Study on the Pseudo Threshold Pressure Gradient Model of Heavy Oil Reservoirs Considering CO2 Dissolution Viscosity Reduction and Thermal Effects

Abstract This study investigates Xinjiang oilfield heavy oil as the research subject, employing a high-temperature, high-pressure rheometer to analyze changes in crude oil viscosity under varying temperatures after CO2 dissolution, thereby exploring the synergistic effects of CO2 and thermal action on heavy oil viscosity reduction. Additionally, core flooding experiments were conducted to examine the influence of this synergy on the flow behavior of heavy oil in reservoirs. The impact of CO2 dissolution at different temperatures on the pseudo threshold pressure gradient (PTPG) of heavy oil was tested, and the relationship between mobility, PTPG, and activation temperature was established. The results indicate that the viscosity of heavy oil decreases with increasing CO2 solubility. However, the viscosity-reducing effect of CO2 diminishes at higher temperatures. Through rheological experiments, constitutive Eq.s for heavy oil under different CO2 solubilities were developed. Core flow experiments yielded expressions for the pseudo-TPG and activation temperature of heavy oil under combined CO2 and thermal effects. A small amount of CO2 can achieve flow performance equivalent to that at elevated temperatures, significantly reducing the thermal energy demand for heavy oil recovery. Furthermore, these expressions enable more accurate predictions of the maximum drainage radius and optimal thermal stimulation transition temperature in heavy oil reservoirs. This study establishes models for the TPG and activation temperature of heavy oil under the synergistic action of CO2 dissolution and thermal effects, clarifying the influence of CO2 and heat on the PTPG. These models improve the prediction of the maximum drainage radius and thermal transition temperature in heavy oil reservoirs. The findings provide theoretical guidance for CCUS and CO2-assisted steam flooding EOR technologies in similar Xinjiang oilfield blocks.