Study of the Non-Equilibrium PVT Properties of Methane- and Propane-Heavy Oil Systems
Authors: Wang Hongyang , Zeng Fanhua , Zhou Xiang 
Published: May 2015

The non-equilibrium properties of foamy oil flow are essential to better understand CSI processes in thin heavy oil reservoirs.

In this paper, non-equilibrium PVT constant composition expansion (CCE) measurement of typical Canada heavy oils saturated with methane and propane respectively has been conducted. The PVT properties were tested under typical heavy oil reservoir temperature 20°C to show the non-equilibrium that different saturation pressures presented at a fixed temperature.

For equilibrium tests (a magnetic stirrer stirring the heavy oil-solvent mixture at the bottom of the PVT cell), an equilibrium bubble point pressure was achieved at 76psi and 270psi for propane and methane, respectively. The initial onset pressures of the non-equilibrium tests were slightly above the equilibrium bubble point pressures.

For non-equilibrium tests, Supersaturation tests and Smooth Pressure Decline tests were conducted on propane-based live oil, while only Smooth Pressure Decline tests were conducted on methane-based live oil. For Supersaturation tests, a sudden pressure drop deviated from the equilibrium bubble point pressure was applied. For Smooth Pressure Decline tests, pressure was declined almost linearly with time. Tests were used to simulate the porous media condition where there is no stirring movement and the first visible free gas appearance was observed to record the foamy oil single phase volume expansion peak before free gas forms continuously. All CCE tests phenomenally recorded different saturation pressures (inflection pressure points) at various pressure decline rates at a fixed temperature.

For propane-based supersaturation tests, pressure deviation from equilibrium bubble point at 6psi and 26psi led to a pseudo bubble point at 50psi and 41psi, respectively, and they led to a different time of reaching the volume peak at 23 hours and 11 hours, respectively. For propane-based smooth pressure decline tests, pressure decline rate 0.04psi/min, 0.11psi/min, 0.263psi/min and 0.386psi/min yielded pseudo bubble pressure at 53psi, 56psi, 59psi and 60psi, respectively. For methane-based smooth pressure decline tests, pressure decline rate 0.149psi/min, 0.442psi/min, 0.856psi/min, 0.937psi/min, 2.046psi/min and 3.363psi/min yielded the pseudo bubble point pressure at 214psi, 170psi, 156psi, 110psi, 75psi and 65psi, respectively. In addition, the in-situ density was measured with different pressure decline rates and it was found that for propane-based live oil, pressure decline rate 0.04psi/min yielded the lowest live oil density 0.83g/cm3 at the lowest pseudo bubble point pressure 57psi, while the highest pressure decline yielded a relatively low density at 0.835g/cm3 at the highest pseudo bubble point pressure 63psi. Also, the higher level supersaturation test yielded a lower density at a much lower pseudo bubble point pressure about 40psi compared with that of the lower supersaturation level test at 52psi. For methane-based tests, higher the pressure decline rate was, lower the densities of the solvent-heavy oil systems at lower pseudo bubble point pressures were the results.