| Title | Fracture Permeability in a Pervasively Fractured Rock Mass: the Role of Regional Tectonics at Te Mihi, Wairakei Geothermal Field, New Zealand |
|---|---|
| Authors | Cecile MASSIOT, David D. MCNAMARA, Sarah D. MILICICH, Pilar VILLAMOR, Katie MCLEAN, Fabian SEPULVEDA, Will F. RIES |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | Structural permeability, Wairakei Geothermal Field, downhole measurement, borehole images, LiDAR, regional tectonics, GPSS |
| Abstract | Surface and downhole fracture mapping in the TaupŠVolcanic Zone (TVZ), New Zealand, highlights that, although fractures and faults occur pervasively, only a portion of these structures supply geothermal fluids. Detailed fault mapping and borehole image logs were acquired prior to commissioning the Te Mihi power station in the Wairakei Geothermal Field to 1) identify areas of active-fault-related seismic hazard; 2) assist with site selection for the power station (commissioned in 2014); and 3) reservoir characterisation. This work also improved our understanding of structural trends and associated permeability of the field and the wider region. The four borehole image logs show pervasive fracturing with orientations and maximum horizontal stress orientations (SHmax) consistent with the regional rifting, and fractures well-oriented for slip in a normal stress regime. Fractures are mainly steeply dipping (more than 70°) with dominant NE-SW strike, and secondary E-W and N-S strike. SHmax is NE-SW to NNE-SSW, with small-scale rotations related to potential recently active fractures and nearby faults. Based on correlation with completion test data, we observe that only a portion of the structures imaged with downhole tools supply geothermal fluids. Fractures in permeable zones are typically 1) of low amplitude and seen on the travel-time image of acoustic images; and 2) of low-resistivity fractures with high-resistivity haloes on resistivity images, although similar fracture characteristics are also observed outside permeable zones. In addition, permeable zones preferentially contain high fracture density and wide aperture fractures. Tectonic structure changes substantially along rift strike (NE-SW) in the Te Mihi area, with the deformation of the NW-dipping, 045° striking Kaiapo and Whakaipo faults becoming more spatially distributed. Detailed surface mapping at Te Mihi using LiDAR reveals that Te Mihi is located at an intersection between two normal dip-slip fault sets: one striking 045° and another 060-070°. 3D fault models defined from local well stratigraphy also show fault terminations, splays and intersections, with permeable zones non-systematically coinciding with individual strands of the Kaiapo Fault. Regional plate motions, indicated by Global Navigation Satellite System (GNSS) velocity vectors in the TaupŠRift-Forearc region, show a clear change from a SE direction within the TaupŠRift to an SSE direction within the forearc. We propose that this change of direction allows the two normal fault orientations mapped on surface to be active simultaneously within the TaupŠRift, although the SE extension is dominantly reflected in the present-day SHmax direction in the boreholes. This biaxial extension may enhance fault dilation, connectivity, and thus permeability, supporting upflow in the Te Mihi area with fluid pathways dominantly controlled by the fault strands and local fracture connectivity. We propose that regions in the TaupŠRift where NE-SW and ENE-WSW to E-W trends intersect are areas that have experienced bi-axial extension, with higher potential for permeability. Considering regional tectonics is thus useful in assessing large-scale controls on permeability. |