Molecular anatomy and macroscopic behavior of oil extraction from nanopores by CO2 and CH4. (15th September 2022)

• Record Type: Journal Article
• Authors: Moh, Do Yoon; Zhang, Hongwei; Sun, Shuyu; Qiao, Rui
• Is Part Of: Fuel. Volume 324:Part B(2022)
• Publication Date: 2022-09-15
• Subjects: Unconventional reservoirs -- Nanopores -- CO2 -- Gas injection -- Enhanced oil recovery; Fuel -- Periodicals; Coal -- Periodicals; Coal; Fuel; Periodicals; 662.6
• DOI: 10.1016/j.fuel.2022.124662
• Languages: English
• ISSNs: 0016-2361

Abstract

Highlights: Gas injection-induced oil recovery from shales is studied at the single-pore scale. A transition of oil recovery mode during gas injection is discovered. The oil recovery mode transition is governed by gas adsorption on pore walls. Oil recovery obeys a diffusive law with a gas type-dependent effective diffusivity. Inferring oil recovery's effective diffusivity via oil self-diffusion is difficult. Abstract: Injecting gas to enhance oil production from unconventional reservoirs dominated by nanoscale pores has been practiced in past decades with varying success, in part due to the lack of a fundamental understanding of the underlying physical processes. Here, we report molecular dynamics simulations of gas-enhanced recovery of decane from single 4 nm-wide calcite pores under reservoir conditions (383 K and 345 bar). Two gases, CO2 and CH4, are considered due to their different adsorption strength on calcite pore walls and practical considerations such as their availability and benefits for carbon sequestration. We show that, upon entering a pore, both gases form two molecular populations (free and adsorbed molecules), and their accumulation leads to the extraction of corresponding decane populations. The CO2 -decane exchange is initially significantly driven by the evolution of the adsorbed populations, but a transition to the dominance by free populations occurs later; For the CH4 -decane exchange, the opposite occurs. Despite this difference, the overall gas accumulation and decane extraction behavior follow the same diffusive law for CO2 and CH4 gases. The CH4 -decane exchange has higher effective diffusivities than the CO2 -decane exchange, i.e., CH4 enables faster decane extraction under the conditions studied here. These effective diffusivities do not always align well with the self-diffusion coefficients of CO2, CH4, and decane in nanopores.