The current models for predicting the phase behavior of gas injection in shale can be highly unreliable because nanopores (with diameters less than 50 nm) form a significant pore volume in many shale formations. Conventional PVT models cannot describe the phase behavior in nanopores. Here, we present a practical framework to regenerate the PVT considering the shale nanopores effect for a more reliable compositional simulation of gas injection in shale reservoirs by using existing commercial simulators.
The pore-size distribution in shale reservoirs can be discretized into a bulk-region (fractures and macropores) and nanopores. We use a pore-size-dependent equation of state (PR-C EOS) to describe the phase behavior of the fluid for each pore. Bulk fluid characterization with laboratory PVT reports determines the bulk fluid parameters for the PR-C EOS. The confinement parameters for the PR-C EOS are from the reported database (Luo et al. 2018a). Further, multi-scale phase equilibria are calculated by minimizing the free energy. We model the multi-scale constant composition expansion and constant volume depletion with volume expansion per stage. The modeling generates multi-scale PVT (formation volume factor, saturation, etc.) for the shale reservoir, which is used to retrain the Peng–Robinson equation of state (PR EOS) by modifying the acentric factor, binary interactions, and critical temperature and pressure. The retrained PR EOS is then applied in a commercial compositional simulator to forecast gas injection improved oil recovery (IOR) in shale. We also use the updated gas saturations in the multi-scale PVT model to modify the relative permeability tables used in the compositional simulation.
We predict significantly higher gas production and lower oil production when the effect of shale nanopores on the phase behavior and updated relative permeability are considered in the compositional simulation of the primary depletion of shale reservoirs. In the gas injection improved oil recovery (IOR) stage, the cumulative oil production is enhanced with both the original and multi-scale PVT models. However, when the effect of nanopores is not considered in the compositional simulation, the increases in the cumulative oil production and cumulative gas production can be underestimated and overestimated, respectively. This can have significant consequences on the economic evaluation of the gas IOR projects in shale reservoirs.
The application of multi-scale phase equilibria in shale reservoirs is challenging in compositional simulators. Our proposed framework enables engineers to incorporate multi-scale phase equilibria from the PR-C EOS in their shale reservoir simulations. It does not require a change in the cubic equations of state in current developed commercial compositional simulators, thus preserving the efficiency of the compositional simulators.