Summary
Fracability characterizes the ease of gas shale to form a complex-fracture network with hydraulic-fracturing treatments. Previous methods of fracability evaluation take into account some mechanical properties of gas shale, such as brittleness and fracture toughness. However, very little work has been performed to verify these methods by comparing the predicted fracability against the actual result of fracturing stimulation. Moreover, the prediction models of fracture toughness used in the previous methods are derived from conventional shale rather than from gas shale, which leads to the low resolution of these methods. In this paper, new prediction models for both the Mode-I and the Mode-II fracture toughness of gas shale are developed by use of straight-notched-Brazilian-disk (SNBD) tests and logging data. Furthermore, an improved fracability-evaluation model is proposed on the basis of the new toughness models. The new fracability model takes into account the brittleness, fracture toughness, and minimum horizontal in-situ stress of the gas-shale reservoirs. Compared with the previous models, the new model has better resolution in identifying fracability. The accuracy of the proposed model is verified with the efficiency of field hydraulic-fracturing jobs.
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