CN112528503A - Numerical simulation analysis method for gas extraction of abandoned mine - Google Patents
Numerical simulation analysis method for gas extraction of abandoned mine Download PDFInfo
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Abstract
A numerical simulation analysis method for gas extraction of abandoned mines comprises the following steps; constructing a three-dimensional geological geometric model of a waste ore reservoir, assigning values, selecting a proper physical field combination, and performing simulation analysis; the large-scale waste ore reservoir contains a plurality of goafs, the caving zones and the fracture zones are not completely parallel and are not planes, a compaction zone, an O-shaped ring zone and each block section and contour of a geometric model are transversely divided, and the height of a top plate and the height of a gas content contour line come from a certain abandoned mine; firstly, desorbing and diffusing the adsorbed gas to a pressure relief area, then extracting the gas under the condition of bottom hole negative pressure, simulating the content of the adsorbed gas and the reservoir pressure of the originally planned well after extracting for a period of time, comparing the gas production rate, and analyzing and searching a position with possibly higher gas production rate; simulating the optimized well position again, and comparing the gas production rate change after the well position changes; and completing the well position optimization of the gas extraction of the abandoned mine. The invention has the characteristics of rapidness, accuracy and capability of changing different conditions.
Description
Technical Field
The invention relates to the technical field of development and utilization of abandoned mine resources, in particular to a numerical simulation analysis method for gas extraction of abandoned mines.
Background
The research on the development and utilization of waste mine resources in China starts late, the basic theory research is weak, the heterogeneity of the waste mine is greatly increased in the early coal mining process by means of experience when the waste mine gas extraction well pattern is arranged, and the optimization of the well pattern is difficult to realize. The following difficulties exist in research:
(1) how to reasonably evaluate the coal bed gas resources of the waste coal mine goaf is a key problem in the development of the waste coal mine goaf. The waste coal mine gas mainly exists in free gas and adsorbed gas in coal pillars and residual coal beds, adjacent unexplored coal beds and surrounding rocks. The gas is mainly present in the waste mine in a free state, an adsorbed state and a dissolved state.
The goaf of the abandoned mine is mainly in a free state and mainly exists in a fracture system of a coal rock layer and the goaf formed by coal mining; the adsorbed gas is stored in coal pillars, residual coal beds, carbonaceous shales and mudstones close to unexplored coal beds and surrounding rocks; dissolved gas exists in underground water in a dissolving mode, and differences of coal bed gas sources and enrichment degrees of goafs are determined by different geological conditions and coal mining methods.
(2) The permeability distribution heterogeneity is strong, and the mining-induced fracture is generated with great randomness, and the support structure formed in the later period and the mine pressure recompression effect can cause great influence on the distribution of the mining-induced fracture, so that the permeability at each position in the goaf has regional difference.
(3) An effective well pattern optimization method is lacked, and the existing optimization method for the coal bed gas extraction well pattern does not consider heterogeneity caused by a goaf and cannot be applied to optimization of the waste mine gas extraction well pattern. The conventional coal mine gas extraction method mainly aims at guaranteeing mining safety aiming at a local goaf, does not consider global well pattern optimization, and therefore cannot be applied to optimization of a waste mine gas extraction well pattern.
Disclosure of Invention
In order to solve the technical problems, the invention aims to provide a numerical simulation analysis method for gas extraction of a abandoned mine, which has the characteristics of rapidness, accuracy and capability of changing different conditions.
In order to achieve the purpose, the invention adopts the technical scheme that:
a numerical simulation analysis method for gas extraction of abandoned mines comprises the following steps;
the first step is as follows:
constructing a three-dimensional geological geometric model of a waste ore reservoir, assigning values, selecting a proper physical field combination, and performing simulation analysis; the large-scale waste ore reservoir contains a plurality of goafs, the caving zones and the fracture zones are not completely parallel and are not planes, a compaction zone, an O-shaped ring zone and each block section and contour of a geometric model are transversely divided, and the height of a top plate and the height of a gas content contour line come from a certain abandoned mine;
the second step is that:
firstly, desorbing and diffusing the adsorbed gas to a pressure relief area, then extracting the gas under the condition of bottom hole negative pressure, simulating the content of the adsorbed gas and the reservoir pressure of the originally planned well after extracting for a period of time, comparing the gas production rate, and analyzing and searching a position with possibly higher gas production rate;
the third step:
then optimizing the well position to simulate again, and comparing the gas production change after the well position changes; and finally, well position optimization of the gas extraction of the abandoned mine is completed.
