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A Different Perspective on Critically Stressed Fractures and Their Impact on Fluid Flow

Abstract

Naturally fractured reservoirs are often characterized by high-density multi-scale fracture networks, organized in multiple orientation families. However, not all fractures contribute to fluid flow under in-situ stress conditions during production. The Coulomb friction criterion is often used to predict reactivation of fractures, and hence quantify which part of the fracture network contributes to flow at in-situ stress conditions. Coulomb friction predicts that only the fractures that are sub-parallel to the maximum horizontal stress are reactivated. However, in both outcrops and reservoirs, it is commonly found that multiple orientation families, often organized in orthogonal patterns, are at least partially open. Partial cementation of fractures may explain how fractures can be open even when they not meet the Coulomb criterion, as cement often forms bridges that prevent the fracture from fully closing. However, in the absence of these cement bridges, roughness of the fracture surface itself, combined with minor amounts of shearing during opening, provides an additional explanation for the occurrence of hydraulically open fractures. The Barton-Bandis model, based on experimental work on outcropping rocks, quantifies the aperture that may remain when irregular fracture walls no longer perfectly interlock, and finds that even when a fracture is not ideally aligned with respect to the horizontal maximum stress, it may still act as a partial conduit to flow. We implement the Barton-Bandis aperture model into a Finite Element code to model the aperture distribution at in-situ stress conditions for natural fracture network geometries with varying degrees of complexity and intensity, digitized from outcropping pavements. The resulting aperture distributions are heterogeneous, with partial opening and closing even along a single fracture. Whether a fracture segment is open depends not only on the orientation of the fracture but also on its length and spacing distribution. The fraction of reactivated fractures in the Barton-Bandis model is on average 80%, versus 50% based on the Coulomb criterion. The average aperture predicted by Barton-Bandis is 0.2 mm in our models, but in a low-permeable matrix, i.e. less than 25 mD, the impact of fractures on flow is significant because of the high fraction of open fractures. The Barton-Bandis model may better explain how multiple fracture orientation sets contribute to fracture flow in the subsurface, compared to Coulomb.