EXPERIMENTAL STUDY OF FRACTURE GENERATION AND PERMEABILITY EVOLUTION DURING INJECTION-INDUCED PRECIPITATION IN CARBONATE-RICH SHALE

Fracture mineralization is commonly observed in nature but has proven difficult to study in the laboratory. Here we present a model system involving reaction of a BaCl₂ aqueous solution with a fine-grained carbonate rock that dissolves calcium carbonate and precipitates barium carbonate within a laboratory-produced fracture network. For the experiments, we use a triaxial direct-shear coreflood system with integrated x-ray tomography and radiography. The experiment was conducted at a confining stress of 3.5 MPa, 20 ^oC and using a 1x1 inch cylinder of an almost pure carbonate core sample from the Utica shale gas play. The specimen was loaded in direct shear to failure, which led to the formation of a through-going, permeable fracture system. We recorded the fracture process using x-ray radiography that showed the formation of a variable aperture fracture coincident with the onset of peak permeability of 0.43 mD. Over the course of a 90-minute injection of a dilute BaCl₂ solution, barium carbonate precipitation was clearly evident in radiography and in subsequent tomography conducted at pressure. X-ray tomography showed that the fracture system was dominated by a primary fracture that connected the shear platens and that had a large but variable aperture.

Back-scattered electron microscopy revealed that precipitation of barium carbonate occurred throughout the fracture system and from inlet to outlet. Precipitation was concentrated in small-aperture regions, areas of fracture gouge, and within an extensive network of very small, subsidiary fractures emanating off of the main fractures. Needles of barium carbonate were observed to grow perpendicular to the fracture walls. Despite the extensive precipitation, permeability dropped by only a factor of 3 during the experiment. This model system may be an analog for precipitation occurring in geothermal systems, reactions between injected fluids and formation waters in oil and gas, or as a method of manipulating permeability through reaction-induced in-filling of fractures in applications such as conformance in oil and gas production or caprock integrity in CO₂ sequestration.