ABSTRACT:

The behavior of intersections is central to the fluid flow and transport evolution of a fracture network in response to chemical and physical processes. Here, laboratory experiments were performed to explore the evolution of network geometry during loading of a 3D printed fracture network composed of two intersecting orthogonal fractures. 3D X-ray microscopy was used to image the changes in the fractures and intersection for two orientations of the network, "+" and "X", relative to an applied normal load. For the "+" orientation, the connectivity along the intersection was maintained by the vertical fracture that did not deform significantly under load. However, when the network was rotated to the "X" orientation, the void volume decreased along both fracture planes and the connectivity between the two orthogonal fractures ceased to exist at the highest load. The orientation of fracture networks relative to a local stress field affects the flow paths in the individual fracture planes and the connectivity of the fracture network.

1. INTRODUCTION

Fracture networks in rock provide the connected flow paths through which fluids are injected or withdrawn from subsurface reservoirs during energy production (e.g. geothermal, oil & gas, H2 storage) and anthropogenic waste isolation (e.g. CO2 sequestration, nuclear waste). While the mechanical and hydraulic behavior of single fractures are well understood (e.g. Pyrak-Nolte & Nolte, 2016), less laboratory research has focused on fractures networks (e.g. Hull, 1986; Montemagno & Pyrak-Nolte, 1999; Johnson & Brown, 2001; Johnson et al., 2006) and how they deform under stress (e.g. Frash et al., 2016, 2017 & 2019). Transitioning from an individual fracture to a 3D fracture network increases the complexity of a fracture system by introducing fracture intersections. Though intersections compose only a tiny fraction of the total void volume of a fracture network, closure or opening of an intersection can radically affect the transport paths and connectivity of a network.

Our view of fracture networks is often based on 2D views from outcrops (Figure 1) or boreholes. Although fracture networks are intrinsically 3D, the 2D sections enable one to infer the topological elements of the system (Bai & Pollard, 2000; Sanderson & Nixon, 2015; Peacock et al., 2018; Laubach et al., 2019). One of the simplest topological elements of a fracture network is shown in Figure 1c and is referred to here as "X". This topological element is composed of 2 intersecting fractures with 1 intersection (Figure 1c).

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