Dynamic permeability change during coalbed methane production and its controlling factors

Abstract

Coal reservoir permeability, which changes constantly during the exploitation process, is a key factor for determining the production effects and ultimate recovery of coalbed methane (CBM) wells. To calculate the dynamic permeability more accurately, we reviewed the controlling mechanism of coal reservoir permeability and then established three mathematic permeability change models considering the effective stress, matrix shrinkage and gas slippage respectively. Relation between the production data and permeability by adopting the material balance theory was established and then a production data-based theoretical model for calculating the permeability change during the drainage process was derived. Seventeen wells from the Fanzhuang–Zhengzhuang block in the southern Qinshui Basin of China were employed for a case study. The calculated results clearly reflect the dynamic permeability change. The dynamic permeability curves can be divided into “decline type”, “rebound type” and “rising type”. Furthermore, the impacts of pressure drop, daily gas production, and final pressure on permeability change were analyzed. The results show that the rate of permeability change induced by effective stress had a significant positive linear correlation with the pressure drops. A quadratic relationship existed between the rate of permeability change induced by matrix shrinkage and daily gas production rate. The lower final pressure will give rise to the faster permeability change that induced by gas slippage, which indicates a typically exponential relationship.