Numerical Simulation Study Of Gas Extraction From Abandoned Coal Mines

There are many characteristics of abandoned coal mines in China,such as large quantity,large area of goaf and many residual coal in the mine.The development of abandoned coal mine gas has multiple benefits of safety,economy,environment and society.However,due to the influence of natural fractures and artificial excavation,the heterogeneity of abandoned coal mine reservoirs in 3D geological model,gas content,permeability and other aspects is stronger than that of conventional coalbed methane reservoirs.At the same time,the occurrence state of gas in each area and the migration rule under the pumping state are also more complex,which greatly increases the difficulty of selecting the appropriate drilling location.In view of the fact that most of the wells location are selected by experience,this paper will study the reservoir characteristics,gas flow law,porosity and permeability heterogeneity distribution and calculation model of abandoned coal mines,master the influence law of abandoned coal mine gas extraction,carry out the numerical simulation of abandoned coal mine gas extraction.After the feasibility is verified by a case,the 3D flow field distribution is analyzed,then a series of cases are set up to simulate and optimize the well location,and the suggestions for optimizing the well location and increasing the well location of a abandoned coal mine are given,and the sensitivity analysis of some parameters affecting the gas production is carried out.The main research contents of this article are as follows:(1)Investigate the distribution and division basis of the fracture scale in the horizontal and vertical direction of the reservoir,analyze the evolution law of mining fracture field,study the characteristics and influencing factors of fracture development in different regions,and qualitatively and quantitatively analyze the law of the porosity and permeability of the abandoned coal mine reservoirs.(2)Investigate the occurrence status and migration characteristics of abandoned coal mine gas,study the flow patterns of gas experienced from the abandoned coal mine to wellbore,establish a suitable mathematical model of fluid flow,and describe the process of gas desorption,diffusion and flow in porous media.(3)Establish a 3D geological model of the abandoned coal mine reservoirs,use COMSOL for numerical simulation,verify the feasibility through the case,design optimization and add well location cases for simulation,and give an abandoned coal mine well location optimization proposal.(4)For the same reservoir,when the well is located in the unexploited area,the gas production efficiency is obviously low;when the well is located in the fracture zone of the lateral fracture zone and the re-compaction zone,the gas production condition is good;when the well is located in the fracture zone of the "O" ring,the gas production is the highest,and the reservoir pressure drops the fastest.When the well is located in the compacted goaf,the gas production is low,and the gas production is good;when the well is located in the fracture zone,the gas production highest.The influence of bottom hole extraction pressure on gas production is small,and gas production decreases with the increase of bottom hole extraction pressure.(5)In addition to theoretical analysis and numerical simulation,this paper combined with optimization algorithm to explore the numerical simulation method of automatic optimization of well location,to realize the automatic optimization of local well location and develop relevant APP,to provide certain theoretical guidance and technical support for realizing the optimization of well location in abandoned coal mines and improving the efficiency of gas production in abandoned coal mines.