Influence of water on thermo-oxidative behavior and kinetic triplets of shale oil during combustion

Abstract

The oxidation reactions between oxygen and shale oil directly affect the feasibility and performance of air injection. In this work, the influence of water on thermo-oxidative behavior of shale oil was studied using high-pressure differential scanning calorimetry (HP-DSC). Subsequently, the kinetic triplets for shale oil oxidation in the absence and presence of water were determined using Friedman and Ozawa-Flynn-Wall (OFW) models as well as one advanced integral master plots method. The HP-DSC results showed that at 5 MPa, the shale oil was vulnerable to obviously higher heat release at the low-temperature oxidation (LTO) region compared to the high-temperature oxidation (HTO) region, resulting in shortening the oxidative induction time and achieving in-situ upgrading of shale oil. Water delayed the initial heat release due to the fact that heat release yielded by LTO compensated the heat adsorption yielded by evaporation of water and light hydrocarbons. Additionally, the heat release was reduced with the elevated water saturation as a result of the reduction in exothermic rates and intensified evaporation of water and light hydrocarbons. The higher energy was needed to trigger off the LTO reactions while increasing water saturation from 10% to 20%. Therefore, it was believed that the presence of water extended the oxidative induction time, thereby hindering the smooth and rapid transition from the LTO to HTO stages. Water changed the reactivity of each oxidation stage for shale oil, resulting in higher energy required for the initial LTO reactions and lower energy required for the subsequent oxidation reactions. The water saturation range of 10% to 15% could accelerate the occurrence of high-temperature combustion most effectively, demonstrated by values of activation energy for the conversion greater than 0.6. The appropriate oxidation reaction models for shale oil alone and water saturations of 10%, 15%, and 20% were determined to be D2, F1, F3, and R3, respectively. The determination of kinetic triplets assisted in building the reaction schemes for modelling the air injection process of shale oil reservoirs.