Experimental Investigation On Permeability Evolution Of Steam-hot Water-gas With Gas Recovery By Thermal Injection

Coal seam gas,also called coalbed methane,is a clean and unconventional gas energy source.The pumping of gas（coalbed methane）is a major national demand for reducing gas disasters,reducing greenhouse gas emissions and increasing natural gas supply.The low permeability of coal seam is the bottleneck restricting gas extraction.One of the important characteristics of gas extraction in low permeability coal seam is insufficient gas pressure driving force in later stage,long extraction period and serious influence on excavation substitution.Increasing gas production by heat injection is an advanced technology which is expected to solve this problem.The study on the permeability of steam,water and gas during heat injection is relatively weak,especially for the evolution law of the permeability of steam in coal.Aiming at the above problems,the seepage experiments of gas,steam and hot water in two typical stages of elasticity and failure are carried out,and the axial strain,radial strain and volumetric strain of coal under temperature are measured simultaneously.The influence of temperature on the permeability and thermal deformation of coal,the law of steam permeability and thermal strain evolution during injection of hot fluids（steam,hot water）,as well as the law of hot water permeability and thermal strain evolution are obtained.Subsequently,the experiment of steam thermal excitation of coal containing gas is carried out,and the regularity of increasing gas production is obtained.Finally,the influence of temperature on permeability is studied theoretically,and the following research results are obtained.Experimental aspect:（1）In elastic stage and failure stage,CH₄permeability decreases with the increase of effective stress at the same temperature but different effective stress.However,under the same effective stress but different temperatures,CH₄permeability decreases initially and then increases with the increase of temperature,and the inflection point temperature of high effective stress increases relatively to that of low effective stress,leading to an explanation that the internal and external expansion of coal caused such experimental phenomenon.（2）In elastic stage and failure stage,under the condition of same effective stress but different temperature,the permeability of hot water and steam decreases initially and then increases with the increase of temperature;by analyzing the experimental results of hot water yield,it is found that under the same effective stress,the liquid yield of hot water increases first and then decreases with the increase of temperature.At the same temperature,the yield of hot water increases with the decrease of effective stress,and the higher the temperature of hot water is,the lower the accumulative yield of hot water becomes.By analyzing the variation of parameters of hot water and steam along with time,it is found that the permeability and instantaneous flow rate of hot water and steam will fluctuate periodically,and with the increase of temperature,the pulsation frequency increases,and the amplitude decreases,which attaches hydrodynamic instability and pulsation to coal’s complex pore structural characteristics such as wide pore size range and the uneven distribution of various pore sizes.（3）In the process of injecting hot steam and hot water,the strain of the coal body increases with the increase of the thermal injection time;the coal body as a whole shows the characteristics of thermal expansion,and the internal expansion is also observed during the process.（4）Comparative experiments of thermal excitation of steam to gas under different mechanical stages and pore pressure conditions are conducted.When the coal sample is in the elastic stage,the effect of steam thermal stimulation is more obvious for low gas which is difficult to extract,and the production can be increased by 27.6%under 0.5 MPa pore pressure.When the mechanical stage is in the failure stage,the thermal stimulation effect of steam is more significant,and the pore pressure of 1.0 MPa and 2.0 MPa can be increased by42.19%and 39.08%respectively,which is due to the more developed pores of the coal body,facilitating the injection of heat.Theoretical aspect:Based on experimental principles,this paper constructs a gas permeability model coupled with temperature and effective stress.