The Effect of Water-Induced Stress to Enhance Hydrocarbon Recovery in Shale Reservoirs

Abstract

Abstract Waterflooding has been an effective improved oil recovery process for several decades. However, stress induced by waterflooding has not been well studied or documented. Water injection typically increases reservoir pressure and decreases reservoir temperature. The increase in reservoir pressure and decrease in reservoir temperature synergistically reduce the effective stress. Because of such decrease in stress, existing healed natural fractures could be reactivated and/or new fractures could be created. Similar effects could enhance hydrocarbon recovery in shale reservoirs. In this paper, we calculated the magnitude of water injection-induced stress using a coupled flow-geomechanics model. To evaluate the effect of water injection in the Bakken, a numerical simulation study for a sector model was carried out. Stress changes due to the volume created by the hydraulic fracture, water injection, and oil production were calculated. Hoek-Brown failure criterion was used to compute rock failure potential. Our numerical results for a waterflooding example show that during water injection, the synergistic effects of reservoir cooling and pore-pressure increase significantly promotes rock failure, potentially reactivating healed natural macrofractures and/or creating new macrofractures, especially near an injector. The rock cooling can create small microfractures on the surface of the matrix blocks. In shale oil reservoirs, the numerical experiments indicate that stress changes during water injection can improve oil recovery by opening some of the old macrofractures and creating new small microfractures on the surface of the matrix blocks to promote shallow water invasion into the rock matrix. Furthermore, the new microfractures provide additional interface area between macrofractures and matrix to improve oil drainage when using surfactant and CO2 EOR techniques. These positive effects are particularly important farther away from the immediate vicinity of the hydraulic fracture where much of the undrained oil resides.