Stress distribution and permeability modelling in coalbed methane reservoirs by considering desorption radius expansion

Abstract

Coal permeability is significantly stress-dependent and changes during gas production. In the previous studies, less attention has been paid to the impact of desorption radius and its expansion on the stress distribution and permeability changes. In this work, a mathematical model is developed to analytically evaluate the dynamic stress distribution and accordingly permeability by coupling the geomechanics, sorption and fluid flow in the cleat system. In this approach, the coalbed is divided into two regions: desorption area and non-desorption area. The desorption area represents the region with a low water–gas ratio, where the pressure squared (P2) approach is applied for flow modelling. The non-desorption area represents the region with a high water–gas ratio with almost no desorption effect, where Darcy’s equation (P approach) is used for flow modelling.The results indicate that previous models, in which either uniform desorption or no desorption was assumed, cannot reflect the correct stress distribution in coalbed and accordingly overestimate or underestimate permeability, respectively. This is attributed to neglecting the varying desorption radius. The results demonstrate that this has a significant effect on stress distribution. The proposed model gives a more realistic evaluation of stress distribution and permeability as it only considers the effect of matrix shrinkage in the desorption area.