Stress sensitivity of coal reservoir and its impact on coalbed methane production in the southern Qinshui Basin, north China

Stress sensitivity has significant negative effects on the permeability and production of coalbed methane(CBM)reservoirs.To effectively minimize these negative effects,the degree of stress sensitivity during the CBM production process should be carefully studied.In this work,the curvature of the stress-sensitivity curve was adopted to explore the degree of stress sensitivity,dividing the stress-sensitivity curve and the drainage process into five stress stages:sharp decrease,rapid decrease,low-speed decrease,slower decrease and harmless with four critical stress points—transition,sensitivity,relief and harmless.The actual stages were determined by the initial permeability,stress-sensitivity coefficient and difference between the reservoir pressure and desorption pressure.The four critical stress points did not completely exist in the stress-sensitivity curve.With an increase in the initial permeability of coal,the number of existing critical stresses increases,leading to different gas-water drainage strategies for CBM wells.For reservoirs with a certain stress-sensitivity coefficient,the permeability at the sensitive stress point was successively greater than that at the transition,relief and the harmless stresses.When the stress-sensitivity coefficient is different,the stage is different at the beginning of drainage,and with an increase in the stress-sensitivity coefficient,the decrease rate of the permeability increases.Therefore,the stress-sensitivity coefficient determines the ability to maintain stable CBM production.For well-fractured CBM reservoirs,with a high stress-sensitivity coefficient,permeability damage mainly occurs when the reservoir pressure is less than the relief stress;therefore,the depressurization rate should be slow.For CBM reservoirs with fewer natural fractures,the reverse applies,and the depressurization rate can be much faster.The higher the difference between the reservoir and desorption pressures,the higher the effective stress and permeability damage after desorption,resulting in a much longer drainage time and many difficulties for the desorption of coalbed methane.The findings of this study can help better understand and minimize the negative effects of stress sensitivity during the CBM production process.

Pore structure characteristics of low-rank coal reservoirs with different ash yields and their implications for recoverability of coalbed methane—a case study from the Erlian Basin, northeastern China

Pores are the main accumulation sites and migration pathways for coalbed methane(also referred to as CBM).Pore structure restricts the content and recoverability of CBM from coal reservoirs.In this study,12 representative coal samples with different ash yields that have similar tectonic characteristics and burial depths were collected from different mining areas in the Jiergalangtu and Huolinhe depressions in the Erlian Basin.These samples were used to study the restrictions of ash yield on the characteristics of coal pore structures and the recoverability of CBM through macroscopic and microscopic structure observation,scanning electron microscope observations,vitrinite reflectance tests,low-temperature N2 adsorption,nuclear magnetic resonance(NMR),and micro-computed tomography.The results show that coal reservoirs in the study area vary greatly in ash yield,based on which they can be divided into three types,i.e.,low-ash-content,ash-bearing,and high-ash-content coal reservoirs.In addition,the ash yield has a certain impact on the development of coal pores;coal samples with lower ash yields indicate the presence of well-developed medium-large pores and better connectivity.Ash yield also has a certain impact on the brittleness of coal wherein a lower ash yield implies the development of brittle coal that is more liable to fracture as compared to less brittle samples at the same pressure.Absorbed gas content also varies significantly with ash yield;a low ash yield impacts the gas saturation of coal.Overall,for coal reservoirs in the study area,their porosity,pore diameter,movable fluid porosity,adsorbed gas amount,and recoverability decrease as the ash yield increases.