Direction-dependent permeability and resistivity of fractured rocks tuned to New Zealand geothermal reservoirs

Alison Kirkby , Cécile Massiot

Understanding permeability in the Earth is vital to optimizing sustainable resource development such as geothermal. In many geothermal fields, permeability is controlled by faults, resulting in high spatial and directional variability, difficult to characterise without costly drilling. Electrical resistivity, however, can be measured from the surface, and like permeability, is sensitive to fluids if they are sufficiently conductive. Modelling of resistivity-based geophysical data has largely focussed on isotropic properties to map clay caps and deep heat sources. However, faults commonly have a preferred orientation, and fluid-filled faults often have lower resistivity than the matrix, so anisotropy of permeability and resistivity is expected. This study models faults from the borehole to field scale to characterise direction-dependent permeability and resistivity. Parameters are derived from active faults and geothermal fields in the Taupō Volcanic Zone (TVZ), New Zealand. Permeability is highest (1×10-12 m2) in the horizontal along-dominant-strike direction, lower vertically (4×10-14 m2), and lowest in the across-dominant strike direction (2×10-14 m2), with significant uncertainty in across-strike permeability (4×10-15 m2 to 9×10-14 m2). For typical TVZ fluids, resistivity is lowest along dominant strike (190 Ωm), slightly higher vertically (230 Ωm) and highest across-strike (465 ± 30 Ωm). Resistivity anisotropy ratios along-strike/across-strike are relatively consistent regardless of fault size distribution (~0.33-0.48). Anisotropy ratios for permeability are more variable (1-300). The calculated petrophysical properties provide numerical inputs to design geophysical surveys to measure direction-dependent resistivity, which may assist with improving fractured reservoir models and resource use.