| Abstract |
New Zealand’s geothermal systems are mostly located in high-temperature, volcano-tectonic settings, but background levels of local natural microseismicity (micro-earthquakes) are quite variable. Triggered or induced seismicity effects from geothermal operations have also been very mixed. Understanding the reasons for these differences leads to better comprehension of the potential seismic risks and rewards (or opportunities) of different development options. Large-scale NZ geothermal projects started in the late 1950’s, but reinjection, the most commonly attributed cause of induced seismicity, only commenced in the mid 1980’s (at Wairakei). Here, initial injection trials were relatively shallow (~1.3 km) and triggered some local low-magnitude microseismicity, at high effective stimulation pressure (~5 MPa). Other, low-pressure, reinjection projects soon followed, with subsequent changes in injection strategy (depth, location, in-situ temperature, pressure and flow-rate). They included: Ohaaki (1988, deep to shallow), Kawerau (1992, shallow to deep), Rotokawa (1997, shallow to deep), Ngawha (1998, deep), Mokai (2000, shallow to deep), and Ngatamariki (2013, deep). A wide range of induced seismicity responses has accompanied these injection strategies and this paper provides an overview of these experiences. The overall objective of much of this research is to illuminate probable mechanisms, identify zones of potential fracture permeability enhancement from microseismic locations, thereby provide information on probable reservoir boundary conditions for simulation models, and help provide possible mitigation options, if (and when) induced seismicity magnitudes and felt event rates exceed acceptable values. |