Abstract
SUMMARYWe investigate the relation between geothermal field production and fracture density and orientation in the Ngatamariki and Rotokawa geothermal fields, located in the Taupo Volcanic Zone, New Zealand using shear wave splitting (SWS). We determine the SWS parameters for 17 702 microseismic events across 38 stations spanning close to 4 yr from 2012 to 2015. We compare the strength of anisotropy to changes in field production and injection. We also compare the orientation of the anisotropy to in situ and regional measurements of maximum horizontal stress orientation. ($S_{\mathrm{ H}_{\mathrm{ max}}}$). Due to the volume of unique events (approximately 160 000), shear wave phases are picked automatically. We carry out automatic SWS measurements using the Multiple Filter Automatic Splitting Technique (MFAST). The SWS measurements are interpreted in the context of stress aligned microcracks. Outside both fields and within Ngatamariki, fast polarizations align with the NE–SW regional orientation of $S_{\mathrm{ H}_{\max}}$. Within Rotokawa a greater complexity is observed, with polarizations tending toward N–S. We observe increases in per cent anisotropy coinciding with the start of production/injection in Ngatamariki and then a later correlated drop in per cent anisotropy and vP/vS ratios in southern Ngatamariki as injection is shifted to the north. This relationship is consistent with pore fluid pressure within the reservoir being affected by local changes in production and injection of geothermal fluids causing cracks to open and close in response.
Highlights
Seismic anisotropy is a material property in which a seismic wave’s velocity is dependent on its direction of propagation and the anisotropic symmetry of that system
Seismic anisotropy at Coso geothermal field, California was examined with shear wave splitting (Lou & Rial 1997) to determine the fracture density and fracture orientations
Fast polarizations at stations surrounding Rotokawa and Ngatamariki are plotted in Figs 4 and 5 respectively
Summary
Seismic anisotropy is a material property in which a seismic wave’s velocity is dependent on its direction of propagation and the anisotropic symmetry of that system. Evans et al (1995) measured shear wave splitting in the Geysers geothermal field in California and determined that fast polarizations were sometimes parallel to stress and other times parallel to rock fabric. They suggested that approximating permeability anisotropy with seismic anisotropy might help optimize fluid injection and steam recovery. Injection has been linked directly with anisotropy at Coso with delay times being observed to increase with the onset of injection (Rial et al 2005) This relationship is observed at Krafla geothermal field in Iceland (Tang et al 2005)
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