Nonequilibrium Exsolution Kinetics of Hydrocarbon Solvents in Heavy Oil

Abstract

Nonequilibrium exsolution kinetics of methane and propane in heavy oil were studied through visual experimentations and continuum simulations. Experimentally, novel Constant Composition Expansion (CCE) tests, combined with a micromodel to visualize the bubbly oil flow, were fulfilled in pressure−volume− temperature (PVT) cells. Two schemes were applied, namely, constant volume extraction (CVE) and stepwise pressure depletion (SPD). From the pressure measurement of CVE, methane first went through a pressure-declining period due to hysteresis of exsolution and later a pressure-maintaining period under a balance between exsolution and volume expansion rate. Meanwhile, the pressure of the propane system dropped rapidly and rebounded back to a constant pressure for a long period of time due to a higher solubility than methane. Compared with equilibrium state, the solvent exsolution of both live oils were always under nonequilibrium. Bubbly oil flow was visually observed to determine the range of pseudobubble point pressure. From the volume expansion behavior from the SPD tests, both methane-and propane-based live oil systems present a close-to-linear relationship between volume and time at each pressure drawdown level before reaching equilibrium. This facilitates the quantification of exsolution rate under given pressure. Numerically, a simulation platform to calculate nonequilibrium exsolution kinetics has been built. The feasibility of the simulator is verified through simulating literature data with various pressure data ranges. The exsolution rates have been achieved by history matching the CVE and SPD tests. For CVE tests, methane shows a higher exsolution rate (1 × 10 −4 to 1 × 10 −2 min −1 ) than propane (1 × 10 −6 to 1 × 10 −3 min −1 ) under the same volume expansion rate, indicating a higher tendency to exsolve and form foamy oil. Propane tends to stay in heavy oil due to high solubility. The exsolution rates of the stable-pressure period present a power relationship with volume expansion rate for both live oils. For SPD tests, methane presents a similar range of exsolution rates (1 × 10 −5 to 1 × 10 −4 min −1 ) as propane under similar P/P sat ratios with pre-existing free gas phase. Compared to tests conducted by a sudden pressure drawdown scheme, pre-existing gas cap yields lower exsolution rates due to smaller chemical potential between phases. SPD tests also verify faster exsolution kinetics for methane than propane.