The gas-water two phase flow behavior in low-permeability CBM reservoirs with multiple mechanisms coupling

Although considerable advances have been achieved in recent years, it is still challenging to describe gas-water two phase flow behavior in low-permeability CBM (Coalbed methane) reservoirs. The pressure propagation characteristics at different production stages are generally overlooked, which play a crucial role to affect overall productivity and are required to be incorporated. In this work, a completely new production prediction model is established. Firstly, to consider pressure and saturation gradients, the pressure-saturation relationship is developed after rigorous derivation process in terms of partial differential equations for gas-water two phase flow. Subsequently, the gas/water phase productivity equations are modified on the basis of the key relationship. Secondly, according to the relationship between formation pressure and critical desorption pressure, the whole coal seam is divided into desorption area and without-desorption area. Moreover, pressure distribution features in desorption area and that in without-desorption area are characterized. Finally, an efficient iterative algorithm is present to predict the production performance and quantify the expansion regularities of drainage and desorption area at different production stages. The proposed model is utilized to shed light on the optimization of production strategy as well. Some meaningful conclusions and practical insights are drawn.