Lab experiments and numerical modeling have indicated that gas injection for IOR in tight oil reservoirs is technically feasible. Several operators have conducted pilots in the Eagleford and Bakken shales for a huff-n-puff IOR strategy with mixed results. Our objective in this work is to study the impact of (a) geomechanical effects during injection and fracture closure during production, (b) injection rate effect during huff-n-puff processes, (c) timing effect in huff-n-puff process, (d) phase behavior effects for huff-n-puff oil recovery, and (e) impact of soak time on oil recovery.
We developed and utilized a fully coupled geomechanical compositional fracturing/reservoir simulator for gas injection in tight oil reservoirs. The model calculates stress changes due to both poroelastic (pressure changes) and mechanical (fracture opening and closing) effects. Permeability hysteresis during loading/unloading cycles is also considered. The simulation procedure involves the following steps: (a) The well is produced for a period of time (b) Gas is injected into the well and this is simulated by specifying the injection rate and gas composition (c) The well is produced again after some soaking period, and the results are checked for any improvement in the oil recovery.
Based on our simulation studies, we observed several important trends. The degradation of permeability over multiple loading/unloading cycles due to hysteresis together with the poro-elastic effect severely impacts the oil recovery in later cycles. It was found that incremental oil recovery decreases after several huff-n-puff cycles. It was found that if higher gas injection rate gives higher oil recovery, but the oil recovery does not increase linearly with the injection rate. Phase behavior is found to be a key factor in the oil recovery and rich gas injection gives the best results. The impact of timing to initiate the huff-n-puff IOR was also studied. The soak time increases the ultimate oil recovery, but its impact is not significant in the study.
Our simulation results provide operators with significant new insights on the design of huff-n-puff IOR. It is shown that fracture widening and closure during huff-n-puff cycles have a significant impact on oil recovery. The novelty of the work is the development and use of compositional geomechanical model for IOR performance evaluation.