GEOMETRICAL MODELLING OF FRACTURE NETWORKS IN AN ANDESITE-HOSTED GEOTHERMAL RESERVOIR

Abstract

Permeability in geothermal reservoirs hosted in andesitic volcanic rocks is dominantly controlled by faults and fractures, but the connectivity and permeability of these systems are generally poorly constrained. We test the assumption of a truncated power-law distribution for fracture length on fracture networks whose linear density (~0.55 m-1) and orientation (subvertical) matches borehole observations in the Rotokawa Geothermal Field, New Zealand.
Low power law exponents (?<2.5) yield uniform, fully connected networks on reservoir scales and anisotropic permeability. Higher power law exponents result in low connectivity, and hence low reservoir-scale fracture permeability because of the predominance of short fractures. However, isolated clumps of connected fractures exist with ~50 m scales. For low power law exponents, numerous connected paths of long fractures are found parallel to the main fracture orientation, and produce dominant vertical permeability. By contrast, horizontal connected paths can be found, but are rare and of high tortuosity, permitting very limited horizontal flow. This work provides a baseline for estimating the power law exponent depending on the observed characteristics of a reservoir’s permeability.