When creating arrays of hydraulic fractures in close proximity, stress field changes induced by previously placed hydraulic fractures can lead to deflection in subsequent fracture paths and coalescence between fractures. Any fracture coalescence can compromise the effectiveness of the treatment array and the fracture geometry will not be appropriately accounted for in reservoir or caving models. Here we present the results of an experimental study consisting of arrays of 4 closely spaced hydraulic fractures grown sequentially in 350x350x350 mm blocks of a South Australian Gabbro under different initial stress states and for notched and un-notched wellbores. In particular we focus on insights gained from 3-dimesional serial sectioning and digital reconstruction of the hydraulic fracture patterns that were formed. The results show that the curving hydraulic fractures typically do not exhibit a high degree of radial symmetry in their paths even though the fractures grew by radiating outward from a centrally located wellbore. The results also confirm model predictions that a subsequent fracture will curve towards a previous fracture when the minimum stress is zero and that this curving is suppressed when the minimum stress is sufficiently large. Finally, fracture initiation is shown to be critical to the symmetry of the fracture pattern and preponderance of branching and therefore effective notches that lead to initiation in the eventual plane of favored propagation have a profound impact on the hydraulic fracture geometry.

As the reduction of carbon emissions becomes an increasingly pressing issue, a larger emphasis is being placed on the need for the development of renewable energy. One such option is geothermal energy which utilizes the heat from the earth’s crust; it presents a vast potential for the production of commercial scale base-load power generation. However, the conventional techniques used in the stimulation of hot dry rocks (HDR) geothermal wells are not very effective in producing a permeable reservoir for heat exchange between the rock mass and the working fluid. To increase the permeability of geothermal reservoirs, a new stimulation technique (developed by CSIRO - Commonwealth Scientific and Industrial Research Organisation) which involves isolating sections of the well for controlled planar fracture growth can be used. However, if these notches/fractures are placed too closely together they will interact with one another, resulting in a deviated fracture path. A two dimensional numerical model has thus been developed to study conditions under which adjacent fractures will interact with one another. This study aims to verify the numerical model through stimulating a number of granite blocks, and drawing comparisons between the observed fracture pattern and that predicted by the model. To achieve this goal, the stimulated and fractured granite blocks were sectioned and their fracture patterns were extracted using a MATLAB code, before being reconstructed in their respective positions. Stimulation was carried out firstly using conventional techniques, and then by trialling the method proposed by CSIRO. Observation of the reconstructed images showed good agreement between the model predictions and the observed fracturing patterns in two-dimensions. However, the three-dimensional pattern in the notched, perpendicular well-bore was observed as a ‘half cylinder’. This was counter intuitive as it was expected that radial symmetry of the fractures would be observed resulting in a ‘bowl’ shape. It was therefore concluded that while the model was unable to accurately predict the three-dimensional geometry of an array of fractures, stimulation through a notched perpendicular wellbore was very effective in the production of a controlled system of fractures with an improved fluid flow and heat exchanging surface area of the reservoir in comparison to the conventional techniques.

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