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

Background

Numerous fractures are observed in fractured geothermal reservoirs on borehole images in the Taupō Volcanic Zone (TVZ), Aotearoa New Zealand. These fractures are necessary to explain the sustained reservoir permeabilities despite the low matrix porosity. However, conventional continuum models do not adequately represent fluid flow through these fractured rocks.

Methods

We present new Discrete Fracture Network (DFN) codes that model fractures and associated fluid flow in 2-D at reservoir scales to represent typical rock types found in TVZ reservoirs. Input parameters are derived from interpretations of borehole images at the Rotokawa and Wairakei geothermal fields where fractures have high dip magnitudes (> 60–70°). This paper focuses on the effect of fracture density along virtual boreholes (P₁₀), that is in average 0.6 m⁻¹ in sheet-like andesites; 0.8 m⁻¹ in ignimbrites and 1.7 m⁻¹ in rhyolite lavas.

Results

The number of fractures in the models scale linearly with the input P₁₀ in virtual boreholes. The percolation threshold, where the backbone of fractures is connected across the entire model domain, is reached for P₁₀ > 0.24 m⁻¹. Above this threshold, mean flow measured along the mean fracture direction scales linearly with P₁₀. For P₁₀ > 0.4 m⁻¹ the permeability anisotropy lies in the interval 13 ± 3, with the scatter decreasing as P₁₀ increases. The pressure distributions in individual DFN realisations are highly variable, but averages of 50 realisations converge towards those given by equivalent continuum models. Probability density functions resulting from DFN realisations can therefore be used to constrain continuum models. Tracing of fluid particles through the DFN shows that particles can take numerous pathways to define a swath of paths. The travel time of particles over 1 km follows a distribution similar to real tracer tests, with arrivals peaking at 1–2 days and a long tail stretching to over 200 days.

Conclusions

The new codes, calibrated to real measurements of fracture geometries in borehole images of the TVZ, reproduce patterns of flows in fractured geothermal systems. Mean flows and permeability anisotropies derived from the DFNs can be used to improve modelling of flows through fractured geothermal reservoirs using continuum models at a limited computational cost.