Interpretation of Microseismicity at the Rotokawa Geothermal Field, 2008 to 2012

Abstract

At the Rotokawa Geothermal Field, the integrated interpretation of microseismic (MEQ) data with other geoscience and reservoir engineering data has helped identify and constrain important elements of the reservoir model, including a major structure that likely influences reservoir fluid flow. MEQ monitoring began when injection moved to deeper wells in 2006, and an 8-10 seismometer array has been operating continuously since mid-2008. To meet goals for characterizing large-scale reservoir permeability structure and tracking the path of injection, two lines of investigation were pursued; 1) obtaining accurate MEQ locations, and 2) inferring reservoir properties by comparing the MEQ locations and timing with relevant reservoir data. Manual phase picking and double-difference relative re-location was used to significantly improve location accuracy from initial automated locations. These more accurate locations were used to support the use of 4D visualization tools to interpret MEQ patterns in space and time with respect to the integrated geoscience and reservoir engineering data sets. Correlation of the MEQs with changes in well flows, pressure and temperature data has been particularly important to understanding likely causes of the MEQs and their implications for reservoir properties. Most MEQs at Rotokawa appear to be triggered by injection cooling and field-wide pressure transients, although other mechanisms are possible. Many MEQs occur in swarms of >10 events/day along a linear, northeast trending structure – the Central Field Fault (CFF). The majority of the remaining MEQs are clustered on the southeast side of this structure that separates injection wells to the southeast and production wells to the northwest. The location and timing of the MEQs relative to major changes in production and injection are consistent with other geoscience data sets suggesting that the northeast trending structure acts as a barrier to fluid flow across strike and a zone of enhanced permeability along strike. This structure and the effective base of the reservoir permeability are the main features interpreted from the MEQ data that have been incorporated into the Rotokawa conceptual and numerical models, which are used to guide field management.