A compositional model for gas injection IOR/EOR in tight oil reservoirs under coupled nanopore confinement and geomechanics effects

Abstract

The nanopore confinement and geomechanics are generally considered as non-negligible in tight oil reservoirs, but their coupled effects are still not well understood for gas injection which is becoming a favorable Improved/Enhanced Oil Recovery (IOR/EOR) technique for tight oil. We present a general compositional model to investigate the complex multiphase and multicomponent behaviors under coupled nanopore confinement and geomechanics in tight oil reservoirs. The in-house simulator developed is validated against a commercial simulator before being applied to simulate dry gas huff-n-puff in Eagle Ford. The simulation reveals that huff-n-puff would improve the recovery factor (RF) of each component versus the depletion. If the reservoir pressure is much higher than the bubble point, the nanopore confinement will have a minimal impact on RF for both the depletion and early cycles of huff-n-puff. Geomechanics is found to be an important factor for RF but not always detrimental, as enhanced compaction drive of matrix could offset the permeability reduction of fracture in certain scenarios. The heavy component would first have a higher RF than the light component in early huff-n-puff cycles, but its RF will be gradually outpaced by the light and finally surpassed after a crossover point which is controlled by the critical gas saturation of matrix. Considering the nanopore confinement in the simulation will delay this crossover and reduce the RF of the light component but increase the RF of the heavy component after huff-n-puff. This work can better assist engineers to understand the complex multiphase and multicomponent behaviors during gas injection in tight oil reservoirs.