ABSTRACT:

Pre-existing natural fracture networks (NFNs) are ubiquitous in many unconventional oil and gas formations. Hydraulic fracturing (HF) is used to stimulate these formations; many traditional HF analyses and modeling efforts have largely ignored (or highly simplified) the NFN effect on HF stimulation results. NFNs are characterized by geometric properties (including spatial distribution, length, and height) and mechanical properties (including tensile strength, cohesive strength, and shear stiffness). This paper explicitly represents NFNs in a validated, state-of-the-art geo-mechanical simulation model, which couples rock mechanics, fluid mechanics, and proppant transport to enable HF stimulation process modeling. Simulations are used to systematically study NFN properties and their effect on HF stimulation outcomes, including fracture length, propped area, and fracture complexity. All NFN properties (and other formation and stimulation treatment variables) together affect the final stimulated network; examples from the US Eagle Ford and Spraberry formations demonstrate some of these effects. In addition to NFN properties, fracture toughness and horizontal stress anisotropy properties were varied to provide additional insight into the results. Study results demonstrate that NFNs may crucially affect HF stimulation outcomes and, therefore, indicate the importance of mapping NFNs and their mechanical properties, and considering them in designing optimal HF stimulation jobs in particular formations.

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