Multicomponent diffusion modeling of cyclic solvent injection in ultratight reservoirs

We present a new semianalytical compositional model designed for primary production of multicomponent oil and cyclic solvent injection in ultratight oil reservoirs that is dependent on diffusion-dominated transport within the matrix (k<200 nd) coupled to advectiondominated transport in the fractures. The semianalytical model consists of a well-mixed tank model for the fractures coupled to diffusive transport within the matrix. Production of oil, gas, and water from the fractures to the well is proportional to its phase mobility. The matrix allows for differing effective-diffusion coefficients for each component. Because there are no gridblocks within the matrix, the analytical solution is computationally less expensive than numerical simulation while capturing the steep, nonmonotonic compositional changes occurring a short distance into the matrix that result from multiple injection cycles. The Peng-Robinson equation of state (PR EOS) (Robinson and Peng 1978) is used to calculate phase behavior with time within the fractures and to initialize density and mass concentrations within the matrix based on the semianalytical framework. The coupled convective (fracture) and diffusive (matrix) model is validated with several laboratory- and field-scale cases. For primary recovery, the results show that the model correctly reproduces the pressure and oil-recovery declines observed in the field. We show that the hydrocarbon-recovery mechanism for solvent huff 'n' puff (HnP) is facilitated by greater density reduction and compositional changes. Two solvents are considered in HnP calculations: Carbon dioxide (CO2) and methane (CH4). Recovery of heavier components is enhanced with CO2 compared with CH4 within the reservoir (matrix and fractures). Furthermore, the results demonstrate that multiple HnP cycles constrained to surface injection are needed to enhance density and compositional gradients, and therefore oil recovery. Although shorter soaks are better for short-term recovery (i.e., 3 to 5 years), longer soaks maximize recovery over a longer time frame (i.e., 10 to 15 years). This paper provides a limiting case model based on diffusive matrix transport and convective fracture transport to determine the optimal number/duration of cycles and when to start the HnP process after primary recovery.