Wet In-Situ Combustion Substantially Increases Oil Recovery in Light Oil Shale Reservoirs

Abstract This study presents a novel application of in-situ combustion (ISC) for enhancing oil recovery in light oil shale reservoirs, using core materials and fluids from the Bakken Formation. ISC is a well-established thermally enhanced oil recovery (EOR) method, traditionally applied to heavy oil reservoirs. This study explores its potential for shale oil plays, a concept that has received limited investigation in U.S. shales. ISC enhances sweep efficiency, maximizes oil recovery, and lowers operating costs, making it a promising technique for U.S. shale reservoirs. We conducted five experiments using a 1-meter-long insulated stainless steel combustion tube packed with Bakken crushed reservoir rock, formation brine, and low-viscosity crude oil at a 1:1 liquid pore volume ratio. Compressed air injection into a preheated tube initiated combustion. Dry ISC relied solely on air injection, whereas during wet ISC, we co-injected air and distilled water at controlled water-to-air ratios. We used three shale samples, two oil-wet and one non-oil-wet, from different Bakken formation wells. Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis with differential scanning calorimetry (TGA-DSC) quantified residual hydrocarbons and water. We applied the Sarathi (1998) and Nelson and McNeil (1961) analytical models to evaluate ISC performance for field-scale implementation. Results showed that rock mineralogy and wettability strongly influenced ignition behavior, with oil-wet shale igniting at 183 °C and non-oil-wet rock requiring up to 350 °C. Wet ISC consistently produced a more stable combustion front, reduced oxygen breakthrough, and enhanced thermal efficiency. Front velocities increased from 10.5 ft/day (dry) to 15.1 ft/day (wet), with significantly lower air injection needs. FTIR and TGA-DSC confirmed near-complete hydrocarbon removal in wet modes. Analytical modeling demonstrated higher oxygen utilization (88–92%) and 40–50% lower power and air demand for wet ISC compared to dry ISC. This study introduces dry and wet ISC as a viable approach for shale oil reservoirs, offering high efficiency and cost-effectiveness as an EOR method. Enhanced combustion stability and substantial oil recovery make ISC well-suited for shale formations. These findings pave the way for wet ISC implementation in U.S. shale reservoirs, providing a scalable and sustainable solution for improving oil recovery while minimizing operational costs.