Relationship Between the Orientation of Maximum Permeability and Intermediate Principal Stress in Fractured Rocks

Abstract

Flow and transport properties of fractured rock masses are a function of geometrical structures across many scales. These structures result from physical processes and states and are highly anisotropic in nature. Fracture surfaces often tend to be shifted with respect to each other, which is generally a result of stress‐induced displacements. This shift controls the fracture's transmissivity through the pore space that forms from the created mismatch between the surfaces. This transmissivity is anisotropic and greater in the direction perpendicular to the displacement. A contact mechanics‐based, first‐principle numerical approach is developed to investigate the effects that this shear‐induced transmissivity anisotropy has on the overall permeability of a fractured rock mass. Deformation of the rock and contact between fracture surfaces is computed in three dimensions at two scales. At the rock mass scale, fractures are treated as planar discontinuities along which displacements and tractions are resolved. Contact between the individual rough fracture surfaces is solved for each fracture at the small scale to find the stiffness and transmissivity that result from shear‐induced dilation and elastic compression. Results show that, given isotropic fracture networks, the direction of maximum permeability of a fractured rock mass tends to be aligned with the direction of the intermediate principal stress. This reflects the fact that fractures have the most pronounced slip in the plane of the maximum and minimum principal stresses, and for individual fractures transmissivity is most pronounced in the direction perpendicular to this slip.

Tracking Fluid Flow in Shallow Crustal Fault Zones: 1. Insights From Single‐Hole Permeability Estimates

“…An increased normal stress may lead to a decreased fracture aperture and permeability, while an increase in shear stress can cause dilational displacement and enhance permeability (Barton et al, ; Lei et al, ; Min et al, ). In such deep crustal systems, the correlation between stress and permeability may be observed (Lang et al, ; Rutqvist & Stephansson, ). However, stress effects on the permeability of fractures in shallow crystalline rocks recently have been shown to be minor based on hydraulic tests at low pressure (Mattila & Follin, ).…”

Heterogeneous Permeability Changes along a Fault Zone Caused by the Xingwen M5.7 Earthquake in SW China

“…The heterogeneous permeability changes along the Huarongshan fault and the anisotropy at a given well location after the Xingwen M 5.7 earthquake may be related to the heterogeneous fracture development and background tectonic stress. Field observations and simulations have shown that fault zones appear to be more permeable in areas where tectonic activity is frequent, with fissured media that are most aligned with the direction of maximum horizontal stress or that have a high ratio of shear stress to normal stress, generally exhibiting permeability (Barton et al, ; Hennings et al, ; Lang et al, ).…”

Permeability of Three‐Dimensional Numerically Grown Geomechanical Discrete Fracture Networks With Evolving Geometry and Mechanical Apertures

“…In order to avoid boundary effects, averages for equations and are computed only on elements with barycenters located within the central 90% of the domain. This approach has been referred to as undersampling or flow jacketing (Lang et al, ) and produces results which better reflect the overall connectivity of the domain (Lang et al, ). Use of Darcy's law assumes that the medium can be replaced by an equivalent homogeneous block of a small size with permeability k , known as a representative elementary volume.…”

“…The methods by which mechanical and hydraulic properties are calculated on DFNs vary widely between implementations, including boundary element (Tuckwell et al, ), discrete element (Lisjak & Grasselli, ), finite difference (Karimi‐Fard et al, ), finite element (Kim & Deo, ; Lang et al, ), finite volume (Koudina et al, ), and virtual element methods (Benedetto et al, ). Combined methods incorporate both continuum and discontinuum approaches to handle solution of forces and displacements, and handling of dynamics and contact, respectively (e.g., Elmo & Stead, ; Lisjak et al, ; Marschall et al, ).…”

“…In layered rocks, fracture spacing is also controlled by layer thickness (Olson, ). (iv) Aperture: Fractures have complex walls and can be filled with different materials, resulting in extremely heterogeneous apertures and flow distributions across large fractures (Black et al, ; Snow, ; Tsang & Neretnieks, ), particularly at intersections (Leckenby et al, ). Apertures vary in response to stresses (Lang et al, ) but also due to chemical effects including dissolution and precipitation (Lang et al, ), and thermal effects (Salimzadeh et al, ). In situ stress and fracture interaction may have a more significant influence on apertures than fracture length (Olson, ; Pollard & Segall, ; Renshaw & Park, ). (v) Length: The distribution of fracture lengths in a population is usually described using power laws (Bonnet et al, ; Davy et al, ; Gillespie et al, ).…”