The Effect of Hydrothermal Alteration on the Mechanical and Physical Rock Properties

Abstract

This paper describes the relationship between hydrothermal alteration and physical/mechanical rock properties of selected formations from three geothermal fields in the Taupo Volcanic Zone, New Zealand. Our characterisation of hydrothermally altered rocks from geothermal reservoirs will lead to an improved understanding of rock mechanics in the geothermal environment, and subsequently support drilling optimization, wellbore stability studies and geomechanical modelling. To characterise the physical and mechanical rock properties of selected formations, samples were prepared from intact core for non-destructive and destructive laboratory testing. The hydrothermal alteration assemblages were characterised using optical mineralogy and existing petrography reports. All samples exhibited secondary mineralisation, but the type and occurrence varied depending on where in the system the sample was collected. The samples from the geothermal fields show a wide variety of physical rock properties. Samples of the same formation that originate from different wells across a field have a spread of uniaxial compressive strength and porosity because of differences in the alteration zones, primary rock properties and depth. Where this data spread exists, we have been able to derive trends for this specific data set and subsequently have an improved understanding of how hydrothermal alteration affects physical/mechanical properties.