The conversion of a highly impermeable medium like shale into numerous gas producing ‘pay zones’, using geomechanics steered, stress-and-structure oriented hydraulic fracturing, is a remarkable achievement. So remarkable in fact that shear mobilization of natural fractures has also to be invoked to explain both the continued though declining production and the sources of larger ‘radius’ microseismic activity well beyond the assumed ellipsoidally-shaped tensile-fractured and sand-propped ‘central’ zones. The microseismic activity is believed to be the remote-sensing sign of shearing initiation of a large number of the natural fractures. The assumed shearing, and the resulting gas drainage, cannot occur in the case of gas-shales unless the shale is of high enough modulus to sustain the shear induced dilation, which results in a coupling with enhanced fracture permeability. The pre-peak mobilization of roughness and permeability due to pre-peak dilation, combined with low in situ shear stiffness due to block-size related scale effects, is part of the rock mechanics reality behind critically stressed fractures, which are simplified as linear Mohr Coulomb events in petroleum geomechanics. In reality a more sophisticated and more favourable series of coupled processes are likely to be involved.

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