| Title | MODELLING FLOWS IN DISCRETE FRACTURE NETWORKS DERIVED FROM A NEW ZEALAND LAVA-HOSTED GEOTHERMAL SYSTEM |
|---|---|
| Authors | W. Kissling, C. Massiot |
| Year | 2018 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | fractures, networks, permeability, fluid flow, borehole imaging, tracers |
| Abstract | Many of New Zealand’s geothermal resources are hosted within volcanic rocks. In these systems, matrix permeability can be very low and the transport of geothermal fluids is dominated by fractures. In this paper we model fluid flow in generic fracture systems based on borehole image data from Rotokawa geothermal system. Fluid flow is modelled using an appropriate flow law to calculate the pressure at each fracture intersection in the network. For the computation the fracture network is reduced to its basic ‘backbone’, where all singly- or unconnected fractures which do not cross boundaries are removed. By applying fixed pressures around the boundaries of this network, and insisting that the flows sum to zero at each fracture intersection, there are sufficient equations to define the pressure at all fracture intersection points. To upscale the models for geothermal engineering purposes, fracture flows are modelled across a backbone network which spans a 350 m x 350 m x 100 m block. A single tunable parameter, the ratio of hydraulic fracture aperture to geometric aperture, is used to match reservoir-scale permeabilities derived from traditional reservoir engineering methods. Multiple realisations of statistically identical fracture networks yield probability distributions for alongstrike and across-strike permeabilities, and thus for permeability anisotropy at reservoir scales. We further identify multiple fluid pathways which connect two widely separated ‘wells’, and their associated fluid residence times. This flow modelling workflow, and identification of multiple pathways with different residence times, provides new opportunities for interpreting tracer tests in the future. |