Rock properties of Greywacke Basement hosting geothermal reservoirs, New Zealand: preliminary results
McNamara, David D.
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McNamara, David D., Faulkner, D., & McCarney, E. (2011). Rock Properties of Greywacke Basement Hosting Geothermal Reservoirs, New Zealand: Preliminary Results. Paper presented at the Thirty-ninth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California.
Geothermal resources in New Zealand are known to be hosted in greywacke basement rocks. Fluid flow in these reservoirs and the wells that access them is controlled by fracture networks. As such it is of vital importance to understand how these structures are impacted by the mechanical and thermal properties of this basement rock, and how these in turn are affected by changing temperature and pressure conditions at depth. This paper details the results of an initial set of laboratory tests of two New Zealand greywacke basement terranes, in which geothermal reservoirs are known to be hosted. The aim of the study was to provide an initial understanding of the rocks mechanical properties so that further, more refined testing could be applied. Low permeability and porosity measurements are consistent with the current understanding that fractures control fluid flow in the basement. Preliminary mechanical testing suggests a systematic difference between greywacke from the Waipapa and Torlesse Terranes, with the Waipapa Terrane being mechanically stronger, potentially as a result of a coarser grain size and/or composition differences. Tensile strength testing of whole rock and fractures in Waipapa greywacke rock show lower tensile strengths for the fractures. This indicates that geothermal fluid flow makes use of existing fracture networks via crack-seal mechanisms rather than through the generation of new fractures. Further work, including mechanical testing at; high temperature conditions, variable grain size, and variable composition will provide us with a better insight into the role increasing pressure and temperature conditions play in fracture controlled geothermal fluid flow as we approach the change from brittle to ductile deformation. This in combination with a rigorous investigation of the effects of the character and heterogeneity of greywacke basement in New Zealand will inform us on how best New Zealand can explore for and utilize deep (>3km), fracture dominated, geothermal resources.
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