Hydraulic Fracture Growth

Hydraulic fracture initiation and propagation in the presence of multiple fluid injection sources and under far-field stresses are investigated experimentally utilizing a novel fracturing cell. Mixtures of plaster and clay are used to cast sheetlike, porous test specimens with injection holes in different configurations. The specimen is placed between two transparent plates that allow us to take videos of the speckled rock sample surfaces to facilitate digital image processing of rock displacement. Far-field differential stress is applied to the specimen via pneumatic jacks. Hydraulic fracture growth during the experiment is recorded using a high resolution digital camera. Key frames are subsequently analyzed using Digital Image Correlation (DIC) to reveal micro-cracks, strains and other features that are difficult to detect with the naked eye. Experiments are conducted to examine fracture growth in proximity to a constant pressure injection source as well as propagation of multiple neighboring fractures in porous media.

It is clearly shown that high pressure injection regions alter stress field appreciably. Consequently, a fracture propagating along the maximum far-field stress tends to turn towards a high pore pressure source. In addition, a secondary fracture can be induced when the magnitude of the pore pressure source is relatively large. Fracture turning and formation of secondary fractures due to the adjacent high pressure injection hole maybe suppressed under relatively high far-field differential stress. It is also observed that fractures tend to grow perpendicular to the maximum far-field stress direction for multiple, closely spaced injection holes. In the case of isotropic far-field stress state, fractures tend to initiate and propagate towards neighboring injection holes. Fracturing pressure is generally observed to be markedly lower for tests with multiple fluid injection sources compared to tests with a single injection hole. Our experimental work confirms and extends previous literature in the topic via direct monitoring of fracture growth and novel application of digital image processing.