Physical Simulation Experimental Study On Coal Rock Permeability Sensitivity

In order to study the dynamic permeability changes of coal reservoirs of different coal ranks during development,using the LFLab-I coal rock triaxial stress-strain(matrix contraction and expansion)test system,carried out physical simulation experiments of coal rock permeability sensitivity,including single-factor physical simulation experiment of effective stress effect,single-factor physical simulation experiment of matrix contraction effect and double-effect physical simulation experiment of effective stressmatrix contraction.Finally,based on the experimental results,permeability characteristics are described quantitatively.The effects of coal rank,coal quality and constant effective stress on the dynamic permeability of coal reservoirs are analyzed,and combined with experimental knowledge,it provides suggestions for the optimization of the coalbed methane target area and the optimization of extraction work system.The experimental results show that:(1)The permeability of coal samples of different ranks decreases exponentially with the increase of effective stress under triaxial stress state,and the change trend of permeability can be described quantitatively.And as effective stress increases,stress sensitivity decreases.Low coal rank coal sample are more stress sensitive than medium coal rank and high coal rank coal samples,and the development of cracks will strengthen the stress sensitivity of coal rocks.(2)Under the influence of matrix contraction,as the reservoir pressure decreases,the permeability of low,medium and high coal rank coal samples increases exponentially.Basically,the reservoir pressure is 3MPa as the critical point,and then the permeability increases rapidly.In addition,coal rank and constant effective stress have important effects on the dynamic characteristics of permeability.(3)Under the double-effect of effective stress and matrix contraction,the permeability of coal samples of low,medium and high coal rank decreases first and then stabilizes or decreases first and then increases as the drainage progresses.The trend is "L"pattern or asymmetric "V "" Pattern change.The permeability is reduced to 50%-60% of the initial permeability,and it will not continue to decrease.And the dynamic permeability rebound effect of coal samples of middle and high coal ranks is better.(4)Combined with the experimental knowledge,it is recommended to add the index of dynamic permeability when the CBM target area is optimized;and based on the dynamic permeability change of coal reservoirs of different coal ranks during the development process,provide suggestions for the optimization of the drainage and production work system in different drainage and production stages.