TRANSIENT WELL TEST MODEL FOR THE WELL WITH GAS AND WATER DISTRIBUTED IN COALBED

Coalbed methane (CBM) is a kind of efficient and clean unconventional gas resources, and its producing process is mainly drawing down the formation pressure by draining water. CBM is primarily adsorbed in the coal rather than the most conventional gases stored in the pore space. CBM is desorbed from the coal when the coal bed pressure is lower than the critical desorption pressure, and is produced out with water. So, the fluids in coalbed are gas and water in different distributions. This paper introduced a set of transient well test model on the basis of desorption characteristic of CBM, the permeability relationships depended on gas and water distributions and the size of desorption area. The flow state of gas and water was described very well by using the separated areas in the CBM producing process. The model was solved by using finite volume method(FVM), and its type curves were obtained. The effects of the desorption coefficient, the desorption composite radius, the saturation distribution of gas and water, etc on type curves are analyzed. The research results of this study are of great guiding significance to develop CBM economically.