Improving recovery efficiency by CO2 injection at late stage of steam assisted gravity drainage

The steam assisted gravity drainage (SAGD) process requires high energy input to maintain the continuous expansion of the steam chamber for achieving high oil recovery. In the late stage of SAGD operation where the oil rate is low and the heat loss is high from a mature steam chamber, maintaining steam chamber pressure with a lower steam injection is the key to maintaining the economic oil-to-steam ratio (OSR). Both laboratory studies and field tests have demonstrated the effectiveness of adding a non-condensable gas (NCG) to the SAGD steam chamber for improving the overall thermal efficiency. In this study, a multi-well reservoir model was built based on the detailed geological description from an operating SAGD project area, which contains thick pay and top water. Grounded with the history matching of more than 20 years of production using CSS (cyclic steam stimulation) and SAGD as follow-up process, the model was applied to optimize the operating strategies for the late stage of SAGD production. The results from this study demonstrated that the co-injection of steam with CO₂ or the injection of CO₂ only has potential to improve the OSR and reduce emissions by more than 50% through the improvement in steam-saving and the storage of CO₂. The results from reservoir modeling indicate that, with the current volume of a steam chamber and an operating pressure of 4.0 MPa, about 55 sm³ of CO₂ could be sequestrated and utilized for producing 1.0 m³ of oil from this reservoir through the replacement of a steam injection with CO₂ in the late stage of SAGD operation.

Supercritical carbon dioxide (SC-CO₂) is suitable to extract low-polar organics and to assist in the dissolution of pores and fractures in shale. In this work, we investigate the effect of temperature on the structure of five shale samples via high pressure reaction assisted with SC-CO₂. Shale samples were analyzed using X-ray diffraction, field emission scanning electron microscopy, and ImageJ software. Due to the extraction of CO₂, after Sc-CO₂ treatment, carbonate and clay content decreased, while quartz and plagioclase increased slightly, which improved gas and oil flow in microscopic pores and shale cracks. Shale samples showed an increase in surface fracture area as experimental temperature increased. Since Sc-CO₂ fluid density and solubility increase with temperature, more organics can be extracted from shale pores and fractures, resulting in newly formed pores and fractures. As a result, the threshold temperature for shale high-temperature Sc-CO₂ cracking was confirmed to be 400 °C, and the fracture area increased by more than 45% at this temperature. Based on the findings of this study, Sc-CO₂ technology can be used to potentially recover low-maturity shale oil efficiently.

The CO₂ miscible flooding in Baikouquan tight formation was thoroughly investigated under reservoir condition of 37 MPa and 89 °C. The low-field nuclear magnetic resonance (NMR) system was used to measure the real time oil saturation under high-temperature and high-pressure environment. The results show that the recovery factors of large, medium and small pores are 66.18%, 45.71%, and 12.71%, respectively. A complementary investigation of CO₂–brine–rock interaction was performed, which indicate that the small pore throats blockage during the displacement process is the main reason for low recovery in small pores.

In order to compare the oxidation kinetics parameters of crude oils with different properties in the process of crude oil oxidation, six different crude oil samples were selected to analyze the oxidation characteristics of crude oils with different properties. In order to study the oxidation of crude oil, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) synchronous analyzer were used for crude oil in the oxygen environment between 25 °C and 900 °C at a heating rate of 20 °C/min. The experimental results were based on crude oil oxidation using TGA and DTG experimental data to evaluate the oxidation mechanism of different crude oils, so as to better understand the situation in the oxidation reaction process. At the same time, the oxidation stage of crude oil was divided according to DSC data. Arrhenius method was used to analyze the oxidation kinetic parameters of crude oils with different properties, and the activation energies and pre-exponential factors of different crude oils were calculated. The experimental results show that the oxidation stage of crude oil can be divided into three stages: low-temperature oxidation, fuel deposition, and high-temperature oxidation. The low-temperature oxidation reaction begins at 280 °C, and the high-temperature oxidation reaction occurs at 400 °C. The low-temperature oxidation activation energy of an oil sample is 39.73–77.74 kJ/mol. The activation energy of the high-temperature oxidation is in the range of 106.27–264.47 kJ/mol. The activation energy of crude oil in the low-temperature oxidation stage increases with the increase of crude oil viscosity and decreases with the increase of crude oil viscosity in the high-temperature oxidation stage. Therefore, during the high-temperature oxidation stage, high-viscosity crude oil is more prone to reactions.

This paper investigates the influence of reaction atmosphere and operation parameters of the lightening process under high temperature and high pressure on high-viscosity shale oil using an experimental approach. Two types of experiments were implemented, one involving a thermogravimetric analyzer and another using an autoclave to carry out the lightening process. By these two kinds of experiments, the effects of reaction atmosphere and operation parameters on the lightening efficiency were clarified. As for the reaction atmosphere, the effects of CO2, N2 and air were separately evaluated. As for the operation parameters, the effects of heating rate and formation rock were investigated. The results indicate that under a CO2 atmosphere, the lightening reaction is more intense than that under the other two gas phases, and it gains the highest reaction rate. Part of the minerals in the formation rock can be treated as catalyst in the shale oil lightening process. With the formation rock being present, the reaction rate increases significantly and higher contents of light components are obtained in both the lightened shale oil and gas phase. For the kinetic parameters in the lightening process, proportional relationships between the kinetic parameters and heating rates under CO2 atmosphere with and without formation rock were identified. The findings of this study can provide guidance for enhancing high-viscosity shale oil using an in-situ lightening process. Document Type: Original article Cited as: Zhou, X., Li, H., Zeng, F., Yu, C., Ouyang, H., Jiang, Q. Lightening of shale oil using high-temperature supercritical CO2: An experimental study. Advances in Geo-Energy Research, 2025, 16(2): 99-113. https://doi.org/10.46690/ager.2025.05.03