The Effect of Fractures on Fluid Flow in Geothermal Systems, Taupo Volcanic Zone, New Zealand.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The goal of this thesis is to evaluate the effect of fractures on the bulk permeability of rocks. Several methods are used to address this problem: 1) surface radon gas measurements, 2) stress induced fracture permeability 3) fracture generation conditions. Each method was variably effective in providing answer to the initial question.
The radioactive radon isotopes (220Rn and 222Rn) were measured in soil gas extracted from 1 m depth in two areas and the concentrations for both isotopes tended to be higher near mapped faults. Soil samples recovered from 1 m depth indicate that the isotopic anomalies are coincident with changes in soil colour and the emanation of 220Rn, but are unrelated to the 222Rn emanation. The lack of a relationship for the latter can be explained by small-scale (~1m) diffusion for >90% of the soil gas measurements. However, diffusion cannot explain all of the observed patterns in the data, and in some specific locations along the fault, 222Rn concentrations are more likely sensitive to advective flow of sub-surface gases, suggesting channelizing of flow along faults.
Stress is estimated using Leak-off Tests, estimating overburden weigth, and using drilling induced features observable in boreholes to model stress conditions. The results of the stress interpretation in the Rotokawa Geothermal Field show a relationship between the differential stress and alteration zones containing smectite, where the presence of smectite lowers the differential stress in the crust. This confirms a well-recorded relationship between the friction of rocks, and the strength of the crust. The magnitude of the principal stress axes, which are determined in this thesis, are used to predict the fracture orientations prone to slip in the Rotokawa Reservoir. The precise range of fracture orientations prone to slip is critically dependent on the poorly constrained intermediate stress. However, analysis of stresses on fracture orientations observed in the Rotokawa Andesite, coupled with independent permeability estimates reveal a complex relationship between fracture slip, and permeability, suggesting that slip on fractures can have both a positive or negative effect on slip. This is will depend on the degree of alteration of the Rotokawa Andesite.
Failure in the Rotokawa Andesite is a result of: 1) the constant tectonic strain and 2) the increase in fluid pressure. Mathematical models used in this thesis show that if failure occurs through increase in fluid pressure, it is unlikely that the overpressures required to induce rock mass failure are solely generated by porosity/permeability reduction in the Rotokawa geothermal reservoir, requiring a constant external flux of fluids to induce the overpressures. Large-scale failure of the Rotokawa Andesite is modelled as a rock mass using the Hoek-Brown failure criterion, and indicates that the current dominant mode of failure is for the Rotokawa Andesite is shear failure at depth. However, small scale changes in stress, or an increase in rock mass strength will favour tensile failure. High fracture densities observed in three wells of the Rotokawa Andesite are oriented consistent with fractures formed in shear mode, consistent with ‘Healy’ faulting being the main mode of fracture formation in the Rotokawa Andesite.