The Impact of Gas-Oil Miscibility on Oil Recovery During Huff-and-Puff EOR in Organic-Rich Shales

Abstract

Injection pressure is a key parameter in the design of a Huff-and-Puff EOR process in organic-rich shales. Reservoir engineering studies have shown that injection pressure and miscibility play a role during Huff-and-Puff EOR. However, during field implementation, the injection pressure is often limited by the number of compressors and in several cases, the gas injected might not reach sufficient pressure to achieve a state of complete miscibility with the oil. These scenarios lead to a condition of partial miscibility or immiscibility which impacts the efficiency of the recovery mechanisms during Huff-and-Puff EOR. The objective of this present study was to quantify experimentally the impact of complete, partial, and immiscible gas injection during Huff-and-Puff EOR in organic-rich shales. For the purpose of this study, we have collected crude oil and wax preserved core samples from the Eagle Ford shale formation. The Eagle Ford shale samples were characterized by measurements of mineralogy, TOC, porosity, pore throat size, and specific surface area. We performed 20 Huff-and-Puff EOR tests in the shale samples at several different injection pressures, using two field gases and an immiscible helium gas. The minimum miscibility pressure (MMP) between the field gases and the crude oil was measured using the vanishing interfacial tension technique. Oil recovery for each Huff-and-Puff EOR cycle was quantified using NMR measurements. The produced hydrocarbon compositions were determined using a multi-step dry Pyrolysis measurement at the end of each Huff-and-Puff EOR test. Our results show that injection pressure is one of the most important factors controlling the oil recovery during Huff-and-Puff EOR in shales using a field gas. We observe a strong linear increase in oil recovery as injection pressure increases. The injection of the field gas in either partially miscible or completely miscible conditions provides significantly larger oil recovery compared to the immiscible gas at the same absolute pressures. The oil recovery is three times larger for the field gas in a completely miscible condition compared to the immiscible gas and two times larger in a partially miscible condition. The multi-step dry Pyrolysis results show a preferential production of light hydrocarbon species regardless of the state of miscibility during Huff-and-Puff EOR. The findings reported in this experimental study will help to optimize the design of field Huff-and-Puff EOR operations in organic-rich shales. The strong linear trend between the injection pressure and the oil recovery factor in the field gas tests can be used for the selection of gas compressors during field implementations. The superior performance of the field gas compared to the immiscible helium gas at the same injection pressures confirms the importance of miscibility during Huff-and-Puff EOR in organic-rich shales.