Experimental And Modeling Studies On Coalbed Methane Recovery Mechanism In Deep Coal Seams

The recovery of coalbed methane(CBM)will trigger a series of coal-gas interactions,including methane desorption,coal deformation,and permeability change.Each physical process will affect the initiation and progress of another.Advances in our understanding of coal-gas interactions help to understand the occurrence characteristics and recovery processes of CBM.These will further improve the drilling,completion,production,and stimulation technologies for coal reservoirs.Upon a comprehensive review of relevant researches,this dissertation focused on the reactions and coupled processes of methane desorption,coal deformation,and permeability change.What follows are the main achievements obtained from this research.(1)Based on gas adsorption isotherm system,methane adsorption tests were conducted on several coal samples from different basins,and the influences of moisture,temperature,and pressure on adsorption capacity were further analyzed.The equations of state and potential energy were evaluated to obtain the optimal simplified local density(SLD)theory.Some parameters should be determined with experimental results,and can be related to proximate analysis results,thus the theory was generalized.In addition,fluid mixing rule was introduced into SLD theory to characterize water and methane competitive adsorption while minimum energy method was introduced to distinguish the primary and secondary water adsorptions,based on which methane-water mixture adsorption model was developed to predict the initial methane and water in place simultaneously.A reserve calculation method for methane and water was proposed to reveal their occurrence state in situ.(2)Based on coal cleat permeability testing apparatus,stress and temperature sensitivity experiments were carried out on several coal samples,proving that the permeability change with temperature may be the opposite of those experienced with different stresses,and the inversion point is where the permeability decrease due to thermal expansion is offset by matrix shrinkage.To develop the original permeabilty expressions,the permeability influence factors were rearranged into three dimensionless variables on the basis of dimensional analysis,and their relation can be described with the fitting of experimental data.The relationship between cleat volume compressibility and effective horizontal stress was re-drived on the basis of dimensional analysis,considering that the porosity changing rate caused by effective horizontal stress gradient is constant.This relationship was then used during the derivation of analytical permeability model,followed by comparison with four widely used models.(3)The adsorption induced coal deformation model was developed on the basis of surface energy theory,considering that coal strain is proportional to the surface energy change.The adsorption-strain-permeability coupled model was then developed on the combination of SLD adsorption theory,coal strain model,and coal cleat permeability model.The coupled model can quantitatively describe the interactions of methane desorption,coal deformation,and permeability change on one another.The coupled model was integrated into the Transport of Unsaturated Groundwater and Heat Simulator,illustrating the gas flow rules during CBM recovery.