| Gob coal seam gas well extraction is a unique way of CBM development in coal mining area,and it is an important way to achieve high efficient surface methane drainage,especially in soft and low permeability coal seams,using the overburden rock to break down and release pressure produced by the mining of coal seam,to improve the permeability of the upper adjacent coal seam.It is also an effective measure to prevent gas overrun in the upper corner of the working face.However,for many years,the numerical simulation of gob coal seam gas well extraction productivity has always been a worldwide difficult problem.This paper is based on the "12th Five-Year" National Science and Technology Major Project "prediction technology of coalbed methane production capacity in coal mine area(2011ZX05040-003)",takes the Huainan mining area of Anhui Province,which is exploited by coal seam group,as the research object,using the theory and method of coal geology,coalbed gas geology,mining science,seepage mechanics,mathematical physics method,oil and gas reservoir numerical simulation technology and so on,through field investigation,theoretical research,analysis test,reservoir modeling,numerical simulation and engineering test,the research work is carried out on the migration and output law of gob coal seam gas well extraction,the spatio-temporal evolution model of permeability caused by mining,the numerical simulation model and simulation method of the gob coal seam gas well extraction,etc.A software for numerical simulation of gob coal seam gas well extraction productivity in coal mining area has been developed.The research results are applied to the history matching and productivity prediction of gob coal seam gas well extraction of Huainan mining area,and the following main research results and knowledge are obtained.(1)The law of occurrence,accumulation,migration and production of coalbed methane in mining area is revealed.With the continuous mining of working face,mining causes the upper strata and coal seam to break,fracture,release pressure,leakage of stratum water and reservoir pressure drop.The coalbed methane desorption and diffusion enter the mining-induced fractures,and then flow and yield to gob coal seam gas well under the action of pressure gradient.The production data analysis of 106 gob coal seam gas wells of 24 working faces in Huainan mining area shows that it is obviously different from conventional CBM wells.The gob coal seam gas wells show unique characteristics of gas production,and the daily gas production is multiplied by several times to dozens of times,and the relationship curve of the daily gas production and time increases rapidly in the early stage until the maximum peak,and then rapidly descends to the lowest level,and maintains a long time stability,it shows a typical characteristic of "single spike dragged long tail".(2)It is found that there is a good correlation between the daily gas production of the gob coalbed methane well and the distance from the gob coalbed methane well to the working face.And its relationship curve also presents the typical morphological characteristics of the " single spike dragged long tail ".That is,when the working face is pushed forward to more than 5m of the gob coal seam gas well,the gas output increases sharply in a steep wall,reaching the maximum peak when the working face exceeds the gob coal seam gas well about 50 m,and the gas output decreases rapidly as the working face continues to advance until the gob coal seam gas well is overtaken 280 m,and the higher gas production is still maintained after 1000 m.The unique gas producing characteristics of the gob coal seam gas well are mainly due to the disturbance caused by the working face mining,which make the upper coal seam and rock strata break down and pressure relief,strengthen the infiltration and gravitational compaction,caused the permeability of upper coal seam to increase sharply in geometric magnitude,and then decrease rapidly.Secondly,it is related to the factors such as water rapid leakage and pressure relief of the upper coal seam and strata.(3)Because the dynamic evolution process of the upper coal seam permeability in the coal mine gob is very complicated,it is difficult to represent comprehensively and realistically with a function.Based on the black box theory and the Boltzmann function fitting,the mathematical expression of the variation of the apparent permeability of the upper coal seam in the coal mine gob with the distance between the working face and the gob coal seam gas well is proposed for the first time,which represents the equivalent permeability of the upper coal seam in the coal mine gob,to comprehensively reflect the actual influence of coal seam permeability on coalbed methane well productivity under mining disturbance.(4)The generalized geological model,three-dimensional two-phase mathematical model and numerical model of gob coal seam gas well productivity simulation are established.And the method of boundary treatment,numerical solution and key parameter fitting is given.Compared with the conventional CBM numerical simulation model and solution method,it has the following characteristics: first,the mining disturbance of the working face causes the dynamic boundary condition,which makes the range of the mining area larger with time;second,the reservoir parameters of the upper coal seam in the mine gob such as permeability,reservoir pressure and porosity are dynamically changed;third,the production of gas in the CBM well is undrained,and the water in the coal reservoir is leaked down through the mining fissure,resulting in reservoir pressure drop;fourth,the method of obtaining the morphological control parameters and the permeability extremum parameters in the mathematical expression of the coal reservoir apparent permeability is put forward,that is to obtain the morphological control parameters in the mathematical expression of the coal reservoir apparent permeability through the data fitting of the gob coal seam gas well production and the distance from the well to the working face,to obtain the permeability extremum parameter in the mathematical expression of apparent permeability of coal reservoir by fitting the production data of gob coal seam gas well.(5)The numerical simulation software CCGS v1.0 for the production capacity of gob coal seam gas well is developed,which has realized the numerical simulation of single well pumping productivity affected by the mining action.Based on the conventional CBM capacity numerical simulation software CBM-SIM,a special computer module has been developed to integrate,process and assign the dynamic parameters of coal seam permeability,reservoir pressure,leakage of water in the upper coal seam of gob.The mechanism of time card interrupting and renovating parameter is innovated,and the function of automatically updating the dynamic parameters of coal reservoir in time step is realized in the operation process.A dual function software platform for gob coal seam gas well productivity numerical simulation and conventional CBM well productivity numerical simulation is formed.The software has friendly interface,flexible operation and convenient operation.(6)The research results are applied to the coal mines of Pan Yi Mine,the Pan San Mine,the Gu Qiao Mine,the Ding Ji Mine and the Zhu Ji Mine,in Huainan mining area.The production data history fitting and the gas production numerical simulation prediction are carried out on 11 gob coal seam gas wells of 8 working faces,the results have been verified and good application results have been obtained. |
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