field, and establishing a geological profile engineering document of a ground vertical drilling extraction extra-thick coal seam gas engineering; the content of the engineering document comprises the position of a working face, the up-down relation of a well, the coal bed condition, the gas condition, the coal bed top and bottom plate condition, the geological structure condition and the hydrogeological condition;
the coal bed conditions comprise coal bed thickness, coal bed structure, coal bed inclination angle, coal bed hardness, mined coal bed, coal type, stability and mining index;
the gas condition comprises the gas content of the coal seam, the gas pressure and the gas emission quantity;
the coal seam top and bottom plate conditions comprise rock names, thickness and lithology characteristics;
the geological structure comprises faults and folds;
the step 2 specifically comprises:
(a) establishing a numerical calculation model for extracting gas from the ultra-thick coal seam by using a ground vertical drilling hole:
specifically determining the construction process, the pore diameter, the extraction negative pressure and the plugging mode of extracting coal seam gas from the ground vertical borehole according to the actual condition of a construction site; firstly, respectively establishing numerical calculation models when the positions of the drilling final holes are positioned at the top of a coal seam, the middle of the coal seam and the bottom of the coal seam by adopting large-scale finite element analysis and calculation software, namely gas coal rock fracture process analysis software, and obtaining an optimal drilling final hole position model through numerical calculation and analysis; then respectively establishing numerical calculation models of different drilling hole intervals when the final hole positions of the drilling holes are optimal on the basis; the selection range of the different drill hole distances comprises 30m, 40m, 50m, 60m, 70m and 80 m;
(b) grid division of a numerical calculation model for extracting gas from an ultra-thick coal seam through a ground vertical drilling hole:
carrying out grid division while carrying out numerical test modeling by using gas coal rock fracture process analysis software, setting the two tasks in the same interface, and setting the number of unit grid divisions of a calculation model and the actual size of the calculation model according to actual needs in the setting interface; the actual requirements include the model dimensions and the actual condition ratios, the memory size of the computer server, and the computing power.
2. The numerical test method for extracting gas from an ultra-thick coal seam through the ground vertical borehole according to claim 1, wherein the step 3 specifically comprises:
according to engineering field test data and physical experiment data, determining key parameters of the numerical calculation model, including homogeneity, elastic modulus, compressive strength, Poisson's ratio, porosity, internal friction angle, compression-tension ratio, air permeability coefficient, gas content coefficient, gas pressure coefficient and permeability coupling coefficient.
3. The numerical test method for extracting gas from the ultra-thick coal seam through the ground vertical borehole according to claim 2, wherein the step 4 specifically comprises:
in order to enable the established numerical calculation model for extracting the gas of the ultra-thick coal seam from the ground vertical borehole at different borehole final hole positions and different borehole intervals to carry out numerical tests under various geological conditions, the boundary conditions and the control conditions of the calculation model are set in a simplified manner, and the method specifically comprises the following steps:
(1) assuming that the rock mechanics parameters conform to Weibull distribution;
(2) judging the rock fracture by adopting a molar-coulomb intensity criterion;
(3) displacement constraint is adopted on two boundaries of the model, rigid constraint is adopted on the bottom boundary, and no matter the displacement constraint or the rigid constraint refers to no displacement deformation;
(4) adding a bedding weak plane with small elastic modulus and small tensile deformation between rock stratums, namely replacing the weak plane between the layers with a linear material;
(5) determining rock components of an old top, an immediate bottom and an old bottom of a coal seam, and correspondingly simplifying the rock stratum part above the old top during modeling, namely replacing the pressure of an overlying rock stratum with 0.25 MPa/m;
(6) the loading mode adopts dead weight loading in the vertical direction, namely, the loading in the vertical direction is carried out from the gas-containing coal bed to the old top according to the volume weight of the rock, and the overlying strata part is replaced by vertically downward uniformly distributed force;
(7) the pressure of gas in the coal body is controlled by a water head, namely 1MPa is equal to the height of the water head of 100m, and the gas pressure of the gas in the goaf is set to be 0;
(8) although gas in a coal seam is extracted, the solution type is still regarded as a plane strain problem, a total calculation step is set, the self weight in the Y direction is considered, the unit calculation is a cavity unit method, and the influence of seepage time on model calculation is ignored.
