Experimental Analysis: The Role of Minerology in Reduction of Salinity on Production Water and CO2 Gases by Improved Wet In-Situ Combustion on Unconventional Light Oil Reservoir

Abstract

Produced-water management remains a major challenge in unconventional reservoirs as hydraulic fracturing generates large volumes of highly saline water that require treatment, reuse, or underground disposal. Wet in-situ combustion (ISC) offers an alternative approach by improving oil recovery while promoting steam distillation, mineral-fluid reactions, and the in-reservoir retention of impurities in hydrocarbons during thermal upgrades. This study investigates the effects of water injections, brine salinity, and reservoir mineralogy on oil recovery, produced-water quality, and combustion-gas composition. The objective is to determine whether wet ISC cannot only improve oil productions in unconventional reservoirs but also reduce the salinity of the recovered water. Three combustion tube experiments were conducted using Permian Basin shale, light crude oil, and formation brine. The experiments consisted of dry ISC (E1), wet ISC with distilled water injection (E2), and wet ISC with formation-brine reinjection (E3). Combustion front temperature, front propagation velocity, liquid production, and gas compositions were monitored throughout each experiment. The initial shale was characterized using X-ray diffraction and thermogravimetric and differential scanning calorimetry analysis. The TDS and pH of the recovered production water were measured. Postmortem samples from the swept zones were analyzed using SEM-EDS to evaluate the redistribution and precipitation of salt and mineral related elements. Wet ISC improved oil recovery from 31.0% in E1 to 54.8% in E2 and 47.9% in E3. The initial formation brine contained 61,200 ppm TDS and had a pH of 5.31. Following combustion, the TDS of collected produced water decreased to 16,500 ppm in E1, 9,600 ppm in E2, and 8,090 ppm in E3, while pH increased to 8.34, 8.68, and 8.22, respectively. The large decrease in salt concentration resulted from steam formation and condensation, dilution by injected and combustion water, and precipitation of salts at the swept regions. The increase in concentrations of both Ca and S in the E2 wet zone demonstrated moisture-assisted retention of sulfur, whereas tremendous amount of Na and Cl concentrations in the E3 wet zone indicated that salts were retained during brine reinjection. E3 generated less CO2 than E2 but also discharged more residual O2, showing a potential of reduced oxidation intensity rather than confirmed mineral carbonation. This study demonstrates that wet ISC can enhance light-oil recovery while partially conditioning the produced aqueous phase through steam condensation and salt precipitation. The results also show that reservoir mineralogy influences the redistribution of sulfur compounds and brine salts during combustion. These findings establish wet ISC as a potential combined oil-recovery and produced-water-management process. Formation-brine reinjection also provides a route for internal water reuse, although salt deposition introduces a direct tradeoff between production water qualities and air flow capacity within the swept zone.