A Permeability Model for Undersaturated Coalbed Methane Reservoirs Considering the Coal Matrix Shrinkage Effect

Abstract

With the analysis of key elements on the strain state of coal, a permeability dynamic prediction model which is divided by the critical desorption pressure for undersaturated coalbed methane (CBM) reservoirs was established on the basis of pore pressure and considering the matrix shrinkage effect of coal. The law between permeability and pore pressure was analyzed during production with the new model. Through case study, the rationality of the model was also verified. The research shows that the degree of permeability changes mainly depends on the relationship between the critical desorption pressure and the rebound pressure which depends on the strength of the matrix shrinkage. Under the condition of equivalent matrix shrinkage, the reservoirs permeability rebounds better with high Young's modulus and low Poisson's ratio. Adjustment factor contributes to improve the influence of matrix shrinkage on permeability and the larger the matrix shrinkage strength is, the higher the permeability rebounds. PM model and CB model are similar to the new model. PM model limits the matrix shrinkage strength, and CB model is a special case of the new model. Comparing with the well test permeability, the new model is more reasonable to characterize the matrix shrinkage effect in the development process.