In the second step, a gas matrix and natural fracture coupling seepage model is established, and the specific formula is as follows:
(1) the seepage in the matrix adopts pseudo-steady flow, and the equation expression is as follows:
in the formula: v is the amount of adsorbed gas, t is the time,. tau.is the gas diffusion time, VEIs the gas volume (varying with pressure) in the natural fracture;
(2) the natural fracture adopts the gas mass conservation law, and the equation is as follows:
in the formula: rhogIs the gas density,. phi.is the porosity,. phi.is the permeability,. mu.is the viscosity,. p is the pressure,. qdIs the gas mass exchange capacity of the matrix pores and natural fractures, wherein the gas density is derived from the gas equation of state:
in the formula: m is the gas molar mass, Z is the gas compression factor, R is the gas constant, and T is the temperature;
gas mass exchange q of matrix pores and natural fracturesdComprises the following steps:
in the formula: rhocIs the density of the coal, pgaIs the density of the coal bed gas, V is the coal bed gas adsorption capacity, and t is the time;
(3) the adsorbed gas content was characterized using langmuir isothermal adsorption equation:
in the formula: vLIs the Langmuir volume constant, pLIs the langmuir pressure constant;
the motion state of the fluid under specific conditions can be determined by giving definite conditions, and for unsteady state seepage, the initial conditions are generally as follows:
p(x，y，z，t)|t＝0＝f0(x，y，z) (6)
the boundary conditions for solving the partial differential equation are divided into three types, the first type of boundary conditions is adopted, the boundary pressure is directly given, and the formula is as follows:
p(x，y，z，t)|Γ＝f(x，y，z，t) (7)
simultaneous quasi-steady state equations, continuity equations, langmuir isothermal adsorption equations, gas state equations, initial conditions, and boundary conditions.
The invention has the beneficial effects that:
according to the invention, by researching reservoir characteristics, gas flow rules, porosity and permeability heterogeneity distribution and calculation models of the abandoned mine, the influence rule of waste mine gas extraction is mastered, the numerical simulation of waste mine gas extraction is carried out, and the simulated well position is optimized.
The invention can be used for determining and optimizing the well positions of gas extraction in a large number of domestic abandoned mines, can promote the development and reutilization of the abandoned gas, and can bring into play multiple benefits of safety, economy, environmental protection, society and the like.
Drawings
Fig. 1 is a flow chart of a simulation scheme for gas extraction from a reservoir of a abandoned mine.
Fig. 2 is a geometric model and grid division diagram of a reservoir of an abandoned mine.
FIG. 3 is a graph showing the initial adsorbed gas content and pressure distribution of the abandoned mine.
FIG. 4 is a schematic diagram of well locations and fracture ranges of 4 wells in the original plan.
FIG. 5 is a diagram of the content distribution of 5-year adsorbed gas extracted before and after fracturing of a waste mine.
FIG. 6 is a pressure distribution diagram of a 5-year-old reservoir before and after fracturing of a abandoned mine.
FIG. 7 shows the change curve of gas production of 5 years before and after fracturing.
FIG. 8 comparison of gas production before and after fracturing.
FIG. 9 is a flow chart of gas extraction in a reservoir at different moments.
FIG. 10 is a schematic diagram of well placement and fracture extent for 4 optimized wells.
And FIG. 11 is a distribution diagram of the adsorbed gas content of extracted 5 years after optimization of the well position.
And FIG. 12 is a pressure distribution diagram of the extracted 5-year reservoir after the well position is optimized.
And FIG. 13 is a gas production change curve of 5 years before and after fracturing after well position optimization.
And FIG. 14 is a change curve of gas production of 5 years extracted when the well position is not fractured after optimization.
And 15, extracting a 5-year gas production change curve after fracturing after well position optimization.
FIG. 16 is a schematic diagram of the new 2 well locations and fracture ranges.
FIG. 17 is a diagram of the content distribution of 5-year adsorbed gas extracted before and after fracturing of a waste mine.
FIG. 18 is a diagram of a 5-year reservoir pressure distribution before and after fracturing of a abandoned mine.
FIG. 19 is a graph of gas production change of 5 years before and after fracturing.
