The Difference between Stress Deformation and Adsorption Deformation of Coal Reservoirs and Its Role in Permeability Prediction.

The key to the variation in permeability within coal reservoirs lies in the stress-induced deformation and desorption-induced deformation during the coalbed methane (CBM) production. The differences in sample scale and measurement methods between stress-induced deformation and desorption-induced deformation significantly affect the accuracy of permeability measurements. Therefore, in order to elucidate the relationship between stress-induced deformation and adsorption-induced deformation, as well as the influencing factors, and to assess the accuracy of permeability evolution prediction, this study conducted a series of parallel experiments, including compression deformation experiments under stress loading (stress-induced deformation), methane adsorption-induced deformation experiments (adsorption-induced deformation), μCT scanning, and overburden permeability measurements.The results of the study indicate that stress-induced deformation and adsorption-induced deformation are negatively correlated but exhibit a relatively weak correlation. Stress-induced deformation encompasses deformation of coal matrix, minerals, and fractures, whereas adsorption-induced deformation primarily reflects coal matrix deformation. While there is some overlap between the two, they are not entirely identical. The main influencing factor of stress-induced deformation is the mechanical strength of coal, with minerals in coal increasing the Young's modulus of coal reservoirs. Among them, minerals that are more dispersed and have smaller particles have a more significant impact on stress-induced deformation. The primary influencing factor of adsorption-induced deformation is the deformation capability of the coal matrix, with minerals and fractures having less significant effects. Permeability changes are controlled by fracture deformation, but stress-induced deformation measurements weakly reflect this aspect, leading to an inability to accurately predict the scale of the impact of effective stress changes on permeability during CBM production and CO2-ECBM processes. In contrast, adsorption-induced deformation relatively accurately reflects the deformation capability of the coal matrix and provides a more accurate prediction of permeability rebound under the condition of almost unchanged effective stress in the late stages of mining. Therefore, deformation parameters under stress loading are challenging to directly apply to the prediction of permeability evolution, while adsorption-induced deformation parameters can be effectively utilized.