The Effect of Natural Fractures on the Hysteresis of Stress-Dependent Permeability in Keshen Tight Gas Reservoirs

Keshen gas field is characterized with well-developed natural fractures, which plays an important role on productivity. Permeability of both matrix and natural fractures decreases with the increasing effective stress and increases with the decreasing effective stress, following different paths due to the discrepancy of hysteresis, depending on the mechanical, petrophysical properties and the pore structures, impacting the strategies of gas production and reservoir stimulation significantly. The permeability hysteresis results from the partially reversible and irreversible deformation of natural fractures and matrix during loading and unloading processes, which can be very substantial for the tight reservoir since the high-permeability zone is not uibiquitous in the tight reservoir. In this study, the permeability hysteresis of natural fractures and matrix were investigated for the tight gas reservoir in Keshen, Tarim basin. Specifically, permeability of plug core samples from Keshen reservoir with natural fractures were measured over a wide range of effective stresses (600psi to 6600psi) for the loading and unloading processes, followed by the evaluation of the permeability hysteresis on the tight gas production. Results demonstrate that the initial permeability of core samples with multiple fractures is much higher compared to those with single fracture and those without fractures. In addition, the hysteresis of stress-dependent permeability of core samples with single fracture running through both end faces is more obvious than that of others. However, the recovery of the fracture permeability is not as easy as that of the pores because the compressibility of natural fractures is relatively higher than that of the pores. The interpretation of the hysteresis of permeability dependence on stress is important to optimize the effect of the spontaneous imbibition of hydraulic fluid due to the well shut-in on the production of tight gas/oil and minimize the loss of fracture conductivity and matrix permeability resulting from the dissipation of pore pressure during long-term reservoir depletion.