Ellis Brian R, Fitts Jeffrey P, Bromhal Grant S, McIntyre Dustin L, Tappero Ryan, Peters Catherine A
Department of Civil and Environmental Engineering, Princeton University , Princeton, New Jersey.
Environ Eng Sci. 2013 Apr;30(4):187-193. doi: 10.1089/ees.2012.0337.
Geochemical reactions may alter the permeability of leakage pathways in caprocks, which serve a critical role in confining CO 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 CO 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.
地球化学反应可能会改变盖层中渗漏通道的渗透率，而盖层在地质碳封存中对封存二氧化碳起着关键作用。对密歇根沉积盆地碳酸盐地层的一个盖层样本进行了压裂，并在高压岩心流动实验中进行了研究。流入的盐水在40°C和10MPa的条件下被二氧化碳饱和，初始pH值为4.6，方解石饱和指数为-0.8。在实验过程中，裂缝渗透率降低，但后续分析并未发现方解石沉淀。相反，实验观察表明，沿裂缝通道的方解石溶解导致了难溶性矿物颗粒的迁移，从而堵塞了流动通道。通过电子显微镜、基于同步加速器的X射线衍射成像以及微束Ca K边X射线吸收近边结构的首次应用对岩心切片进行分析，提供了证据表明这些堵塞物是母岩的碎片而非次生沉淀物。X射线计算机断层扫描显示，优先流动路径内的岩体有显著损失，这表明溶解作用还去除了关键的粗糙度并导致裂缝的机械闭合。尽管沿裂缝通道有物质的净去除，但裂缝渗透率仍降低，这表明在裂缝性碳酸盐盖层中，溶解与渗透率演化之间存在一种非直观的反比关系。
