Investigation on effective stress coefficients and stress sensitivity of different water-saturated coals using the response surface method

•The effect of different water saturation on the effective stress coefficient is discussed.•The accuracy of the effective stress coefficients calculated using two methods is compared.•Correlation between stress sensitivity coefficient and water saturation is evaluated.•A model for determining the permeability control factors of CBM wells is established. The relative magnitude of confining pressure and pore pressure, a key parameter for stress sensitivity of coal permeability, can be characterized by the effective stress coefficient (ESC). In this study, the stress sensitivity experiments of different water-saturated coals were carried out according to the dewatering of coalbed methane (CBM) wells. The ESC was calculated based on the response surface method (RSM), revealing the effect of water saturation on the ESC and stress sensitivity of high-rank coal. The results showed that the stress sensitivity coefficient of these high-rank coals ranged from 0.8136 to 1.5631, indicating a strong stress sensitivity. With growing net pressure and water saturation, the gas permeability of coals changed in a negative power exponential form. Compared with the binary linear regression (BLR), the effective stress calculated by the ESC using the RSM could better reflect the stress sensitivity of permeability. In addition, the ESC increased significantly along with water saturation. Finally, a mathematical model considering ESC and reservoir pressure was established to calculate the “turning point” of dynamic permeability, where the controlling factors of permeability transform from effective stress to matrix shrinkage.