| Abstract |
Fractures are often a key control on geothermal reservoir permeability. Knowing their orientation can help design wells with the best chance of intersecting them. Borehole image log analysis can provide this information but relies on boreholes being drilled first and provides information only in a small region around the borehole. Electrical resistivity and acoustic velocity are both direction-dependent properties that may be sensitive to the orientation of subsurface features at wider scales and without the need for drilling. Shear wave splitting has been applied in some New Zealand geothermal fields and may provide an estimate of fracture orientation but is not directly sensitive to permeability. Electrical resistivity of a rock can vary with orientation, and the electrical resistivity of a reservoir containing aligned open fractures with a preferred orientation may be resolved by geophysical methods as effective anisotropy. However, most subsurface resistivity models assume isotropic resistivity. Geothermal fields in the Taupo Volcanic Zone (TVZ), New Zealand, have been developed for electricity for >50 years and as such, the TVZ contains a wealth of scientific publications from which to define fracture parameters and fluid properties. This paper presents numerical modelling of the permeability and resistivity of a fractured rock volume containing a fluid with typical salinity and temperature for TVZ fields, to determine effective (direction-dependent) resistivity and permeability, and ground truth the models by comparing with permeability values calibrated from reservoir models. This is the first step to designing a geophysical survey that could image fracture orientation. |