In the quest to enhance shale gas recovery and sequester carbon dioxide, a groundbreaking study led by Haonan Wei from the National and Local Joint Engineering Research Center for Carbon Capture Utilization and Sequestration at Northwest University in Xi’an, China, has shed new light on the intricate interactions between supercritical CO2 (ScCO2), water, and shale rock. Published in the journal *Fractals and Fractional Calculus*, the research delves into how these interactions alter the pore structure of shale reservoirs, offering critical insights for the energy sector.
The study focuses on shale samples from three marine shale formations in southern China, subjected to conditions simulating burial depths of 1000 meters (45 °C and 10 MPa) and 2000 meters (80 °C and 20 MPa). Using a suite of analytical techniques, including X-ray diffraction (XRD), mercury injection capillary pressure (MICP), nitrogen and CO2 gas adsorption (N2GA and CO2GA), and field emission scanning electron microscopy (FE–SEM), the researchers meticulously examined the mineral composition and pore structure changes.
“We found that ScCO2–water–shale interactions lead to significant dissolution of minerals like kaolinite, calcite, dolomite, and chlorite,” Wei explains. “As the reaction progresses, substantial secondary mineral precipitation occurs, particularly under the 2000-meter simulation conditions.” These changes in mineral composition and precipitation patterns have profound implications for the pore structure of the shale.
The study reveals that mineral dissolution and precipitation cause notable shifts in pore structure parameters. Macropores, for instance, exhibit increased pore volume (PV) and decreased specific surface area (SSA), while mesopores show decreased PV and SSA. Micropores, however, remain relatively unchanged. “Mineral precipitation effects are stronger than dissolution effects,” Wei notes, highlighting the complex interplay between these processes.
One of the most intriguing findings is the alteration in multifractal parameters, which quantify the heterogeneity and connectivity of the pore structure. Mineral precipitation reduces pore connectivity, thereby enhancing pore heterogeneity. Correlation analysis further confirms that reduced connectivity corresponds to stronger heterogeneity, with mineral composition strongly influencing the multifractal responses of macropores and mesopores. Micropores, on the other hand, mainly undergo morphological changes, with modifications primarily affecting their internal space.
The study also highlights the structural stability of siliceous shale samples compared to argillaceous shale, attributing this to the mechanical strength of the quartz framework. By integrating multifractal theory with multi-scale pore characterization, the research achieves a unified quantification of shale pore heterogeneity and connectivity under ScCO2–water interactions at reservoir-representative pressure–temperature conditions.
This novel approach not only advances the methodological framework but also provides critical support for understanding CO2-enhanced shale gas recovery mechanisms and CO2 geological sequestration in depleted shale gas reservoirs. “Our findings underscore the complex coupling between geochemical reactions and pore structure evolution,” Wei states, emphasizing the importance of these insights for the energy sector.
As the energy industry continues to explore innovative methods for enhancing gas recovery and reducing carbon emissions, this research offers a compelling roadmap for future developments. By unraveling the intricate dynamics of ScCO2–water–shale interactions, Wei and his team have paved the way for more effective and sustainable energy practices, ultimately shaping the future of shale gas extraction and carbon sequestration.