The Effect of SCCO2 Treatments on the Petrophysical Properties of Continental Shale with Different Mineral Compositions

ABSTRACT CO2 Pre-pad Energized Fracturing has been widely used in China in recent years to effectively develop shale oil. The petrophysical properties of continental shale vary with different sedimentary conditions. However, it is unknown how CO2 pre-injection affects these properties for different continental shale reservoirs. In this study, three sets of continental shale from the Qikou Sag, Jiyang Depression, and Yanchang Formation were selected for the experiment. The effects of SCCO2 on the petrophysical properties of different samples were analyzed by N2GA, XRD, and uniaxial compression tests. According to the results, the SCCO2-water-shale interaction induced the dissolution and precipitation of shale minerals during the immersion test. In addition, the average pore size of QK samples increased due to the dissolution of minerals, while the strong swelling effect and mineral precipitation reduced the average pore size of JY and YC samples. After interacting with SCCO2, the pore structure complexity of the QK and JY samples decreased slightly. Furthermore, the changes in mineral matrix and pore structure led to significant deterioration of the mechanical properties of the three shale groups, especially for the JY sample. These findings provide an experimental basis for applying CO2 Pre-pad Energized Fracturing technology to different continental shale reservoirs. INTRODUCTION In recent years, with the advancement of oil and gas exploration and development technology, China has made major breakthroughs in unconventional oil and gas development, and the proven reserves of unconventional oil and gas account for 41% of the country's oil and gas resources (Yang et al., 2015). As an important unconventional oil and gas resource, shale oil is mainly divided into two types: continental and marine. Continental shale is the main shale oil reservoir in China, and its development and utilization are different from those of marine shale oil in North America in terms of geological environment and development methods (Wenzhi et al., 2020). At present, shale oil enrichment layers have been discovered in numerous basins in China, such as the Junggar Basin, the Ordos Basin, and the Bohai Bay Basin (Feng et al., 2020; Ju et al., 2020). In 2022, Wu Xiaozhi (Xiaozhi et al., 2022) predicted that the national shale oil geological resources would reach 335.3 × 108 t. Therefore, realizing the efficient and economical development of continental shale oil is of great significance for ensuring China's energy security.