Near-Miscible CO2 Application to Improve Oil Recovery
Carbon dioxide (CO2) injection for enhanced oil recovery is a proven technology. CO2 injection is normally operated at a pressure above the minimum miscibility pressure (MMP), which is determined by crude oil composition and reservoir conditions. This is the lowest pressure at which the injected CO2 becomes dynamically miscible with the crude oil remaining in the reservoir. However, many reservoirs are located at depths or under geologic conditions such that they must operate at pressures below the MMP. When CO2 is injected at below the MMP, displacement efficiency decreases as a result of the loss of miscibility. CO2 injection is usually not considered as an enhanced oil recovery process in these reservoirs. Near miscible displacement generally refers to the process that occurs at pressures slightly below the MMP, but the actual pressure range has never been clearly defined.
The objectives of this study were to investigate the feasibility of near-miscible CO2 application and improve our understanding of the mechanisms of near-miscible CO2 flooding by conducting appropriate experimental work and reservoir simulation. The pressure range of interest was from 0.8 MMP to MMP in our study. The Arbuckle formation of Kansas was used as an example to demonstrate our approach to evaluate CO2 flooding at near-miscible conditions. The suite of laboratory experiments used to evaluate the feasibility of operating at pressures below MMP for Arbuckle reservoirs included phase behavior studies, core flow tests and phase behavior model construction using CMG software package.
Phase behavior studies were carried out to characterize the near miscible conditions. Slim tube displacements and swelling/extraction tests were performed to identify the near miscible range and the mass transfer mechanisms which were responsible for the oil recovery within this range. A phase behavior model was constructed and well-tuned to simulate oil properties, CO2/crude oil interactions and slim tube results. Core flow tests were conducted to evaluate the oil recovery efficiency in the near miscible range.
Initial laboratory works indicated that miscibility was not achievable, however at least 65% to 80% of the waterflood residual oil for dolomite cores and lesser from 45% to 60 % for sandstone core in the near-miscible region was observed. The principal oil recovery mechanism in the near-miscible range appeared to be extraction/vaporization of hydrocarbon components from crude oil into the CO2 rich vapor phase, coupled with enhanced mobility control due to the reduction of oil viscosity. This suggested that application of carbon dioxide in the field would require injection and recycling of large volumes of carbon dioxide. Further study is needed to determine if such a process is economically feasible. However the prospect of recovering up to 1 billion barrels of oil from Arbuckle reservoirs offers significant economic potential.
