Dynamic Triaxial Study of Direct Shear Fracturing and Precipitation-Induced Transient Permeability Observed by In Situ X-Ray Radiography

摘要

Precipitation and dissolution processes can modify fracture permeability in applications such as CO_2 sequestration, geothermal energy, oil and gas production, and nuclear waste disposal. In this study, we conduct a triaxial coreflood experiment while imaging with in situ x-ray radiography. The carbonate-rich Utica specimen was fractured using direct shear methods at 20 °Cand confining pressure of 3.4 MPa. Permeability was measured continuously using a dilute BaCl_2 solution. The specimen's permeability increased significantly to 0.43 mD immediately after fracturing at 60 MPa shear stress then decreased with continued fluid flow due to precipitate infilling of the fracture. Precipitation was observed in real-time using x-ray radiographs. Subsequent analysis showed that calcite in the shale had reacted with BaCl_2 to produce BaCO_3, resulting in a 67% reduction in permeability. The precipitate was distributed throughout the fracture network but concentrated in regions with relatively narrow fracture apertures. While BaCO_3 is an unlikely precipitate in a natural environment, the use of BaCl_2 greatly facilitated x-ray contrast with results having applications to other chemical systems that may be kinetically inhibited or difficult to study experimentally. This study sheds light on the use of chemical injectates as mitigation techniques for plugging undesired permeability.