A semi-analytical solution to optimize single-component solvent coinjection with steam during SAGD

摘要

Coinjection of a low concentration of solvent with steam has been studied as an alternative to steam-assisted gravity drainage (SAGD). This research presents a semi-analytical method for comparing oil drainage rates of SAGD and coinjection processes using different single-component solvents for a given set of reservoir/operating conditions. The oil recovery in coinjection involves complex interaction of energy and mass balances with the effects of gravity, phase behavior, and multiphase flow. We simplify the complex interaction without loss of fundamental mechanisms, while retaining the phase behavior details near the chamber edge. The new method begins with solution for thermodynamic conditions at the chamber edge, where the phase transition occurs between two and three phases. Three components are considered; oil, solvent, and water. The chamber-edge conditions that are solved for are used to estimate distributions of solvent and temperature beyond the chamber edge. Darcy's law and material balance are then applied to derive an analytical expression for oil-drainage ratio, the ratio of oil drainage in coinjection to that in SAGD. Since the chamber-edge temperature and composition are interdependent for this ternary phase behavior problem, oil-drainage ratio is solved for as a function of solvent concentration in the oleic (L) phase at the chamber edge (X_(sL)~(edge)). Case studies with the semi-analytical method show that oil-drainage ratio is higher in the higher X_(sL)~(edge) range than in the lower X_(sL)~(edge) range for a given coinjection solvent. This indicates that efficient oil recovery in coinjection requires high accumulation of solvent at the chamber edge. Oil-drainage ratios calculated for different coinjection solvents are compared in the high X_(sL)~(edge) range for preliminary screening of single-component coinjection solvents. This offers significant time savings in selecting a coinjection solvent by reducing the need for numerical reservoir simulation. The semi-analytical method also indicates that highly volatile solvents, which are relatively less expensive in general, tend to be more effective for less viscous reservoir oil and higher operating pressure. Less volatile solvents may offer more flexibility in operating conditions since they remain effective at lower pressures. These results are validated using fine-scale numerical reservoir simulations.