Lab experiments, field pilots and numerical modeling focusing on fluid flow aspects have indicated that gas injection in tight oil reservoirs is technically feasible. Several operators have conducted pilot tests in the Eagleford and Bakken shales for a Huff-n-Puff IOR strategy with mixed results. Our objective in this work was to study the impact of geomechanical effects, such as permeability changes and opening and closure of fractures during injection and production, on such huff-n-puff processes. We developed a fully coupled geomechanical compositional reservoir simulator to model Huff-n-Puff gas injection for improving liquid recovery in tight oil reservoirs. The simulator solves component mass balances, a suitable equation of state and pressure equations which are coupled with rock deformation and calculates stress changes due to both poroelastic (pressure changes) and mechanical (fracture opening and closing) effects. The phase behavior of the injected gas with a specified composition (which is different from the in-situ reservoir fluid) is accounted for using phase stability and flash calculation algorithms.

We present results from simulations using representative rock and fluid data from an unconventional reservoir and observe the following trends using our simulation studies: (a) Selection of the gas injection rate is very important in order to achieve a substantial pressure increase during the injection period. (b) Geomechanical effects, permeability decrease during increasing effective stress and permeability increase during decreasing effective stress has a significant impact on overall oil recovery. (c) Huff-n-Puff IOR will be more successful in certain reservoirs than in others depending on the composition of the reservoir fluid, saturation pressure, producing GOR and geomechanical reservoir rock properties. Our simulation results provide operators with significant new insights on the design of gas injection IOR processes. It is shown that geomechanical effects during huff-n-puff cycles have a significant impact on oil recovery, particularly if natural fractures and planes of weakness are present. Specifically, several design questions can be answered such as (a) gas injection rates and volumes (b) impact of injection fluid and reservoir fluid composition (c) selection of candidate reservoirs (d) impact of operating bottomhole temperature and pressure.