FIG. 20 is a comparison of gas production before and after well stimulation without fracturing.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
Example (b):
as shown in fig. 2: the large-scale waste ore reservoir contains a plurality of goafs, the caving zones and the fracture zones are not completely parallel and are not planes, and areas such as compaction zones, O-shaped rings and the like are transversely divided. When three-dimensional geological modeling of a waste mine reservoir is carried out, functions such as an interpolation function and a parametric surface of COMSOL are required to be applied, advanced operation is carried out on a geometric model through tools such as copying, moving and virtual operation, operations such as cutting and bonding among geometric structures are carried out by using a Boolean operation mode, and the quality of grid division is ensured through operations such as facet deletion and grid configuration. And each block, contour, top and bottom plate height and gas content contour line data of the geometric model come from a certain abandoned mine.
Some of the parameters used in the model are shown in table 1.
Table 1 analog use partial parameter table for abandoned mine
When the coal seam is unearthed or disturbed by adjacent layer development, the fissures are constantly in mass exchange with gas in the matrix and do not macroscopically exhibit significant mass exchange characteristics. And when the coal seam is disturbed by stress, the gas in the adsorption state and the gas in the free state immediately shows different flowing rules. The adsorption gas starts to spread and the free gas starts to seep under the drive of the pressure gradient and the concentration gradient; since the seepage is much faster than the pore diffusion, and no longer in equilibrium, the pores exchange mass with the fractures, the pore system can be considered as an internal mass source of uniform distribution in the fracture system.
When the waste mine gas is extracted under negative pressure, the free gas seeps in the fracture towards the direction of a wellhead, and the pressure of a fracture system is reduced. The gas in the adsorption state in the reservoir is disturbed by pressure and begins to be desorbed, and the desorbed free gas immediately enters a fracture system with lower pressure and is converged into the bottom of the well, and is extracted to the ground through a shaft. In the matrix, the gas is desorbed and diffused; in the mining fracture, the gas flow satisfies Darcy's law. Describing gas adsorption characteristics by using a double-hole medium model and a Langmuir isothermal adsorption model, converting the geological data of the existing waste ore gas into an initial value of a numerical model, establishing a gas matrix and natural fracture coupling seepage model, and using the following formula for calculation:
(1) the seepage in the matrix adopts pseudo-steady flow, and the equation expression is as follows:
in the formula: v is the amount of adsorbed gas, t is the time, is the gas diffusion time, VEIs the amount of gas (as a function of pressure) in the natural fracture.
(2) The natural fracture adopts the gas mass conservation law, and the equation is as follows:
in the formula
Gas density, is porosity, k is permeability, is viscosity, p is pressure, q isdIs the gas mass exchange capacity of the matrix pores and natural fractures. Wherein the gas density is derived from a gas equation of state:
in the formula: mgIs the gas molar mass, Z is the gas compression factor, R is the gas constant, and T is the temperature.
Gas mass exchange q of matrix pores and natural fracturesdComprises the following steps:
in the formula: rhocIs the density of the coal, pgaIs the density of the coal bed gas, V is the coal bed gas adsorption capacity, and t is the time.
(3) The adsorbed gas content was characterized using langmuir isothermal adsorption equation:
in the formula: vLIs the Langmuir volume constant, pLIs the langmuir pressure constant.
The model established by the application is a general mechanism for describing the fluid motion under the ground drilling negative pressure extraction condition, the motion state of the fluid under a specific condition can be determined only by giving a definite condition, and for unsteady state seepage, the initial conditions are as follows:
p(x，y，z，t)|t＝0＝f0(x，y，z) (3.6)
the boundary conditions for solving the partial differential equation are divided into three types, the mathematical model constructed by the method adopts the first type of boundary conditions, the boundary pressure is directly given, and the formula is as follows:
p(x，y，z，t)|Γ＝f(x，y，z，t) (3.7)
and simultaneously establishing a quasi-steady-state equation, a continuity equation, a Langmuir isothermal adsorption equation, a gas state equation, an initial condition and a boundary condition to obtain a partial differential mathematical model for gas extraction of the abandoned mine.
In the parameters, the bottom of the well is set to enter a fractured zone by 8 m; the porosity is obtained according to a formula, the pressure of each caving zone and fracture zone is obtained by calculating the given resource amount, mining area and thickness and the calculated porosity, the pressure of the lateral fracture zone and the non-mining zone is obtained by converting the given gas content according to an adsorption equation, and the adsorbed gas content is assigned according to the actually measured stratum gas content.
