Feasibility and Efficiency of Dry and Wet In-Situ Combustion in Low-Viscosity Oil Reservoirs

Abstract In-situ combustion (ISC) is a complex thermal enhanced oil recovery (EOR) method traditionally applied to heavy and extra-heavy oil reservoirs. Its application to low-viscosity oil reservoirs presents challenges due to the higher ignition temperature requirements, resulting from the stable molecular content of such oils, and the lack of heavy fractions, such as asphaltenes and resins, which serve as fuel for sustaining combustion fronts. This study investigates low-viscosity oil combustion mechanisms and examines the interactions between reservoir rock, brine, and crude oil in sustaining combustion fronts during dry and wet ISC processes. The experiments utilized a 1-meter-long insulated stainless steel combustion tube packed with a mixture of reservoir rock, formation brine, and low-viscosity crude oil at a 1:1 liquid pore volume ratio. Combustion was initiated by injecting compressed air into the preheated tube. Dry ISC experiments involved air injection alone, while wet ISC co-injected air with distilled water at specific water-to-air ratios (WAR). The tested reservoir rocks included quartz-rich Canadian sand (Sample 1), U.S. reservoir rock 1 (Sample 2), and U.S. reservoir rock 2 (Sample 3). Post-experiment analyses were performed using Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) to evaluate elemental composition and mineral transformations. The water produced was analyzed for pH and Total Dissolved Solids (TDS). Results emphasize the crucial role of reservoir rock reactivity and brine ionic strength in sustaining combustion fronts in low-viscosity oil reservoirs. Dry ISC led to less stable fronts, with peak temperatures ranging from 550 to 600°C, whereas wet ISC exhibited superior performance, achieving stable combustion fronts, higher combustion velocities, and lower peak temperatures. Produced water analyses indicated a salinity reduction from 120,000 ppm to as low as 30 ppm, supporting its feasibility for re-injection. Additionally, wet ISC resulted in cleaner reservoir rocks with minimal carbon residues, underscoring its advantages in efficiency, stability, and operational feasibility over dry ISC for low-viscosity oil recovery. This study provides a novel approach to ISC in low-viscosity oil reservoirs, demonstrating that despite the high ignition temperature requirement, reservoir rock and brine interactions play a key role in sustaining combustion. Unlike conventional ISC studies that focus on heavy oil, this research highlights how wet ISC can significantly enhance combustion stability and efficiency in low-viscosity oils, where conventional fuel sources (asphaltenes and resins) are insufficient. Furthermore, the study introduces the re-injection of treated produced water as a sustainable method to improve ISC efficiency, minimize environmental impact, and reduce operational costs. These findings offer new insights into optimizing ISC for low-viscosity oil reservoirs, making it a more viable and eco-friendly thermal EOR technique.