| The Shanxi province is rich in coalbed methane.Developing industry of coalbed methane has an important role in the economic and social development and environment protection.And coalbed methane production depend on hydraulic fracturing and drainage gas recovery both of which can effect on the environment of ground water.Hydraulic fracturing can lead to some artificial fracture with some regularity distribution.Those fracture together with the original hole and fracture system of coal seam make up dual medium which changes the structure of original coal seam and permeability characters.Process of drainage gas recovery can drainage a lot of formation water which can lead to decline of ground water level and impact on regional water dynamic field.Aiming at the problem that coalbed methane mining has an impact to hydrodynamic characters of dual medium,this paper is based on the research of Baode block of East coalbed gas field,based on the investigation,collection and analysis of geology,hydrogeology,gas recovery program and the hydraulic fracture growth law of the block.Firstly,using the COMSOL Multiphysics which is the multiphysics simulation software build the aquifer flow model of dual medium.After simulation,the results of water flowing regulation and the coefficient of permeability of horizontal direction in the aquifer can be got before inter-well interference.Secondly,the conceptual hydrogeological model and mathematical model can be built,then the model was simulated using the Visual MODFLOW simulation of groundwater numerical model computer software based on the finite difference method.The results of the impact on groundwater flow field in the process of group wells drainage.According to the simulation results,the main conclusions are as follows:?1?In the process of flowing,flow velocity change divide into 3 stages which is the hard descend,the stabilized descent,the slow descend.But under the different pressure conditions,flow velocity change is different.Overall,the pressure difference is more greater,the time of the flow velocity declining to the stage of the slow descent take more time,and the last flow velocity is more greater.?2?The waterhead change also divide into 3 stages which is the hard descend,the stabilized descent,the slow descend.But under the different pressure conditions,the waterhead change is different.The pressure difference is more greater,the last head is more small.?3?Before the inter-wells interference,in the single well drainage case,the process of coalbed water flowing is following.At the beginning of drainage,waterhead of the end crack end rapidly lower and the flow velocity of coalbed water of reaching the well along the crack decreased.At this time,water of draining is the water of fracture,and due to the permeability difference,there has been a little of water of coal seam adding to the fracture.When water of the crack end is in the stage of the slow descend,most of original coalbed water stored in the fracture is finished.Driven by pressure difference,the water in the coalbed seam constantly and slowly seep into the fracture and flow into the well along the fracture.the seepage law of the fracture in the both sides of the well is the same.?4?For the coal seam of 4+5#,permeability coefficient of dual medium is0.0135 m/s in the x direction,and permeability coefficient is 8×10-8 m/s in the y direction.For the coal seam of 8+9#,permeability coefficient is 0.011 m/s in the x direction,and permeability coefficient is 6×10-8 m/s in the y direction.Permeability coefficient in x direction is well past the value in y direction,and this results show that permeability of dual medium which is formed by fracturing the coal seam is different in different directions.?5?In the case of the group wells drainage,the level of aquifer in the coal stratum has shown an overall downward trend,but overall current direction which is flowing from the east to the west has not changed.The current direction has changed in the local region and those area is mainly concentrated in the east of the simulation area.Bounded by the coal seam gas mining area,the level of groundwater has little land in the west of the mining area and current direction has little changed.In the mining area and the east of the mining area,with the extension of the time for drainage water level drop significantly and water-table contours are continuously varying thinly which show hydraulic gradient gradually slowed. |
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