Effect of gas contamination and well depth on pressure interval of CO2 near-miscible flooding

Abstract

Minimum miscible pressure (MMP) of a representative oil sample is always taken as a leading indicator for screening the application of CO2 flooding in the targeted oilfield. However, a growing amount of evidence shows that in the vicinity of the conventional MMP, there exists an area (namely near-miscible region) which is significantly different from both miscible region and immiscible region. It can obtain comparable oil recovery as miscible flooding due to the improvement of sweep efficiency. In this paper, oil samples from two discovery oil wells (four well intervals) and gas samples from two discovery gas wells were obtained in QHD 29-2E oilfield, which is a newly discovered low permeability offshore reservoir of China. In view of the thick oil layer and varying CO2 contents of the gas layer, study on the determination of near-miscible pressure interval, the effect of injection gas contamination on the pressure region, and the variation of the specific pressure area with the change of well depth were conducted successively. To begin with, both slim tube test and slim tube simulation with denser test points were conducted to obtain the relationship of oil recovery and interfacial tension (IFT) with pressure. On this basis, near-miscible pressure regions under different CO2 contents were obtained. The lower limits of CO2 content for implementing near-miscible flooding can be determined by comparing with the reservoir pressure at a certain well depth. It is found that the lower limits vary from 34% to 64% for the four well intervals. Composition and location differences of the reservoir fluids are the main reasons for the pressure interval variation. It indicates that MMP and near-miscible pressure region which determined by just one set of oil and gas sample cannot represent the targeted well, and certainly not the whole oilfield with the oil layers of over 150 m. It is concluded that the deeper the well, the lower the CO2 content is needed to achieve near-miscible flooding for QHD 29-2E oilfield. Finally, several potential measures help to achieve near-miscible flooding and suggesting area for further study are presented.