| Title | CHARACTERISTICS OF FLOWS IN PERVASIVELY FRACTURED ANDESITES, ROTOKAWA GEOTHERMAL FIELD, NEW ZEALAND |
|---|---|
| Authors | W. Kissling and C. Massiot |
| Year | 2019 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Fracture flow, permeability anisotropy, fluid dispersion, Rotokawa |
| Abstract | In New Zealand’s high temperature geothermal systems, fluids flow dominantly through fractured rocks with low matrix permeability. It is important to understand the nature of these fracture systems, and how fluids flow through them, so that the geothermal systems may be more efficiently and sustainably used to generate electricity. Here we present fluid flow calculations in several distinct discrete fracture models, each of which is broadly consistent with the fracture density and high dip magnitude angle distributions interpreted from borehole image logs at the Rotokawa Geothermal Field, a New Zealand andesite-hosted reservoir. Our initial flow calculations are carried out using a cubic flow law, which is derived directly from Darcy’s linear relationship between fluid velocity and pressure gradient for flows between smooth parallel plates. For comparison, we then follow with calculations using the Forchheimer flow law, which includes an additional term to account for the friction due to fracture wall roughness. Most models show pervasive connectivity at reservoir scales, with fluid flow and tracer transport predominantly along the mean fracture orientation. As expected, the models show that permeability anisotropy increases as the standard deviation of the dip distribution decreases, with the fracture system eventually forming into distinct unconnected groups so that across-dip permeability (and hence fluid flow) drops to zero. The models also confirm that significant dispersion of fluid will occur as it is transported through a fractured reservoir. The amount of dispersion shows a broad inverse relation to the permeability anisotropy and reaches a maximum for a uniformly distributed population (unrealistic for Rotokawa) where fracture dip magnitudes vary from horizontal to vertical. Our calculations using the Forchheimer flow law with a subset of the fracture population models show that, as expected, the flow rates drop rapidly as the friction reaches a threshold value and, more surprisingly, the permeability anisotropy decreases at the same time. |