Analysis of Reservoir Fracture Sealing in an Active Geothermal System, Kawerau Geothermal Field, New Zealand

Abstract

The microscopic crystallographic structures of geothermal minerals precipitated in fractures located within active geothermal reservoirs have the potential to record significant information concerning the kinematics of fracture formation and sealing over time. It is vitally important to understand these processes when developing reservoir models, especially when defining the influences of secondary permeability for high temperature geothermal systems.

This study highlights a multi-staged analysis of the mineral calcite that has been identified to control permeability in the reservoir of the Kawarau Geothermal Field, TVZ, New Zealand. Outlined is the methods of carrying out EBSD, EDX, Cathodoluminesce, ICPMS and a variety of other techniques to understand the process of fracture sealing using field samples. The initial study of the fracture identified two separate sealing mechanisms occurring between the different phases of calcite deposition. In addition the two phases of calcite deposition had different internal crystallographic structure and elemental composition. A 3-D analysis was then carried out by serial sectioning, this includes the process of 3-D EBSD by serial sectioning and builds on the initial findings. Finally the ICPMS analysis was unable to accurately date the bladed calcite, due to inadequate U-Th ratios. However, it did show that it is possible to carry high-precision geochemical analysis on geothermally precipitated bladed calcite. The Eu anomaly identified in the calcite highlights the complex history of geothermal fluids in the reservoir of Kawerau. These multiple aspects assist in providing a deeper understanding of how the geothermal system has changed over time.

By reconstructing calcite filled fractures using a range of alternative techniques in 2-D and 3-D, an advanced insight into the complex nature of vein crystal growth evolution and related chemical patterns can be extracted. These can then be used to constrain how veins control fracture permeability over time on a microscopic scale.