• Villeneuve, Marlene
• Siratovich, Paul
• Wyering, Latasha
• Cant, Joe
• Kennedy, Ben
• Heap, Michael J.
• Gravley, Darren
• Mordensky, Stanley

Abstract

<p>Conventional geothermal resources are associated with high-temperature fluid convection that induces alteration of the host rocks. In New Zealand the active geothermal reservoirs are hosted in lavas and volcaniclastic rocks that have undergone various grades and intensities of alteration. Through a number of studies in two geothermal fields and a fossil geothermal system we have developed an understanding of the relationship between alteration and porosity and the resultant permeability and mechanical behavior of the reservoir rocks. Porosity is a key factor affecting permeability and strength, where increasing porosity increases permeability and decreases strength. Porosity is generally gained through dissolution associated with smectite/argillic alteration and lost through precipitation associated with both smectite/argillic and propylitic alteration. We show that deeply-buried rocks with propylitic alteration tend to have low porosity, low permeability and deform through microcracking and dilation, resulting in flow zones localized in fractured intervals. We argue that reservoir models must include not only petrophysical properties such as porosity and matrix permeability, butmechanical properties, such as strength, and deformation mode to allow assessment of the changes to the reservoir rocks during burial, tectonic stress, fluid extraction and injection. The results presented in this research are also relevant for mining, slope stability and tunnel design in altered volcanic rocks.</p>

Topics

• impedance spectroscopy
• extraction
• strength
• precipitation
• permeability
• porosity