Experimental and numerical studies on non-equilibrium gaseous solvent dissolution in heavy oil: A continuum-scale approach based on bubble-scale dissolution behaviors

Abstract

Throughout experimental studies, non-equilibrium solvent dissolution kinetics were studied in a micromodel PVT system. Dissolution tests were conducted by pressure decay method with a fixed mass feed of solvent and heavy oil. The non-equilibrium solvent solubility was calculated based on material balance in the closed system. The vapor–liquid phase ratio at the end of the diffusion process was approaching equilibrium state, indicating an efficient solubility measurement. Bubble-scale (Taylor and spherical bubble) dissolution tests were conducted in a visual cell with long microchannel. The simulation-input-oriented bubble dissolution rate [T−1] was found to be in an exponential relationship with pressure. The dissolution rate of spherical bubble (6×10-3 ∼ 2.4×10-2min−1) was one order of magnitude higher than Taylor bubble (9×10-4 ∼ 3×10-3min−1) because of a larger contact area with oil. Numerically, aiming to determine the continuum-scale dissolution-relevant parameters based on bubble-scale experimental findings, a continuum-scale reservoir simulator was developed by coupling MATLAB and CMG STARS to fulfill a transient optimization to obtain dissolution-relevant parameters by history-matching pressure decay curve. The established exponential relationships between dissolution rate and pressure were used as numerical inputs. Two different thermodynamic boundary conditions, namely quasi-equilibrium and non-equilibrium condition, were implemented in the continuum-scale modeling. Under quasi-equilibrium condition, the pressure decay curve was shown to have 3 distinct regimes, namely early/incubation stage, transition stage and late stage. The apparent effective diffusion coefficient was adjusted for every stage to match the pressure decay curves and was calculated to be 3×10-9m2/s in molecular diffusion-dominated late stage, which is in agreement with the analytical/graphical solution and within the common range of literature data. Under non-equilibrium condition, the dissolution kinetic rates were described to a non-equilibrium interphase mass transfer source/sink term. Same with the experimental findings, the segmentally optimized dissolution rate against pressure was calculated to be an exponential relationship. The early-stage dissolution kinetic rates were achieved to be within 10-4 ∼ 10-3min−1, resembling to Taylor bubble dissolution rates. The late-stage dissolution kinetic rates approached zero, indicating that the solvent solubility approached close to equilibrium. The feasibility of the exponential relationship established from bubble-scale dissolution tests to simulate solvent dissolution behavior using continuum-scale modeling technique was effectively proved.