Microseismicity at Rotokawa geothermal field, New Zealand, 2008–2012

• Rotokawa has had persistent microseismicity since deep injection began in 2005.
• More than 1000 events of magnitude ≥0.8 and 50 of magnitude ≥2 have occurred.
• Microseismicity has been induced mostly by contraction due to cool injected fluid.
• Most events are in a fault-bounded compartment between injection and production zones.
• Microseismicity has not caused any public concern.

Rotokawa geothermal field, Taupo Volcanic Zone (TVZ), New Zealand, has experienced persistent microseismicity since deep injection of condensate and brine began in 2005. Microseismicity is thought to be induced mostly by contraction due to injected fluid, c. 200 °C colder than natural reservoir temperatures, causing a reduction in the normal stress on pre-existing faults and fractures, which then slip. This mechanism is consistent with the ability to improve the permeability of Rotokawa injection wells over several years by stimulating them with cold water injection. Any wide-spread increase in reservoir pressure due to injection has been less than 1 bar (0.1 MPa) and has been restricted to areas close to injection wells; pressure change is therefore less likely to drive induced microseismicity.Since mid-2008 more than 1000 events of magnitude ≥0.8, 50 of magnitude ≥2, and 2 of magnitude ≥3 (largest M3.1) have been located in the field, over 70% in a sharply bounded zone approximately 1 km2 in area and 1.5–3 km deep located between injection and production zones. This zone is believed to outline a compartment in the reservoir that influences the pressure–temperature distribution during production and injection. A three-fold increase in deep injection rate accompanying the start-up of a second power station in early-2010 was accompanied by an almost equivalent increase in the rate of microseismicity. The location of microseismicity appears to be controlled by several factors, including: the presence of fractures, the locus of injection, injectate flow driven by pressure gradients, and by fault-controlled permeability and barriers to lateral fluid flow. The boundary of the main microseismicity zone closest to the production reservoir is controlled by the Central Field Fault (CFF), which acts as a partial barrier to lateral flow that slows injected fluid reaching the production zone. Many of the magnitude ≥2 events occur immediately adjacent to the CFF. The location of the CFF and the depth to the base of reservoir permeability, estimated from the depth of the deepest microseismicity, have been incorporated in numerical models of the field and have influenced decisions such as siting of new wells. Although two events of magnitude ≥3 occurred at Rotokawa in 2012, microseismicity of similar magnitude is common throughout the TVZ, so activity at Rotokawa has not caused any public concern.