4. The numerical test method for extracting gas from the ultra-thick coal seam through the ground vertical borehole according to claim 3, wherein the step 5 specifically comprises:
and performing numerical calculation by using gas coal rock fracture process analysis software, wherein the numerical calculation process of the gas coal rock fracture process analysis software comprises stress calculation analysis, phase change analysis and element phase change analysis.
5. The numerical test method for extracting gas from an ultra-thick coal seam through the ground vertical borehole according to claim 4, wherein the step 6 specifically comprises:
firstly, according to numerical calculation models of the positions of the final holes of the drilled holes at the top, the middle and the bottom of a coal seam, respectively obtaining a coal seam gas extraction stress failure diagram, a coal body stress curve diagram, an acoustic emission diagram, a gas vector cloud diagram and a gas flow curve diagram of the three calculation models through numerical calculation of gas coal rock fracture process analysis software, and comparing to obtain the most reasonable positions of the final holes of the drilled holes when the coal seam gas is extracted through the ground vertical drilled holes;
secondly, after the best and reasonable drilling final hole position is determined in the last step, according to the preliminarily set numerical calculation models with different drilling intervals, and through numerical calculation of gas coal rock fracture process analysis software, a coal bed gas extraction stress failure diagram, a coal body stress curve diagram, an acoustic emission diagram, a gas vector cloud diagram and a gas flow curve diagram of the numerical calculation models with different drilling intervals are respectively obtained for comparative analysis; meanwhile, the drilling input cost and the extraction effect are comprehensively considered, so that the most reasonable drilling distance is obtained when the coal seam gas is extracted by the ground vertical drilling; the selection range of different drill hole spacing comprises 30m, 40m, 50m, 60m, 70m and 80 m.
6. The method for testing the value of gas drainage from the ultra-thick coal seam through the ground vertical borehole according to claim 5, wherein in the step 6, whether the value test result achieves an expected reasonable result is judged according to the following standards:
a coal seam gas extraction stress failure diagram: the stress failure range of the stress failure diagram under the reasonable final hole position and the drill hole interval is the largest, the whole coal seam containing gas can be swept, the stress failure diagram is not concentrated around the drill holes, namely, the coal seam between the two drill holes is subjected to through failure, and the aim of reducing the gas content and the gas pressure of the coal seam to the maximum extent is fulfilled;
coal body stress curve diagram: stress values of a stress curve at a reasonable final hole position and a reasonable hole drilling distance between two drill holes in the initial extraction stage are uniform, and a stress peak value is expanded towards the middle of the two drill holes along with the increase of extraction time; in a gas extraction medium-term stress curve, the stress value is maximum; in the later period of extraction, the stress peak value disappears, the stress values of the coal body containing gas are all lower than the same value and approximate to a horizontal straight line, and the fact that the coal body cracks between two drill holes are penetrated and damaged is shown, so that the arrangement mode of the drill holes can effectively reduce the gas content in the coal bed, and a good gas extraction effect is achieved;
acoustic emission diagram: acoustic emission at a reasonable final hole position and at a drilling hole interval is uniformly distributed in the whole coal seam containing gas, which shows that cracks can be expanded to the thickness of the whole coal seam after gas extraction; rather than being concentrated at the periphery of the drill hole or scattered at a certain part of the coal seam;
gas vector cloud picture: the reasonable gas vectors at the final hole position and the distance between the drill holes are changed from concentrated distribution due to large gas content at the initial stage of extraction to sparse distribution due to small gas content at the later stage of extraction, which shows that the final hole position of the drill hole can play a role in reducing the gas content of the coal seam and cannot be in a serious gathering or sparse state;
gas flow curve chart: the reasonable gas flow curve at the final hole position and the drill hole interval is mainly in a gentle stage, namely a stable extraction stage, and the time is 10-14 days, so that the coal seam gas can be effectively and fully extracted by the drilling arrangement mode; otherwise, it is not reasonable, and the drilling arrangement is not preferable.
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