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

• New multi-scale models link faults, permeability and resistivity from the core to reservoir scale. • Permeability is highest along dominant strike, followed by vertical and across-strike permeability, both over an order of magnitude lower. • Along-strike and vertical resistivity are similar and about half to a third of the across-strike resistivity. Understanding permeability in the Earth is vital to optimizing sustainable geothermal development. 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. In our base-case model, permeability is highest (6 × 10 –13 m 2 ) in the horizontal along-dominant-strike direction, lower vertically (5 × 10 –14 m 2 ), and lowest in the across-dominant strike direction (2 × 10 –14 m 2 ), with significant uncertainty in across-strike permeability (4 × 10 –15 m 2 to 5 × 10 –14 m 2 ). For typical TVZ fluids (0.5 Ωm) and 1000 Ωm matrix, resistivity is lowest along dominant strike (235 Ωm), slightly higher vertically (285 Ωm) and highest across-strike (575 ± 30 Ωm). Resistivity anisotropy ratios along-strike/across-strike are relatively consistent regardless of fault length distribution (∼0.38–0.5), but vary strongly with the proportion of along-strike faults. Anisotropy ratios for permeability are more variable (1–300). The calculated petrophysical properties help link reservoir models with geophysical data, which may guide fractured reservoir models and resource use.