According to the permeability space distribution rule of the reservoir, a fluid flow equation is matched, a proper physical field is selected, partial parameters in the selected physical field are changed according to requirements, and the simulation precision is improved. FIG. 3(a) is the initial distribution of the adsorbed gas content, and the obtained adsorbed gas content map after assigning values is consistent with the gas content contour map. FIG. 3(b) shows the initial distribution of free gas pressure, from which it can be seen that the unexplored zone remains at a higher pressure, rich in adsorbed gas, while the lateral fissured zone, the "O" -ring fissured zone, recompacted zone, is in the relief zone, little or no adsorbed gas, due to mining effects.
Fig. 4 shows the well position arrangement of 4 originally planned wells in the area of a certain abandoned mine and the fracture radius of the fracture design.
FIG. 5 is a comparison graph of the distribution of 5-year adsorbed gas extracted before and after fracturing of a certain abandoned mine, and it can be seen that the content of the adsorbed gas in the whole reservoir after fracturing is obviously reduced, the content of the adsorbed gas in the reservoir after fracturing is reduced more, and the goaf and the surrounding rock thereof have good connectivity. Color range: 0-0.0036m3/t。
FIG. 6 is a comparison graph of pressure distribution of 5 years of extraction before and after fracturing of a abandoned mine, pressure drop of a reservoir is accelerated in the same extraction time after fracturing, cracks develop in the range adjacent to a goaf, and the reservoir has good connectivity and the color range is 0.02-3.2 MPa.
The gas flow velocity around and on the bottom of the well is carried outThe integral of the curved surface can obtain the gas production value. FIG. 7 is a graph of 5-year gas production rate of original planned 4-well non-fractured extraction along with time, wherein 2#And the well is drilled and extracted, the gas production rate is basically consistent with the simulation result, and the simulation result is well verified. As can be seen from the figure: the initial gas production rate of part of the wells fluctuates because the free gas content at the bottom of the well is unstable. When the free gas is extracted quickly and the quick supplement of the gas desorbed from the periphery can not be obtained, the gas yield can be reduced quickly until the extraction and the supplement reach a relative balanced state. And when the free gas content at the bottom of the well is more and the near goaf can be rapidly supplemented, the initial gas production can be relatively stable and even increased, and finally tends to balance along with the continuous extraction.
Analysis of gas production from each well: 3#The well has higher gas production when not fractured because of 3#The well is positioned in the O-shaped ring area, the permeability is high, the gas flowing speed is high, but the gas desorbed from a far position cannot be supplemented in time due to the high permeability, so that the gas production rate is reduced quickly. 1#、4#Well gas production without fracturing is less than 2#The well should be because the initial formation adsorbed gas content distribution is not uniform. After fracturing, the gas production lines of 4 wells all show a rapid descending trend because fracturing improves the bottom hole environment and the gas flow rate is accelerated, 2#The gas production of the well is significantly increased, 1#、3#、4#The resulting gas production from the well tends to be consistent, which is related to the heterogeneity of the adsorbed gas content of the formation and the number of adjacent goafs.
FIG. 8 is a comparison graph of gas production before and after fracturing of an original plan 4 wells, wherein after fracturing of the 4 wells, the yield is improved, the initial yield is obviously improved, and 3#The well gas production is promoted less because of 3#The well is positioned in the O-shaped ring fracture area, and the bottom hole environment is slightly improved before and after fracturing.
The yield graph before and after fracturing can be obtained: 1#、2#、4#The yield increasing effect is better after well fracturing. Comparing the production increasing condition and the well arrangement position, finding: can communicate with more goafs and the shaft bottom is positioned in a lateral fracture zoneThe well of (a) can achieve higher gas production because the location is in the lateral fracture zone and has higher permeability. And 3 located in the "O" ring#The gas production is large when the well is not fractured, but the gas production cannot be improved greatly after the fracturing, and if the fracturing is not considered, the well can be drilled and extracted in an O-shaped ring fracture area. In conclusion, the position of the original well position is properly adjusted, and trial simulation is carried out to compare the gas production rate changes of the new well position and the old well position.
Fig. 9 is a flow diagram of a reservoir at different times of gas extraction, and from the perspective of the space geometry of a flow field, the gas flow in the reservoir and around a borehole is complex due to the different natural conditions of the coal seam, such as heterogeneity and gas content, and the flow state includes unidirectional flow, radial flow and spherical flow. The three-dimensional flowing direction and the desorption-diffusion-seepage path of gas at each point in the model can be intuitively reflected on the streamline chart, and in addition, the density of the streamline at a certain point also reflects the flowing speed of the gas at the point.
