Heterogeneity of structure and stress in the Rotokawa Geothermal Field, New Zealand

Abstract

Geometric characterization of a geothermal reservoir's structures, and their relation to stress field orientation, is vital for resource development. Subsurface 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 borehole televiewer logs. 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 clusters within certain depth intervals have antithetic dip directions to the well's dominant fracture dip direction. These patterns are consistent with expected antithetic faulting in extensional environments. A general S Hmax orientation of NE-SW is determined from induced features on borehole walls. However, numerous localized azimuthal variations from this trend are evident, constituting stress field orientation heterogeneity. These variations are attributed to slip on fracture planes evidenced by changes in the azimuth of drilling-induced tensile fractures either side of a natural fracture. Correlation of observed fracture properties and patterns to well permeability indicators reveal that fractures play a role in fluid flow in the Rotokawa geothermal reservoir. Permeable zones commonly contain wide aperture fractures and high fracture densities which have a dominant NE-SW strike orientation and NW dip direction. Studies of this kind, which show strong interdependency of structure and stress field properties, are essential to understand fluid flow in geothermal reservoirs where structural permeability dominates.

Estimating in situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)

“…More massive rocks (i.e. those with fewer fractures) tend to have more regular walls and circular diameters, resulting in better quality images (McNamara et al 2015). We highlight that potential discrepancies between fracture densities derived from BHTV logging data and those measured directly from core must be considered when deriving GSI.…”

“…This is partly due to the fact that the televiewer resolution (1-2 mm) is sometimes below the fracture aperture. Fracture statistics from BHTV logging is also dependent on borehole circularity and the quality of the borehole walls, which can depend on rock type, rock mass strength, and alteration amongst others (McNamara et al 2015). More massive rocks (i.e.…”

A review of the Rotokawa Geothermal Field, New Zealand

“…The structure of the Rotokawa Geothermal Field is documented in a number of publications (Anderson, 2011;Bannister et al, 2008;Bowyer and Holt, 2010;Collar and Browne, 1985;Davidson et al, 2012;Downs et al, 2014;Grindley, 1961;Grindley et al, 1985;Krupp and Seward, 1987;Massiot et al, 2012Massiot et al, , 2015McNamara et al, 2014;Rae, 2007;Wallis et al, 2013). The surface geology at Rotokawa contains ample evidence of an active structural environment.…”

“…This has utilised research into the geological nature and properties of the field (Anderson, 2011;Davidson et al, 2012;McNamara et al, 2014;Rae et al, 2011;Siratovich et al, 2014;Wallis et al, 2013), and detailed monitoring of reservoir pressure, temperature and chemistry along with other monitoring (e.g. tracer testing, microgravity, microseismic, levelling, thermal feature and groundwater) (Hunt and Bowyer, 2007;Sewell et al, 2013bSewell et al, , 2015Sherburn et al, 2015;Winick et al, 2015).…”

Play-fairway analysis for geothermal resources and exploration risk in the Modoc Plateau region

“…Throughout the Basin and Range and around the world, it is well recognized that geothermal systems occur at discontinuities along faults (Curewitz and Karson, 1997, Faulds et al, 2006, Hinz et al, 2008, Faulds et al, 2011, Hinz et al, 2011, McNamara et al, 2015. Horse-tailing fault terminations, fault intersections, fault step-overs, and accommodation zones are the most common structural settings in the Great Basin and host the majority of known geothermal systems , Faulds et al, 2006, Hinz et al, 2011, Hinz et al, 2010, Hinz et al, 2008.…”