Quantification of dissolution and exsolution dynamics of gaseous solvents in crude oil systems under reservoir conditions

Abstract

With a growing demand for fossil fuels, it is of a great importance to improve the oil recovery factor from both conventional and unconventional reservoirs. Among different enhanced oil recovery (EOR) methods, injecting gaseous solvents, including CO2, N2, flue gas, and alkane solvents, is considered as a more effective and efficient method in both light and heavy oil reservoirs, during which mass transfer is the key underlying recovery mechanism. In order to optimize the solvent injection method and achieve a higher oil recovery, it is of fundamental and practical importance to quantify both dissolution and exsolution dynamics of solvents in light and heavy oils under reservoir conditions. Firstly, a pragmatic method has been developed and applied to quantify the mutual mass transfer in different solvent(s)-light oil systems. Experimentally, diffusion experiments have been conducted for flue gas-light oil systems at constant pressures and temperatures. The dynamic liquid volume is monitored and recorded continuously during the experiments, while gas samples are collected at the beginning and end of each test to measure the gas fractions with gas chromatography (GC) analysis. Theoretically, by combining the Fick’s law and Peng-Robinson equation of state (PR EOS), the preferential and mutual diffusion between the flue gas and light oil can be quantified once the deviations between the measured and calculated parameters (i.e., dynamic swelling factor and gas composition) are minimized. Both individual diffusion coefficients for each gas component of a gas mixture in an oil phase and that of the extracted oil components in the gas phase are increased with pressure and temperature. Both the experimental and theoretical methods are modified and then extended to CO2/C3H8-heavy oil systems to quantify the mutual diffusivity between the gaseous solvents and heavy oil with the consideration of natural convection at high pressures and elevated temperatures or coupled with heat transfer process. Similarly, diffusion experiments are conducted with a PVT setup, during which the dynamic swelling factors of the heavy oil are measured continuously. Both oil and gas samples are collected at end of each test for oil compositional and GC analyses, respectively. The diffusivities of both solvents (i.e., CO2 and C3H8) in heavy oil and the extracted oil components in the gas phase are found to increase with pressure and temperature. Also, there exists an obvious extraction process from the oil to gas phases at elevated temperatures as light-medium components have been detected in the collected gas samples at end of the experiments. While coupling heat and mass transfer to determine the diffusivity of hot solvent in the heavy oil, thermal equilibrium is found to be achieved earlier than mass equilibrium. Combined heat and mass transfer will accelerate the oil swelling effect. Then, experimental and theoretical techniques have been developed to predict gas exsolution dynamics of CO2/CH4-heavy oil systems on the bubble level to reflect the physical behaviour of foamy oil. Experimentally, constant-composition-expansion (CCE) experiments have been performed in a sealed PVT system for a CO2-heavy oil system and a CH4-heavy oil system, respectively. Theoretically, the classical nucleation theory, population balance equations (PBEs), Fick’s law, and PR EOS has been integrated to predict the gas bubble number and size by reproducing the experimentally measured parameters (i.e., liquid volume and pseudo-bubblepoint pressure). It has been observed that both temperature and diffusivity of the gas component play an important role in the foamy oil behaviour. Compared with CO2, CH4 can induce a stronger and more stable foamy oil since more CH4 bubbles are dispersed in the oil phase.