Abstract

The inclusion of fracture networks in reservoir models is generally based on the concept of failure associated with sub-vertical fractures. In general, it is surmized that fractures can grow irregularly in a stress field that is perturbed by a hydraulic fracture injection. It has also been considered that structural weaknesses in the rock such as pre-existing fractures and naturally occurring laminations commonly found in shale-gas reservoirs can be conduits for fracturing during stimulation and active pathways for fluid flow. We postulate that local stress perturbations through stress transfer allows for fractures to propagate and initiate failure along pre-existing fracture sets, which include sub-vertical and sub-horizontal fractures. Additionally, the degree of fracture interconnectivity and the type of fracturing will play a role in whether effective proppant transport is achieved. Through moment tensor inversion of microseismic events related to stimulation in the Horn River Basin utilizing well-conditioned geophone arrays, we have been able to define a three dimensional discrete fracture network consisting of sub-horizontal and sub-vertical fractures. Geologic data from the site provided corroborative evidence to the validity of the observed discrete fracture network, the presence of sub-horizontal fractures and fracture orientations in-line with current regional stress field. The fracture intensity and complexity appeared to be directly related to the degree of interaction between the sub-horizontal and sub-vertical fractures. Regions dominated by sub-horizontal fractures were also regions exhibiting poor fracture intensity and complexity. Based on these observations and moment tensor derived failure modes (opening component of failure), we were able to identify regions of enhanced fluid flow, further identifying regions of effective fluid transport. Regions with poor connectivity and dominance of sub-horizontal fractures also were identified as regions of poor fluid flow; these then become regions for potential re-stimulation. Based on these analyses, it can be suggested that sub-horizontal fractures can play an important role in the overall fracture development.

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