Shale Gas Huff-N-Puff with Live Oil: Effect of Huff Pressure

Abstract In recent years, the Huff‑n‑Puff (HnP) gas injection technique has been tested in shale and tight reservoirs to enhance oil recovery. Laboratory studies have been conducted to understand the effects of these parameters, but mostly with dead oils. We have developed a workflow that generates live oil in cores in situ that is compatible with our NMR analysis. The goal of this work is to study the effect of huff pressure (4000 vs. 6000 psi) in CO2 and hydrocarbon gas-based huff-n-Puff process in shale cores and identify the underlying mechanisms. The HC gas employed consisted of 72% methane (C1), 16% ethane (C2), and 12% propane (C3), closely matching the HC gas composition used in field operations. A primary depletion stage simulating field production and six huff‑n‑puff cycles were performed, including a soaking period of 24 hours. Oil recovery was quantified by three techniques: T2 NMR, gravimetry, and fluids collected at each cycle. The liquids and gases produced at each stage were collected separately and analyzed via gas chromatography (GC) to assess compositional changes throughout the process. The results indicate that the oil recovery decreases significantly with the huff pressure in the first two cycles. The later cycles (3-6) makeup for the loss in the first 2 cycles partly. As the huff pressure decreases, the extraction of the intermediate components decreases. At a huff pressure of 4000 psi, both HC gas and CO2 effectively enhance oil recovery, but CO2 shows a higher oil recovery (~64%) compared to HC gas (~56%) after six cycles. CO2 consistently recovered more intermediate hydrocarbon components (C10–C20) than the HC gas because its stronger interaction with these intermediate components.