Physical property relationships of the Rotokawa Andesite, a significant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand

Abstract

Background: Geothermal systems are commonly hosted in highly altered and fractured rock. As a result, the relationships between physical properties such as strength and permeability can be complex. Understanding such properties can assist in the optimal utilization of geothermal reservoirs. To resolve this issue, detailed laboratory studies on core samples from active geothermal reservoirs are required. This study details the results of the physical property investigations on Rotokawa Andesite which hosts a significant geothermal reservoir. Methods:We have characterized the microstructure (microfracture density), porosity, density, permeability, elastic wave velocities, and strength of core from the high-enthalpy Rotokawa Andesite geothermal reservoir under controlled laboratory conditions. We have built empirical relationships from our observations and also used a classical micromechanical model for brittle failure. Further, we compare our results to a Kozeny-Carman permeability model to better constrain the fluid flow behavior of the rocks. Results: We show that the strength, porosity, elastic moduli, and permeability are greatly influenced by pre-existing fracture occurrence within the andesite. Increasing porosity (or microfracture density) correlates well to a decreasing uniaxial compressive strength, increasing permeability, and a decreasing compressional wave velocity. Conclusions:Our results indicate that properties readily measurable by borehole geophysical logging (such as porosity and acoustic velocities) can be used to constrain more complex and pertinent properties such as strength and permeability. The relationships that we have provided can then be applied to further understand processes in the Rotokawa reservoir and other reservoirs worldwide.Keywords: Geothermal; Uniaxial compressive strength; Permeability; Physical properties; Elastic modulus; Microstructure BackgroundFractures on multiple scales are the dominant control on fluid flow in most geothermal systems worldwide. Geothermal environments are prone to variable heat fluxes, dynamic fluid flow regimes, and active tectonics which impact the physical and mechanical properties of the reservoir rocks in which they are hosted. The influence of such a dynamic environment can render the host rocks highly altered, fractured, and microstructurally complex. As a result, the empirical correlation of physical properties to

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

“…Geothermal resources in the Taupo Volcanic Zone (TVZ), New Zealand, are often hosted in lithologies with low primary permeability such as volcanics (andesites and rhyolites, 9.95 × 10 −9 − 1.68 × 10 −7 md), welded and indurated ignimbrites and tuffs, pervasively silicified volcaniclastic deposits, and greywacke basement rocks (4.89 × 10 −7 md) [ Rowland and Sibson , ; Rosenberg et al ., ; Bignall et al ., ; Milicich et al ., ; Siratovich et al ., ; McNamara et al ., ]. Structure and stress characterization in these reservoirs is increasingly employed to assess and optimize resource utilization; however, this characterization is hindered by a lack of surface exposure, few drill cores, difficulty with geophysical imaging (e.g., seismic reflection) of the subsurface, and operational challenges and technological constraints on traditional logging technology.…”

Evidence for tectonic, lithologic, and thermal controls on fracture system geometries in an andesitic high‐temperature geothermal field

“…The Rotokawa Andesite is an 800–2100 m thick unit of andesite lavas and breccias and is inferred to form a large buried andesitic volcano which is also partly located under the nearby Ngatamariki Geothermal field [ Browne et al , ; Chambefort et al , ]. The deep, hot (320°C) aquifer is hosted in these low matrix porosity (4–15%) andesites [ Siratovich et al , ; Hernandez et al , ], in which fluid flow is mostly controlled by fractures and faults [ McNamara et al , ]. Three large (0–400 m throw) NE‐SW striking normal faults have been inferred within the geothermal field based on stratigraphy defined from 35 boreholes, reservoir temperatures, and microseismicity (Figure a) [ Wallis et al , ; Sherburn et al , ].…”

“…Spot core fracture analyses (presented here for the first time) give details on the rock types and extend the scale of thickness and density observations made on BHTV logs. Previously reported thin‐section data further inform fracture thicknesses and densities at subcore scales [ Siratovich et al , ].…”

“…Presented thin section data contains the range (minimum‐maximum) and mean values measured in the 10 thin sections analyzed in Siratovich et al []. P 33 for thin sections was calculated from the P 32 and a mean fracture thickness of 5 μm.…”

The development and application of the alteration strength index equation

“…Micro-and macrofractures, whether pre-existing or induced during testing, coalesce during uniaxial compression ultimately leading to failure of the sample (Bieniawski, 1967;Bieniawski et al, 1969). Samples that contain pre-existing fractures require less energy to propagate the fractures, resulting in lower peak strength values in these samples (Walsh, 1961;Martin, 1997;Siratovich et al, 2014).…”