To examine the features of fractures induced by hydraulic fracturing and the surrounding stimulated regions, a hydraulic fracturing experiment was conducted using shale samples. In this experiment, the fracturing fluid was a thermosetting resin, methyl metaacrylate (MMA), mixed with a fluorescent paint, and just after fracturing the resin was immediately fixed within the sample by heating. Cut sections of the samples were observed under ultraviolet light irradiation. It is expected that the hydraulically induced fractures and the surrounding fractured region will be detected because the induced fractures filled with resin should emit light, while the other parts will not.
The samples, which were collected from the Kushiro Coal Mine in Hokkaido, Japan at the depth around275m,were roughly 85mm in diameter,170mm in length, and cored normal to the sedimentary planes. An injection hole with a10-mm diameter was drilled onto the center of the sample parallel to the sedimentary plane to simulate hydraulic fracturing in shale gas development. The experiment was conducted under a uniaxial loading condition of 3MPa and the fracturing fluid was injected into the sealed injection hole at a constant flow rate.
The main fracture induced by hydraulic fracturing, which was subsequently filled and fixed with the resin, is clearly observed. Detailed microscopic observations show that the main fracture is accompanied by many thinner secondary branches. Furthermore, fractured regions around the induced main fracture, which penetrate with the resin and emitted light under ultraviolet light irradiation, are also observed. It is confirmed that the region influenced by hydraulic fracturing around the main fractures exists. This fractured region is considered to be the stimulated region where the permeability is improved, presumably because the main fracture forms a new fracture network and/or activates a pre-existing one.