As can be seen from fig. 9, at the time t ═ 0.1s, the equilibrium state of the gas in the reservoir is broken, the gas in the unexplored zone and the lateral fractured zone starts to desorb, a part of the gas is diffused under the driving of the density difference (the position of the gas-containing line boundary), and a part of the gas is permeated to the goaf, the caving zone and the fractured zone under the action of the pressure; when t is 100 days, gas in a reservoir close to the bottom of the well flows to the bottom of the well through a lateral fracture area, a fracture zone and an caving zone, and gas far away flows under the action of extraction pressure and formation pressure; when t is 5 years, a specific seepage channel from the reservoir to the bottom of the well is formed in the waste ore reservoir, and the flow lines in the O-shaped ring area and the lateral fracture area are dense, so that the permeability distribution rule of the reservoir is met. It can be seen that higher gas production can be achieved near the goaf boundary.
According to the result of goaf distribution and three-dimensional flow field analysis, well positions of 4 wells are properly adjusted by combining the ground condition, and the adjustment result is shown in figure 10.
As can be seen from the graph 11, the gas content of the whole reservoir layer is reduced faster after fracturing, and the adsorption gas amount of the adjacent goaf and the mining surrounding rock is changed synchronously because longitudinal and transverse fractures in the surrounding rock are developed, so that the gas permeability is improved. Color range: 0-0.0036m3/t。
FIG. 12 is a comparison graph of 5-year pressure distribution of extraction before and after fracturing of a certain abandoned mine, and the reservoir pressure is reduced quickly in the same extraction time after fracturing. The adjacent goaf and its goaf wall pressure remain consistent also because of the high connectivity within the zone. The color range is 0.02-3.2 MPa.
FIG. 13 is a graph of gas production of 5 years extracted before and after fracturing after well placement optimization as a function of time. As can be seen from the figure: 2#The well is used as a reference, the well position is unchanged, and the change trend before and after fracturing is consistent with that before and after fracturing. Optimization 3#The well is still positioned in the O-shaped ring, the initial flow rate is high, and the gas production rate is reduced quickly. After fracturing, the gas production of 4 wells is greatly improved, and the gas production change trends before and after optimization are basically consistent, because the optimized well position is near the original planned well position, and the quantity of adjacent goafs and the gas content of the area where the well is located are not changed.
FIG. 14 is a comparison of 5-year gas production changes for 4-well extraction before and after well placement optimization at the time of fracking, 1#、4#The gas production rate is greatly improved after the well is optimized; 2#The well position is unchanged, but is influenced by mining, and the gas production rate in the later period of extraction after optimization is reduced to some extent; 3#The well is positioned in the same O-shaped ring fracture area before and after optimization, and the gas production rate is reduced because the well position is lower after optimization and gas at the top of the fracture zone is not well extracted.
FIG. 15 is a comparison of the 5-year gas production changes of 4 wells before and after the well position optimization after fracturing, optimization 1#The well can still achieve higher gas production because of optimization 1#The well can communicate with more goaf areas after fracturing, and the gas source is sufficient and can be supplied in time. 2#、3#、4#Well gas production does not vary much, 2#Wells are due to unchanged well location, and gas production variations are affected by other well pressure fluctuations, 3#、4#The well is characterized in that the lateral fracture area is located at the same position as the well before optimization, the bottom hole environment is improved after fracturing, and the same adsorption state can be quickly achieved before and after optimization.
In order to research whether a new well needs to be additionally arranged in the abandoned mine, 2 areas of distributed wells which can communicate with more goafs are selected according to the distribution of the goafs and the distribution of flow fields, gas extraction simulation of the abandoned mine is carried out, and the output change condition before and after fracturing is observed. Fig. 16 is a diagram showing the new 2 well positions and the fracture ranges of a certain abandoned mine.
Fig. 17 is a comparison graph of the distribution of adsorbed gas extracted for 5 years before and after well fracturing in a certain abandoned mine, and it can be seen that the gas content of a reservoir after well fracturing is obviously reduced compared with that before well fracturing, and the gas content of the whole reservoir extracted for 5 years after fracturing is obviously reduced. Color range: 0-0.0036m3/t。
FIG. 18 is a pressure distribution comparison graph of 5-year extraction before and after fracturing of a waste mine well, and the pressure of a reservoir is reduced quickly within the same extraction time after fracturing. The color range is 0.02-3.2 MPa.
