The Effects of Hydrothermal Alteration on Mechanical Rock Properties of the Andesite Breccia and Tahorakuri Formation from the Ngatamariki Geothermal Field, New Zealand and Empirical Relations Between Rock Strength and Physical Properties

Abstract

This paper describes the adopted testing methodology and preliminary results of a study assessing the relationship between hydrothermal alteration and the physical and mechanical rock properties of two key units (Andesite Breccia and Tahorakuri Formation) from the Ngatamariki geothermal field. These results are valuable for geotechnical mechanical characterisation of hydrothermally altered rocks from geothermal reservoirs, and are relevant to drilling optimization, wellbore stability studies and geomechanical modeling. The Ngatamariki geothermal field is located approximately 25 km northeast of Lake Taupo. Ngatamariki at present has had seven wells drilled, four in the 1980’s and three since 2005. Drilling of development wells is currently underway. Core samples collected from these units were tested in-situ and in the laboratory, and the physical rock properties were compared to the alteration assemblage to assess the relationship. To characterise physical and mechanical rock properties of the Andesite Breccia and Tahorakuri Formation, plugs from intact core were prepared for non-destructive and destructive laboratory experiments. In addition to strength testing parameters, the physical rock properties analysed included porosity, density, P and S wave velocities. The hydrothermal alteration assemblage of the Andesite Breccia and Tahorakuri Formation was characterised using petrography and existing geological reports. We compared existing empirical relationships established primarily by the petrochemical industry to convert physical properties measured by wireline geophysics to mechanical rock strength. The results show neither of the empirical equations applied here matched the measured UCS. Once the relationship between hydrothermal alteration and rock strength has been established for a number of formations, we will assess drilling performance data so as to investigate the relationship between drilling performance and rock properties in a hydrothermally altered environment. The improved understanding of rock mechanics in a geothermal environment will support the construction of geomechanical models in improve understanding of structural permeability in geothermal reservoirs.