Modelling of flow through naturally fractured geothermal reservoirs, Taupō Volcanic Zone, New Zealand

Rocks in New Zealand geothermal systems are highly fractured. Hot water flows through networks of these fractures and is collected in boreholes to generate electricity. It is important for the geothermal industry to understand how hot water flows through fractured rock so that expensive boreholes can be optimally located and energy generation maximized. Rocks in New Zealand geothermal systems are highly fractured. Hot water flows through networks of these fractures and is collected in boreholes to generate electricity. In this paper we use data collected from boreholes in two New Zealand geothermal fields to study how easily hot water flows through the main types of volcanic rocks commonly found in these fields. The observations made in boreholes tell us how many fractures cross the borehole, how big they are, and in what direction they lie. We have developed computer programs that extrapolate these measurements in a realistic way from close to the borehole to the whole geothermal reservoir, and then to calculate the flows through the resulting fracture networks. Because these networks can be so irregular, we calculate thousands of different examples to understand the likely variability of the flows through them. Averages of many fracture models, at least 50, give similar results to traditional models used by the geothermal industry, which don’t consider fractures. This tells us that the traditional models are missing a lot of important details. Results from our fracture models can therefore be used in traditional models to improve the way they consider flows in fractured geothermal reservoir.