Recently there has been an increasing interest in Enhanced Oil Recovery (EOR) from shale oil reservoirs, including CO2 and field gas injection. For the performance assessment and optimization of CO2 and gas injection processes, compositional simulation is a powerful and versatile tool because of the capability to incorporate reservoir heterogeneity, complex fracture geometry, multi-phase and multi-component effects in nano-porous rocks. However, flow simulation accounting for such complex physics can be computationally expensive. In particular, field scale optimization studies requiring large number of high resolution compositional simulations can be challenging and sometimes computationally prohibitive. In this paper, we present a rapid and efficient approach for optimization of CO2 and gas injection EOR in unconventional reservoirs using the Fast Marching Method (FMM)-based flow simulation. The FMM-based simulation is analogous to streamline simulation and utilizes the concept of ‘Diffusive Time-of-Flight (DTOF)’. The DTOF is a representation of the travel time of pressure ‘front’ propagation and accounts for geological heterogeneity, well architecture and complex fracture geometry. The DTOF can be efficiently obtained by solving the ‘Eikonal equation’ using the FMM. The 3-D flow equation is then decoupled into equivalent 1-D equation using the DTOF as a spatial coordinate, leading to orders of magnitude faster computation for high-resolution and compositional models as compared to full 3-D simulations. The speed of computation enables the use of robust population-based optimization techniques such as genetic or evolutionary-based algorithm that typically require large number simulation runs to optimize the operational and process parameters. We demonstrated the efficiency and robustness of our proposed approach using synthetic and field scale examples. We first illustrate the validation of FMM-based simulation approach using an example of CO2 Huff-n-Puff for a synthetic dual-porosity and heterogeneous model with a multi-stage hydraulically fractured well. In the field-scale application, we present an optimization of operating strategies for gas injection EOR for a depleted shale oil reservoir in the Eagle Ford formation. The rapid computation of the FMM-based approach enabled intensive simulation study involving high-resolution geological models with million cells resulting in a comprehensive evaluation of the EOR project including sensitivity studies, parameter importance analysis and optimal operating strategies. This study shows the novelty and efficiency of the systematic optimization workflow incorporating the FMM-based compositional simulation for the field-scale modeling of CO2 and gas injection in shale oil reservoirs. Not only can it account for relevant physics such as reservoir heterogeneity, fracture geometry and fluid phase behavior but also lead to orders of magnitude saving in computational time over commercial finite difference simulators.
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