Mechanism of pore expansion and fracturing effect of high-temperature ScCO2 on shale

Abstract

To elucidate the mechanisms underlying the alteration of shale pore fractures by high-temperature supercritical carbon dioxide (ScCO2), this study employed medium–low maturity shale samples from a specific region as research subjects. The samples were subjected to high-temperature ScCO2 reactions in a corrosion-resistant high-pressure reaction vessel at various temperatures. A suite of analytical techniques, including X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), ImageJ image processing software, N2 and CO2 adsorption experiments, thermal gravimetric-differential thermal analysis (TG-DTG) of CO2 pyrolyzed shale oil, and gas chromatography (GC) analysis, were utilized to investigate and analyze the microstructural changes in the shale samples before and after ScCO2 treatment.The results reveal a subtle increase in the content of clay minerals and quartz, accompanied by a decrease in the content of carbonates, potassium feldspar, and calcite. These mineral transformations, encompassing dissolution, precipitation, and swelling processes, exert a substantial impact on the shale surface microstructure and modify the internal pore structure. Consequently, a shift in the mechanical properties of the rock is observed, which bears significance for its fracturing behavior.The experimental results reveal a steady growth in fracture area following high-temperature ScCO2 treatment. Importantly, the extent of initial fracture development plays a crucial role in determining the efficacy of hydraulic fracturing, indicating a positive correlation. Moreover, the connection between fracture area and temperature exhibits a clear trend marked by an initial rise, a subsequent decline, and finally, a resurgence. In particular, the thermal fracturing threshold temperature for the shale samples was found to be around 400 °C.From the analysis, it can be inferred that ScCO2, when subjected to an elevated temperature range of 100-200℃, exhibits mutual interactions with the shale. This results in effective extraction of organic matter and dissolution of mineral constituents within the shale matrix. Consequently, this treatment process leads to a significant reduction in the number of micropores (pore sizes less than 2 nm), while the quantities of mesopores (pore sizes between 2 and 50 nm) and macropores (pore sizes greater than 50 nm) display a noticeable increase.Upon exposure to a temperature range of 200-300℃, a remarkable affinity of the shale for CO2 adsorption becomes evident. The adsorption process contributes to the expansion of the matrix material and deposition of minerals, effectively filling the pore spaces. As a result, the contraction of mesopores and macropores is mitigated, with the predominant change being an increase in the number of micropores.When temperatures reach 300-500℃, the intensified heat initiates thermal decomposition of the kerogen, culminating in the release of pore spaces. This phenomenon prompts an increase in the number of micropores and mesopores, thereby augmenting the pore volume and specific surface area of the shale.