Hydraulic Parameters from Microseismicity in the Karapiti Geothermal Reinjection Field, New Zealand

Abstract:

The Wairakei geothermal field is located in the south-western end of the Taupo Volcanic Zone (TVZ), a NNE-trending rifting arc, New Zealand. Commercial geothermal power generation commenced at the Wairakei filed in 1958, with large scale infield reinjection started in eastern Wairakei (Oputu) in 1998 and expanded to southern Wairakei (Karapiti) in 2011.

Since 2009, Contact Energy Ltd. has operated 13 borehole seismometers installed in the Wairakei area and observed micro-earthquakes, generally magnitude < 2. Seismic activity in the Karapiti reinjection field has noticeably increased after the start of reinjection in 2011. The hypocenter locations are mainly in the range of between 500m and 1,500m depth coinciding with the permeable Wairakei ignimbrite geological unit.

In this study, we investigate hydraulic parameters of the geothermal reservoir and faults from micro-seismicity occurred in the Karapiti area between August 2011 and July 2014 using the r-t plot proposed by Shapiro (2002) and Shapiro and Dinske (2009). To estimate hydraulic diffusivity of the reservoir, the Karapiti area was divided into 3 geological layers (Waiora formation, Wairakei ignimbrite, and Tahorakuri formation). A total of six fault structures showing pore pressure propagation patterns were identified from an event cluster analysis using CURATE. In Karapiti area, seven injectors have been operated with three different reinjection time stages. To consider this situation, we made r-t composite plots for the hydraulic diffusivities of the reservoir and faults.

The estimated hydraulic diffusivities of the three geological layers show a narrow range of 0.02 and 0.08 m²/s. The calculated hydraulic diffusivities of the faults range between 0.8 and 4.5 m²/s which show higher values than those for the reservoir. A noticeable figure is that the faults embedded in the Wairakei ignimbrite have higher value than others. We were not able to find any remarkable relationship between fault direction and hydraulic diffusivity.

In order to estimate hydraulic diffusivity more accurately, we are re-calculating the hypocenter locations of Karapiti events using the non-linear location program, NonLinLoc (Lomax et al. 2000). In addition, through measured or estimated rock and fluid properties, we will convert hydraulic diffusivity to permeability.