Diffusion-Based Multiphase Multicomponent Modeling of Cyclic Solvent Injection in Ultratight Reservoirs

Abstract The cyclic solvent (gas) injection has been proved as an economical and effective method to enhance oil recovery in ultratight reservoirs such as shales. However, accurate modeling of cyclic solvent injection has been challenging due to the complex nature of fluid transport in nanopores. Most models are developed based on Darcy's and Fick's laws, which do not capture some critical transport phenomena within nanopores at elevated pressure. Accordingly, we propose a predictive model encapsulating the essential transport mechanisms for cyclic solvent injection in ultratight reservoirs. The model adopts the binary friction concept to incorporate friction between different molecules as well as molecules and pore walls. The Maxwell-Stefan approach is employed to account for the friction among species. The friction between molecules and pore walls is incorporated through partial viscosity and Knudsen diffusivity. A general driving force, chemical potential gradient, and the compressibility factor are used for the high-pressure non-ideal fluid mixture. The Peng-Robinson equation of state with confinement effect is used for the phase behavior calculations. The total flux consists of multicomponent molecular diffusion flux resulting from the chemical potential gradient and pressure diffusion flux driven by the pressure gradient. The governing equations for composition and pressure are solved implicitly using the finite difference method. The developed model is validated against analytical solutions and laboratory experiments. The primary production and solvent injection process are then simulated for a trinary oil (CH4, C4H10, and C12H26) and two solvent types, including CH4 and CO2. The results show that hydrocarbon components’ transport in the vapor phase is much higher than in the liquid phase. Accordingly, light and heavy components are produced at different fluxes during primary production because the vapor phase mainly consists of lighter components. For the single-cycle solvent injection cases, both CO2 and CH4 improve hydrocarbon recovery, with CO2 slightly performing better than CH4. This is attributed to CO2's ability to extract more heavy components into the vapor phase, producing more heavy components within the vapor phase. The recovery factor of heavy components of CO2 injection (3.40%) is higher than that of CH4 injection (3.21%). For multi-cycle solvent injection cases, CO2 injection can improve hydrocarbon recovery with a 2.77% increment, slightly better than multi-cyclic CH4 injection with a 2.73% increment. The injected CO2 can extract more heavy components near the fracture. However, the injected CH4 can penetrate deeper into the matrix to extract more light components within a more extensive region.