Oil Recovery from Tight Reservoirs during Natural Depletion and Cyclic Gas Injection: An Experimental Study Using Live Fluids on a Fractured Reservoir Whole Core Sample

This study presents a highly novel and meticulously designed experimental approach to investigate oil recovery due to natural depletion and cyclic huff-n-puff gas injection in fractured, ultratight reservoirs. To this end, we performed a series of core-flooding experiments on a fractured reservoir whole core sample under elevated pressure and temperature conditions, and using the reservoir live fluids. The whole core setup provides a sufficiently large pore volume in low-porosity core samples and helps minimize uncertainties associated with fluid saturation measurements. The experimental procedure included flow experiments that eventually restored the core back to the reservoir conditions, in terms of pressure, temperature, and fluid saturation, in the presence of propped/unpropped fractures. The core sample was subsequently subjected to several depletion stages to pressures above and below the bubble point pressure. Ultimately, a cyclic huff-n-puff gas injection was employed to quantify incremental oil production after the depletion processes. An initial depletion of 13.75 MPa at a pore pressure above the bubble point resulted in a recovery of approximately 5.8% of the original oil in place (OOIP). This value was consistent with the theoretical estimate based on the rock compressibility and fluid expansion. As the pore pressure dropped below the bubble point, the hydrocarbon gas evolved from the oil phase in the matrix and displaced oil toward the fracture by the solution gas drive mechanism, eventually resulting in an additional recovery of 12.71% of OOIP. After the primary production, a single huff-n-puff gas injection cycle was conducted over the same pressure range of the depletion process, which yielded an incremental recovery of nearly 8% of OOIP. This finding clearly highlighted the promisingly profitable oil volume additionally recovered from the tight matrix system by the huff-n-puff gas injection mode. It was also observed that a subsequent cycle of gas injection did not significantly increase oil production, which indicated that the initial cycle effectively maximized the recovery potential in this ultratight reservoir system.