Structure, stress, and fluid flow characterization of the Rotokawa Geothermal Field, NZ

Abstract:

Characterisation of a geothermal reservoir’s structures, and their relation to stress field orientation, is vital for resource development. Sub-surface structure and stress field orientations of the Rotokawa Geothermal Field, New Zealand have been studied, for the first time, using observations obtained from analysis of three acoustic image logs, acquired using a high temperature BHTV. While an overall NE-SW fracture strike exists, heterogeneity in fracture dip orientation is evident. Dominant dip direction changes from well to well, due to proximity to variously oriented, graben-bounding faults. Fracture orientation heterogeneity also occurs within individual wells, where fractures patterns are consistent with expected antithetic faulting in extensional environments. A general SHmax orientation of NE-SW is determined from induced features on borehole walls. However, localised perturbations from this trend are evident, constituting stress field orientation heterogeneity. This heterogeneity is attributed to slip on fracture planes. Correlation of observed fracture properties and patterns to well permeability indicators reveal some fractures play a role in fluid flow in the Rotokawa geothermal reservoir. Permeable zones commonly contain wide aperture fractures, high fracture densities, with dominant NE-SW strike orientations and NW dip directions. Additional consideration of stress magnitudes on fracture permeability reveals some, but not all, fractures prone to slip occur in permeable zones.