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Dissolution-Driven Permeability Reduction of a Fractured Carbonate Caprock

机译：裂解碳酸盐盖岩的溶质驱使渗透率降低

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Geochemical reactions may alter the permeability of leakage pathways in caprocks, which serve a critical role in confining CO2 in geologic carbon sequestration. A caprock specimen from a carbonate formation in the Michigan sedimentary Basin was fractured and studied in a high-pressure core flow experiment. Inflowing brine was saturated with CO2 at 40°C and 10 MPa, resulting in an initial pH of 4.6, and had a calcite saturation index of −0.8. Fracture permeability decreased during the experiment, but subsequent analyses did not reveal calcite precipitation. Instead, experimental observations indicate that calcite dissolution along the fracture pathway led to mobilization of less soluble mineral particles that clogged the flow path. Analyses of core sections via electron microscopy, synchrotron-based X-ray diffraction imaging, and the first application of microbeam Ca K-edge X-ray absorption near edge structure, provided evidence that these occlusions were fragments from the host rock rather than secondary precipitates. X-ray computed tomography showed a significant loss of rock mass within preferential flow paths, suggesting that dissolution also removed critical asperities and caused mechanical closure of the fracture. The decrease in fracture permeability despite a net removal of material along the fracture pathway demonstrates a nonintuitive, inverse relationship between dissolution and permeability evolution in a fractured carbonate caprock.
机译：地球化学反应可能会改变盖层泄漏路径的渗透性，这在限制二氧化碳在地质碳固存中起关键作用。来自密歇根沉积盆地碳酸盐岩地层的盖层标本被压裂并在高压岩心流实验中进行了研究。流入的盐水在40°C和10 MPa下用CO2饱和，初始pH值为4.6，方解石饱和指数为-0.8。在实验过程中，断裂渗透率降低，但随后的分析未显示方解石沉淀。取而代之的是，实验观察结果表明，沿着裂缝路径的方解石溶解导致了流动性较弱的矿物颗粒的动员。通过电子显微镜，基于同步加速器的X射线衍射成像以及首次在边缘结构附近进行微束Ca K边缘X射线吸收的分析，对岩心截面进行了分析，这提供了证据，表明这些闭塞是基质岩的碎屑而不是次生沉淀物。 X射线计算机断层扫描显示优先流动路径内的岩石质量显着损失，这表明溶解作用还消除了关键的凹凸不平并导致了裂缝的机械闭合。尽管沿裂缝路径净除去了材料，但裂缝渗透率却下降，这表明在碳酸盐岩盖层破裂过程中，溶解度和渗透率演化之间存在非直观的反比关系。

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