FIG. 19 is a graph of the gas production of 6 non-fractured gas extraction 5 years after the well is added, which can be seen from the graph: after the well is added, the gas production rate of all wells can quickly tend to be stable, which indicates that the number of the wells is increased, and the large goaf and the surrounding rock area can be quickly influenced, so that the desorption-diffusion-migration of the adsorbed gas and the gas extraction can quickly reach a relatively balanced state. Comparing fig. 7(a) and (b) shows that: before and after fracturing, add 1#The well has high yield, and the gas production rate is equal to 4 before and after fracturing#Equivalent well, large influence on total gas production, add 2#The well production effect is poor.
As can be seen from fig. 20: when fracturing is not carried out, the yield of the original four wells is influenced after 2 wells are additionally drilled, but the influence degree is not large; plus 1# Well pair optimization 1#、2#The influence degree of the well is more than plus 2# Well pair 3#、4#Extent of influence of well, because of addition of 1#The gob area adjacent the well is larger.
According to the simulation result of the gas extraction condition of the planned construction well position, the following conclusion is obtained:
under the condition of no fracturing, 3#The well has a high gas production 1#、4#Gas production of the well is less than 2#A well;
secondly, after fracturing, the gas production lines of 4 wells all show a rapid descending trend, 2#The gas production rate of the well is remarkably increased byBecause fracturing communicates adjacent goafs; 3#The well gas production is promoted less because of 3#The well is positioned in the O-shaped ring fracture area, and the bottom hole environment is slightly modified before and after fracturing; 1#、3#、4#The resulting gas production from the well tends to be consistent, which is related to the heterogeneity of the adsorbed gas content of the formation and the number of adjacent goafs.
(2) According to the optimized simulation result of the well position gas extraction condition, the following conclusion is obtained:
(i) when fracturing is not performed, 1#、4#The gas production rate is greatly improved after the well is optimized; 2#The well position is unchanged, but is influenced by mining, and the gas production rate in the later period of extraction after optimization is reduced to some extent; 3#The well is positioned in the same O-shaped ring fracture area before and after optimization, and the gas production rate is reduced because the well position is lower after optimization and gas at the top of the fracture zone is not well extracted.
② optimization 1 under the condition of fracturing#The well can still achieve higher gas production because of optimization 1#More goaf areas can be communicated after the well is fractured, and the gas source is sufficient and can be supplied in time; 2#、3#、4#Well gas production does not vary much, 2#Wells are due to unchanged well location, and gas production variations are affected by other well pressure fluctuations, 3#、4#The well is characterized in that the lateral fracture area is located at the same position as the well before optimization, the bottom hole environment is improved after fracturing, and the same adsorption state can be quickly achieved before and after optimization.
(3) According to the simulation result of increasing the gas extraction condition of the well position, the following conclusion is obtained:
firstly, the yield of the original four wells is influenced after 2 wells are additionally drilled, but the influence degree is not large; plus 1# Well pair optimization 1#、2#The influence degree of the well is more than plus 2# Well pair 3#、4#Extent of influence of well, because of addition of 1#Gob adjacent to wellThe area is large.
② if the increase of the drilling quantity is considered, it can be suggested to add 1#The location of the well increases.
The numerical simulation method is used for carrying out gas extraction numerical simulation on a large-scale abandoned mine comprising a plurality of goafs, analyzing the three-dimensional flow field distribution of the reservoir and optimizing the well position arrangement for re-simulation after the feasibility of the gas extraction numerical simulation is verified, and carrying out well position optimization.
Finite element software COMSOL Multiphysics (COMSOL for short) selected by the application is simulated to simulate the waste mine gas extraction process of the dual-pore medium. COMSOL originally serves as a data packet Femlab of MATLAB, and then is gradually improved and separated from the MATLAB to be capable of independently operating. COMSOL has a good software interface, and provides a series of geometric modeling tools, operations and other related functions, including a large number of geometric objects that can be used to construct common shapes, and parameters that can be used to define the features and positions of objects, and users can also create complex geometries using geometric operations such as Boolean operations, transformations, and segmentation. Meanwhile, COMSOL also has a CAD kernel, and can accurately import two-dimensional and three-dimensional model drawings drawn by most professional CAD software nowadays. Compared with other professional three-dimensional modeling software, COMSOL has defects on some detailed operations, but has the advantages that the interface is very friendly, and a general three-dimensional geologic body visualization model can be quickly established.
