A Numerical Study of Thermal-Hydraulic-Mechanical Simulation With Application of Thermal Recovery in Fractured Shale-Gas Reservoirs

Abstract

Summary Shale gas is playing an important role in transforming global energy markets with increasing demands for cleaner energy in the future. One major difference in shale-gas reservoirs is that a considerable amount of gas is adsorbed. Up to 85% of the total gas within shale may be found adsorbed on clay and kerogen. How much of the adsorbed gas can be produced has a significant effect on ultimate recovery. Even with improving fracturing and horizontal-well technologies, the average gas-recovery factors in US shale plays are only approximately 30% with primary depletion. Adsorbed gas can be desorbed by lowering pressure and raising temperature and reservoir-flow capacity can be also influenced by temperature, so there is a big prize to be claimed by use of thermal-stimulation techniques to enhance recovery. To date, not much work has been done on thermal stimulation of shale-gas reservoirs. In this study, we present general formulations to simulate gas production in fractured shale-gas reservoirs for the first time, with fully coupled thermal-hydraulic-mechanical (THM) properties. The unified-shale-gas-reservoir model developed in this study enables us to investigate multiphysics phenomena in shale-gas formations. Thermal stimulation of fractured gas reservoirs by heating propped fractures is proposed and investigated. This study provides some fundamental insight into real-gas flow in nanopore space and gas-adsorption/desorption behavior in fractured gas shales under various in-situ conditions, and sets a foundation for future research efforts in the area of enhanced recovery of shale-gas reservoirs. We find that thermal stimulation of shale-gas reservoirs has the potential to enhance recovery significantly by enhancing the overall flow capacity and releasing adsorbed gas that cannot be recovered by primary depletion. However, the process may be hampered by the low heat transfer rate if only the surfaces of hydraulic fractures are heated.