Mechanism Of Heat-Moisture-Fluid-Solid Interactions In Coal Seam Gas Recovery

Coal seam gas(CSG/CBM)is an important part of clean energy.It plays considerable role in promoting the energy production and consumption revolution.In China,the reserves of coal seam gas is abundant,however the reservoir conditions are extremely complex,leading to very inefficient gas recovery.Coal seam gas recovery is a process that coupled with multi-physics.The interactions among temperature,humidity,gas seepage and coal deformation are becoming more and more remarkable with the deepening of coal seam gas recovery and the exploring of special mining methods,which seriously limits the development of coal seam gas production in our country.Therefore,it has scientific background and engineering significances to study the interactions in coupled multi-physics.Based on the characteristics of reservoir structure,gas storage and transport,this paper discussed the interactive phenomena and their mechanism among heat,moisture,fluid and solid in coal seam gas recovery.The scientific methods of experimental testing,theoretical analysis as well as numerical simulation are synthetically used here.The major advances and conclusions can be drawn as follows:(1)The permeability of raw coal increases exponentially with gas pressure,decreases exponentially with confining pressure,and it evolves variously with increasing temperature.The seepage behavior under variable temperature,confining pressure and gas pressure of raw coal samples from Pingdingshan mine,Changzhi mine and Jinjia mine were tested in laboratory,respectively.The experimental results demonstrate that raw coal permeability increases exponentially with the increasing gas pressure,the exponential function pattern of permeability evolution is obvious under low confining pressure,whereas the evolution trends to be linear-like pattern under high confining pressure.Raw coal permeability decreases exponentially with the increasing confining pressure,when the confining pressure is low,coal permeability is very sensitive to its change,permeability decreases rapidly with the increasing confining pressure firstly and then tends to be stable.The permeability of most raw coal decreases with increasing temperature,whereas decreases firstly and then increases sometimes.(2)Theoretical modellings and laboratory verifications on temperature and humidity change induced microscopic physical processes such as thermal fracturing,thermal volatilization,thermal sorption and thermal expansion were respectively conducted.These work reveal the micro mechanism of permeability evolution under coupled multi-physics conditions essentially.Thermal fracturing increases the matrix permeability but decreases the fracture permeability as it not only promotes the growth of micro cracks/pores in coal matrix but also induces overall expansion of coal matrix that compacts fracture network.The volatilization model of residual moisture in form of water film on fracture surface is established,and it illuminates the physical mechanism that water volatilization enhances fracture permeability as it increases the fracture aperture directly.Thermal sorption enhances the matrix and fracture permeability because gas desorption induces matrix shrink.Thermal expansion restrains both the matrix and the fracture permeability.(3)A new general dual permeability model was derived,it divides the evolution of coal permeability with increasing temperature into three categories theoretically;A thermal stimulation index KT was defined to evaluate the thermal stimulation efficiency of temperature to coal permeability.Based on the model,a plenty of permeability testing results were generally described,different evolution patterns were also summarized and evaluated.The results show that the evolution of coal permeability with temperature increment can be well classified into three categories:the thermal expansion dominated decreasing type,the thermal volatilization dominated increasing type,and they orderly dominated 'U' letter type.Moreover,the presences of secondary influencing factors make the first two types have three sub-classes:linear pattern,upward bending pattern and concave bending pattern.KT can be used to evaluate the stimulation efficiency of temperature to coal permeability,when KT>0,the increase of temperature enhances coal permeability,whereas KT<0,the increase of temperature restricts coal permeability.(4)It is discovered that moisture loss plays dual role of promoting and inhibiting in coal seam gas migration,moreover basic principle of water block induced non-Darcy flow of coal seam gas is revealed.A fully coupled moisture-fluid-solid model was set up based on a water film-permeability model and a non-Darcy flow law with threshold pressure gradient(TPG).The model analyzed the phenomenon of water block and the coupling mechanism for coal gas interactions in wet coal seam gas recovery.The results show that gas recovery in wet coal seam partitions with production distance,only when the actual gas pressure gradient in a certain distance is larger than the threshold pressure gradient,gas flow forms.Moisture loss causes many effects,on the one hand,it improves the permeability thus promotes gas production;on the other hand,it restrains gas desorption thus hinders production.In all,moisture loss induced improvement of coal permeability plays a leading role.(5)The coordinating and promoting effects of thermal fracturing,thermal desorption and thermal volatilization on dual coal permeability are the essential stimulations of heat injection to enhanced coal seam gas recovery.A fully coupled heat-moisture-fluid-solid model that suitable for heat injection enhanced coal seam gas recovery was established.Based on the model,the interactions in multi-physics were discussed.The results show that heat injection can effectively enhance gas production.For the matrix porosity and permeability,thermal sorption and thermal fracturing provide promotion contribution due to the high desorption and the pore growth caused by rising temperature,whereas thermal expansion provides restriction contribution.For the fracture porosity and permeability,thermal volatilization of residual moisture and thermal desorption induced reaction of matrix shrink provide promotion contribution.The enhancement of thermal fracturing to gas production is to coordinate the matrix permeability and the fracture permeability effectively.