Characterization of the greywacke basement of Taupo Volcanic Zone geothermal fields - New Zealand -

摘要

Geothermal energy usage is spreading around the world in both electricity generation and direct use. In New Zealand geothermal resources are known to be hosted in greywacke basement rocks. The fracture networks control fluid flow in these reservoirs and the wells that access them. Hence, it is very important 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 the change of temperature and pressure conditions at depth. Faults and fractures are a key mechanism for the transport of fluids through the crust, providing a pathway for the emplacement and development of geothermal systems. This project utilizes geomechanics, geophysics and petrology to investigate the dynamics of fluid flow within the greywacke basement at Taupo Volcanic Zone (New Zealand), in relation to the exploitation of the geothermal energy. The experimental work describes the response of this lithology under different conditions of stress and temperature, which are factors influencing the permeability of the reservoir rocks. The results have implications for permeability models of this area, which are fundamental for the planning and development of geothermal fields. The tests that have been carried out are targeted to describe the physical properties of this material and their relation to fluid flow. Uniaxial tests, seismic waves velocity measurements, Brazilian tests, porosity measurements, triaxial testing, permeability measurements, and thermo-analysis have been utilized in order to acquire data on elastic properties (E= 54 - 85 GPa; ν= 0.19 - 0.36), strength (UCS= 205 - 384 MPa; tensile strength= 14 - 32 MPa), seismic wave velocity (Vp= 5.89 - 6.41 km/s; Vs= 3.20 - 3.61 km/s), porosity (ɸ= 1.035%), permeability ( k= ~10-21 m^2), thermal properties (α = 11.617·10-6 °C^-1). These data are accompanied by information on the rock composition and microstructure. Comparisons of the results are made with the purpose of understanding the similarities or differences between previously acquired data and the new data generated here. The interpretation of the laboratory results is important in order to have a better idea about the greywacke basement, in terms of physical properties. Following on from the characterization of the basement in this work, it will be possible in the future to understand better the failure mechanisms of these greywackes using numerical simulations of failure and to evaluate the future implications for the geothermal fields in New Zealand.