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.