Abstract: The coalbed methane reservoirs are characterized by strong heterogeneity, uneven distribution of fracturing fractures, and usually asymmetric distribution. At present, there is no evaluation model for productivity of coalbed methane reservoirs with asymmetric fractures. In view of the above problem, Langmuir isotherm adsorption equation and Fick diffusion law were used to explain the adsorption, desorption and diffusion in coalbed methane reservoirs, and a special pseudo-time function was introduced to solve the strong nonlinearity in the material balance equation considering adsorption and desorption. The new defined parameters were used to characterize the asymmetry of hydraulic fractures in the fracturing process, and the productivity evaluation model and its analytical solutions under different symmetry of fractures were obtained. With the principle of time superposition, the actual production data was fitted with the theoretical data. It was found in the study that the asymmetry of fractures greatly affected the results of productivity evaluation and prediction of gas reservoirs; the energy utilization rate of symmetric fractures was the highest; the asymmetry of fractures mainly affected the productivity evaluation in the early production; with the increase of fracture conductivity, the effect of the fracture asymmetry on gas well productivity became lower. The study results are of great important guidance for the accurate evaluation and productivity prediction of fractured vertical wells and the efficient development of coalbed methane reservoirs.

Key words: coalbed methane, asymmetric fracture, fracturing, productivity evaluation, production prediction, seepage model