Does the symmetry of absolute permeability influence relative permeability tensors in naturally fractured rocks?

Relative permeability is a tensorial property directly dependent on absolute permeability tensor which has been taking both symmetric and non-symmetric (full tensor) forms in the literature. Imposing symmetricity during upscaling absolute and effective phase permeabilities could alter both diagonal and off-diagonal terms of relative permeability tensors, specifically in naturally fractured reservoirs (NFRs) where the diagonal components of the permeability tensor could take values with different orders of magnitude. Utilizing a discrete fracture and matrix (DFM) modelling approach, in this paper, we quantify the effects of forcing the symmetricity on absolute permeability tensors on relative permeability tensors of fractured samples in different scales. We also determine the circumstances at which this change in methodology causes a huge difference in the diagonal components of relative permeability tensors

Inherited structural controls on fault geometry, architecture and hydrothermal activity: an example from Grimsel Pass, Switzerland

Exhumed faults hosting hydrothermal systems provide direct insight into relationships between faulting and fluid flow, which in turn are valuable for making hydrogeological predictions in blind settings. The Grimsel Breccia Fault (Aar massif, Central Swiss Alps) is a late Neogene, exhumed dextral strike-slip fault with a maximum displacement of 25–45 m, and is associated with both fossil and active hydrothermal circulation. We mapped the fault system and modelled it in three dimensions, using the distinctive hydrothermal mineralisation as well as active thermal fluid discharge (the highest elevation documented in the Alps) to reveal the structural controls on fluid pathway extent and morphology. With progressive uplift and cooling, brittle deformation inherited the mylonitic shear zone network at Grimsel Pass; preconditioning fault geometry into segmented brittle reactivations of ductile shear zones and brittle inter-shear zone linkages. We describe ‘pipe’-like, vertically oriented fluid pathways: (1) within brittle fault linkage zones and (2) through alongstrike- restricted segments of formerly ductile shear zones reactivated by brittle deformation. In both cases, low-permeability mylonitic shear zones that escaped brittle reactivation provide important hydraulic seals. These observations show that fluid flow along brittle fault planes is not planar, but rather highly channelised into sub-vertical flow domains, with important implications for the exploration and exploitation of geothermal energy