Understanding the Mechanisms of Huff-n-Puff, CO2-EOR in Liquid-Rich Shale Plays: Bakken Case Study

A revolution of unconventional reservoirs is a turning point in the global oil and gas industry since these resources have massive reserves with large potential in contributing to hydrocarbon production. Previous EOR laboratory experiments and simulation studies in the literature illustrated promising results in terms of recovery factor for different EOR applications, such as CO₂, surfactant, and natural gas. However, pilot tests performance reported contrast behavior due to misleading predicting for the EOR physics processes. This paper presents the experimental work to evaluate the feasibility of CO₂-EOR using the huff-n-puff (HNP) protocol in the Middle Bakken (MB) Formation, the Mountrail County, Williston Basin, ND. We evaluate the oil recovery from CO₂-EOR under several scenarios of operational and well/reservoir conditions. The parameters considered in the sensitivity study include temperatures, pressure, soak time, and number of injection cycles to obtain optimum conditions under which the incremental oil recovery from the MB Formation is increased. The wettability alteration (i.e. contact angle) was also studied using rock-chip samples before and after the HNP experiment at the Bakken reservoir conditions (present for example P & T in psi/F). The outcomes indicated on the effect of the reservoir temperature and pressure on the performance of the CO₂, where the recoverable oil increases as the temperature and pressure increase until reach the optimum. As a previous research outcome, the number of cycling and soaking time are crucial design parameters for the HNP experiment and on the field as well to let the CO₂ time to diffuse into the deep formation and swell more oil. In addition, the wettability alteration was changed by CO₂-EOR as injection pressures increase and the wetting phase move from the oil-wet toward the water-wet system. As overall outcomes from this research, the CO₂ HNP process has a good potential in the lab, and could be succeeded economically in field applications that might reduce the need for refracturing stimulation or infill drilling.