A new experimental method for measuring the bubble-point pressures of the light crude oil–CO2 system at different CO2 concentrations

Abstract

In this paper, a novel experimental method, i.e., the dynamic fluid-flow (DFF) method, was developed to measure the bubble-point pressures of the light crude oil–CO2 system at different CO2 concentrations at the actual reservoir temperature. The measured bubble-point pressures were compared with and validated by the measured data from the conventional static constant-composition-expansion (CCE) tests. First, a series of the CCE tests were conducted by using a pressure–volume–temperature (PVT) system to measure the bubble-point pressures of the light crude oil–CO2 system at seven different CO2 concentrations. Second, a series of new DFF tests were undertaken to measure the pressure gradients along a micro-tubing at different pre-specified outlet pressures when a given light crude oil–CO2 mixture was injected at a constant volume flow rate. It was found in the DFF test that the pressure gradient along the micro-tubing was an excellent indicator to identify the related phase changes and that it could be used to measure the bubble-point pressure. More specifically, the pressure gradient remained small and almost constant if the outlet pressure was above the bubble-point pressure of the light crude oil–CO2 mixture. However, the pressure gradient started to increase once the outlet pressure was below the to-be-measured bubble-point pressure. The outlet pressure, where a slope change occurred, was determined to be the bubble-point pressure of the light crude oil–CO2 mixture. It was also found that the measured bubble-point pressure of a given light crude oil–CO2 mixture from the DFF test was close to but slightly lower than that measured from the CCE test due to the gas-bubble re-entrainment and micro-tubing confinement in the former test. In addition, the phase change was found to occur near the outlet of the micro-tubing even if the outlet pressure was far below the bubble-point pressure. Furthermore, the micro-tubing length and the light crude oil–CO2 mixture injection rate in the ranges tested in this study did not noticeably affect the measured bubble-point pressure from the DFF test. In summary, the newly developed dynamic experimental method based on the DFF tests is much more time-saving and cost-effective, and easier to use, in comparison with the conventional static CCE tests by using an expensive and large PVT system.