Transient Flow in Naturally Fractured Reservoirs and its Application to Devonian Gas Shales

For many years gas has been produced naturally from Devonian shales throughout the Appalachian Basin at rates which made them commercially feasible. In 1975, because of the deteriorating gas supply situation in the United States and increasing demand for energy, the U.S. Department of Energy (DOE) established the Eastern Gas Shales Project (EGSP) as part of the Unconventional Gas Recovery (UGR) program to study and characterize and to develop economically feasible technology for further exploitation of Devonian shale gas reservoirs. Devonian shale gas reservoirs typically are characterized by a low storage, high flow-capacity natural fracture system fed by a high storage, low flow-capacity rock matrix. In this study analytical solutions are developed to analyze the basic fractured reservoir parameters that control well productivity. These parameters include fracture system porosity and permeability, matrix porosity and permeability, and matrix size. It is shown that the conventional well test method does not usually work for fractured Devonian shale gas reservoirs. For most cases, the semilog plot of the drawdown and buildup data does not show two parallel straight lines with a vertical separation. Numerical solutions are also used to include the Klinkenberg effect and desorption in the shale matrix.