| Abstract |
Manfred P Hochstein, IESE, Univ. of Auckland, New Zealand Keyan Zheng, Geothermal Council of China Energy Society, Beijing, China Suzan Pasvanoglu, Geological Eng. Dept, Univ. of Kocaeli, Turkey, Peter Vivian-Neal, Kalahari Geoenergy, Lusaka, Zambia Advective (heat sweep) geothermal systems occur in tectonically active regions within suitable and specific hydrological settings. They transfer heat from a temperature field in the upper crust over large distances to small discharge centres at the surface in a hydrological sink setting. The systems are often associated with large mountain terrains, extensive granite complexes, and tectonic shear zones - often in a plate boundary setting. Deep infiltration is enhanced by the difference in average altitude of the piezometric level in the infiltration region attained by precipitation and the discharge level of thermal springs. The fluid temperature at deep sweep levels between infiltration area and discharge point can some times be assessed from geothermometry analysis of unmixed, equilibrated ascending fluids. These assessments point to deep fluid equilibration temperatures between 100 and 200 deg C (equivalent to crustal sweep depths of c. 3 to 6 km) in non-volcanic settings. Many advective systems discharge thermal water through narrow, usually steeply dipping fractures or fracture zones, resulting often in aligned thermal manifestations. Bunched manifestations occur over intersecting fractures. Most systems referred to in this paper discharge thermal waters at T > 60 deg C and transfer anomalous heat, usually at a rate between 1 and 10 MW. Discharge rates can be significantly lower for isolated single spring systems (heat transfer rate < 0.1 MW). Travertine deposits are characteristic manifestations for many prospects. Reservoir modelling of advective systems has been attempted; it has limited predictive power since only a few characteristics of the discharge (spring) areas are usually known and data are not available which describe characteristics along deep sweep paths. However, secular flow patterns covering sweep areas of the order of > 3,000 km2 have been predicted where a narrow range of temperature gradients in the Upper Crust can be inferred. Hot springs of a few well studied advective systems in Asia exhibit discharge temperatures close to boiling point. These prospects were originally selected for exploitation to generate electric power. However, temperatures in fracture zone reservoirs increase only gradually with depth and long-term productivity has been limited due to small yields and the limited storage volume of productive reservoirs. Hence, small binary power plants constructed in Thailand and Tibet, but also in Zambia, failed or could not be enlarged. Other thermal springs, associated with narrow fracture zones of advective systems in China and Turkey, are still being used for balneology with economic returns, maintained in one case by drilling of deep production wells. Studies of the prospects show that predictions of apparent reservoir temperatures using cation-based geothermometres are unreliable if applied to non-equilibrated constituents. |