A fractal production prediction model for shale gas reservoirs

A production forecasting model for shale gas reservoirs is proposed based on the continuum medium theory (organic/inorganic matrix) and discrete fracture model (fracture system). The fractal property of the pore size distribution of shale matrix is considered. The stability and accuracy of the model are successfully validated with the field data. Variations of different flow mechanisms during the exploitation of shale gas are investigated. The sensitivities of gas production to the pore size distribution of shale matrix, natural fracture parameters and hydraulic fracture parameters are analyzed. Our results show that: (1) The contribution of the convective flow and surface diffusion to total gas flow increases with the depletion of shale gas, while that of the bulk diffusion and Knudsen diffusion decreases gradually. (2) Among the fractal pore-size parameters, the maximum inorganic pore diameter has the most significant impact on the cumulative gas production of up to 13.9%. (3) After the natural fracture number increases to a certain extent, gas production increases slowly even if more natural fractures exist in the reservoir. The complex fracture network is beneficial to increase shale gas production. (4) The cumulative gas production rises as the increase of the hydraulic fracture half-length, flow conductivity, spacing and number. The rapid production rate in the early production stage may result in a fast reservoir energy depletion, which has a negative effect on production.