Mechanical and physical properties of hydrothermally altered rocks, Taupo Volcanic Zone, New Zealand

• We provide detailed descriptions of hydrothermal alteration in volcanic rocks.
• The type of alteration impacts the porosity and density of the altered rocks.
• Hydrothermal alteration and secondary minerals impact ultrasonic wave propagation.
• Lower UCS in shallow, low temperature regions of the field (27.7 ± 10.3 MPa).
• Supports ongoing research into new geotechnical models in geothermal systems.

Mechanical characterization of hydrothermally altered rocks from geothermal reservoirs will lead to an improved understanding of rock mechanics in a geothermal environment. To characterize rock properties of the selected formations, we prepared samples from intact core for non-destructive (porosity, density and ultrasonic wave velocities) and destructive laboratory testing (uniaxial compressive strength). We characterised the hydrothermal alteration assemblage using optical mineralogy and existing petrography reports and showed that lithologies had a spread of secondary mineralisation that occurred across the smectite, argillic and propylitic alteration zones. The results from the three geothermal fields show a wide variety of physical rock properties. The testing results for the non-destructive testing shows that samples that originated from the shallow and low temperature section of the geothermal field had higher porosity (15 – 56%), lower density (1222 – 2114 kg/m3) and slower ultrasonic waves (1925 – 3512 m/s (vp) and 818 – 1980 m/s (vs)), than the samples from a deeper and higher temperature section of the field (1.5 – 20%, 2072 – 2837 kg/m3, 2639 – 4593 m/s (vp) and 1476 – 2752 m/s (vs), respectively). The shallow lithologies had uniaxial compressive strengths of 2 – 75 MPa, and the deep lithologies had strengths of 16 – 211 MPa. Typically samples of the same lithologies that originate from multiple wells across a field have variable rock properties because of the different alteration zones from which each sample originates. However, in addition to the alteration zones, the primary rock properties and burial depth of the samples also have an impact on the physical and mechanical properties of the rock. Where this data spread exists, we have been able to derive trends for this specific dataset and subsequently have gained an improved understanding of how hydrothermal alteration affects physical and mechanical properties.

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