MODELLING FLOWS IN DISCRETE FRACTURE NETWORKS DERIVED FROM A NEW ZEALAND LAVA-HOSTED GEOTHERMAL SYSTEM

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.