Predicting The Effect Of Hydrothermal Alteration On Rock Properties

Abstract

Hydrothermal alteration records physicochemical changes to a rock, including dissolution and/or precipitation of new mineral phases, by its interaction with hot mineralised fluid. Transient geochemical processes can alter thermo-physical and thermo-physical rock properties. The prognosis and quantification of hydrothermal alteration processes in reser-voir rock properties is crucial to enhance the ability of THM reservoir models. To predict the effect of hydrothermal alteration on rock properties a database detailing hydrother-mal alteration, matrix permeability, porosity, rock strength and thermo-physical parameters (e.g. thermal conductivity, heat capacity) has been established for various volcanic rock types. The database includes data from a) ~450 drill cores from hydrothermally altered volcaniclastic, sediments and lavas from wells THM12, THM13, THM14, THM17, THM18, THM19 and TH18 in the Tauhara Geothermal System, and b) from outcrop analogue studies of fresh and altered volcaniclastics and lavas throughout the Taupo Vol-canic Zone and Coromandel Volcanic Zone. Thermo-physical rock properties have been determined by applying a combination of new and existing methods. New methods include a mobile permeameter and thermal conductivity scanner, a hydrothermal triaxial device and μ-computer-tomography. Standard tools include X-ray fluorescence, shortwave infrared analysis, optical microscopy, clay miner-al characterisation and gas porosity.