The commercial exploitation of shale resources requires the design of hydraulic fracture programs to optimize the stimulated reservoir volume. Challenges include the engineering of staging, sequencing, injection volume, and proppant selection. This paper describes the development of a 3-D finite-element model to simulate growth of fracture complexes as functions of rock stress, brittleness properties, and injection conditions. The fracture complex is modeled as a major fracture, which consists of a cohesive crack and microcracks that form with variable continuum mechanical damage. Variations of permeability and porosity affect the continuum mechanical damage, which is used to investigate the fracturing sequence of three stages of injections in a horizontal well and generates three sets of fracture complexes. Inputs include a geomodel, the initial geostress, and pore pressure. Elastoplastic deformation coupled with both porous and gap flows are simulated.

The model is also used to investigate the influence of rock brittleness on the fracture complexity. Material brittleness is the product of the elasticity modulus and Poisson’s ratio. Three sets of values of material brittleness were used in the calculation for comparison. The same loads and initial conditions of the first example were used in the model. The results include two sets of distributions of fracture propagation and damage evolution for two different injection sequences, pore pressure distribution after fluid injection stimulation, three sets of numerical results of distribution of fracture, and mechanical damage corresponding to different values of brittleness. Numerical results indicate that adjusting the sequence of multi-stage fracturing significantly influences the stimulated formation volume fractured. The model can be used to design more optimal fracture programs.

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