CO2 Huff-n-Puff: An Experimental and Modeling Approach to Delineate Mass Transfer and Recovery from Shale Cores

Abstract

Gas injection has been demonstrated to be an effective approach to enhance recovery from ultra-tight fractured reservoirs where the role of molecular diffusion often becomes dominant. The open literature offers a large collection of work concerned with gas injection studies and projects, employing carbon dioxide (CO2), methane (CH4) and other gases, and reports a considerable improvement in oil recovery over primary production. CO2 injection has an additional advantage over other gases through the potential for geological sequestration. This explains the growing interest in studying diffusive mass transfer during CO2 injection to delineate the sequestration potential in concert with enhanced oil recovery from unconventional resources. However, additional work is needed to arrive at a comprehensive understanding and representation of diffusive mass transfer in ultra-tight fractured formations. In this paper, we study diffusive mass transfer in shale cores by conducting and simulating CO2 Huff-n-Puff (HnP) experiments at high pressure and temperature. Two cores from a formation in the Middle East were evacuated and then saturated at 3500 psi and 50°C with a synthetic oil consisting of decane (nC10), dodecane (nC12), tetradecane (nC14) and hexadecane (nC16). We performed multiple HnP cycles at varying injection conditions: 2900-4000 psi and 70 °C. Diffusive mass transfer was then investigated via (1) evaluating the effect of injection pressure on oil recovery, (2) analyzing produced oil compositions, and (3) studying the pressure decline during the soaking period. Our experimental observations show that a higher oil recovery is achieved when injecting at a higher pressure. We also observe that molecular diffusion acts as a dominant recovery mechanism in the HnP experiments, as evident from analyzing the produced oil composition and from examining the pressure behavior versus time during the soaking periods: The observed decline rate in the pressure during soaking signify that molecular diffusion dictates the mass transfer during the HnP experiments. Additionally, we note that miscibility conditions will change from one HnP cycle to another, as the injected gas mixes with an oil composition that changes between cycles. We have used the CMG-GEM compositional simulator to interpret the HnP experimental results. When multicomponent diffusion coefficients were computed using the correlation of Sigmund (1976) the simulator is unable to provide a reasonable prediction of oil recovery and produced oil compositions. To achieve a better prediction of diffusive mass transfer in ultra-tight fractured reservoirs, a representation that is based on a more fundamental description of the multicomponent diffusion coefficients is hence required, as discussed in-depth by Alahmari and Jessen (2021) and Shi et. al. (2022).