According to the estimation result of the gas resource amount of a certain abandoned mine, an original coal seam thickness contour map of an estimation range, an original gas content contour map and a working surface layout map, meanwhile, the influences of factors such as ground settlement, surface water and the like are comprehensively considered, and a three-dimensional geological geometric model of a waste mine reservoir is constructed and assigned; selecting a proper physical field combination, and simulating in two steps, namely desorbing and diffusing adsorbed gas to move to a pressure relief area, then extracting gas under the condition of bottom negative pressure, simulating the content of the adsorbed gas and the reservoir pressure of the originally planned 4 wells after extracting for a period of time, comparing the gas production, and analyzing and searching a position with possibly high gas production; simulating again according to the newly arranged well position, and comparing the gas production rate change after the well position changes; and finally, well position optimization of the gas extraction of the abandoned mine is completed. The flow chart of the simulation scheme is shown in figure 1.
The well location automatic optimization is realized by means of an optimization module built in COMSOL, and by combining two functions of global ordinary differential and differential algebraic equation and optimization, a global equation is established, the gas production is used as a target function, a proper optimization algorithm and optimization parameters are selected, and a better optimization result is obtained through multiple debugging.
Claims (2)
1. A numerical simulation analysis method for gas extraction of abandoned mines is characterized by comprising the following steps;
the first step is as follows:
constructing a three-dimensional geological geometric model of a waste ore reservoir, assigning values, selecting a proper physical field combination, and performing simulation analysis; the large-scale waste ore reservoir contains a plurality of goafs, the caving zones and the fracture zones are not completely parallel and are not planes, a compaction zone, an O-shaped ring zone and each block section and contour of a geometric model are transversely divided, and the height of a top plate and the height of a gas content contour line come from a certain abandoned mine;
the second step is that:
firstly, desorbing and diffusing the adsorbed gas to a pressure relief area, then extracting the gas under the condition of bottom hole negative pressure, simulating the content of the adsorbed gas and the reservoir pressure of the originally planned well after extracting for a period of time, comparing the gas production rate, and analyzing and searching a position with possibly higher gas production rate;
the third step:
then optimizing the well position to simulate again, and comparing the gas production change after the well position changes; and finally, well position optimization of the gas extraction of the abandoned mine is completed.
2. The method for numerical simulation analysis of gas extraction in the abandoned mine according to claim 1, wherein a gas matrix and natural fracture coupling seepage model is established in the second step, and the concrete formula is as follows:
(1) the seepage in the matrix adopts pseudo-steady flow, and the equation expression is as follows:
in the formula: v is the amount of adsorbed gas, t is the time,. tau.is the gas diffusion time, VEIs the gas volume (varying with pressure) in the natural fracture;
(2) the natural fracture adopts the gas mass conservation law, and the equation is as follows:
in the formula: rhogIs the gas density,. phi.is the porosity,. phi.is the permeability,. mu.is the viscosity,. p is the pressure,. qdIs the gas mass exchange capacity of the matrix pores and natural fractures, wherein the gas density is derived from the gas equation of state:
in the formula: m is the gas molar mass, Z is the gas compression factor, R is the gas constant, and T is the temperature;
gas mass exchange q of matrix pores and natural fracturesdComprises the following steps:
in the formula: rhocIs the density of the coal, pgaIs the density of the coal bed gas, V is the coal bed gas adsorption capacity, and t is the time;
(3) the adsorbed gas content was characterized using langmuir isothermal adsorption equation:
in the formula: vLIs LangeMuir volume constant, pLIs the langmuir pressure constant;
the motion state of the fluid under specific conditions can be determined by giving definite conditions, and for unsteady state seepage, the initial conditions are generally as follows:
p(x，y，z，t)|t＝0＝f0(x，y，z) (6)
the boundary conditions for solving the partial differential equation are divided into three types, the first type of boundary conditions is adopted, the boundary pressure is directly given, and the formula is as follows:
p(x，y，z，t)|Γ＝f(x，y，z，t) (7)
simultaneous quasi-steady state equations, continuity equations, langmuir isothermal adsorption equations, gas state equations, initial conditions, and boundary conditions.
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