Thermal and hydraulic aspects of the KTB drill site

The extensive data sets obtained by the KTB drilling project (lithological and structural information, BHT values, temperature logs, rock thermal properties) provide a unique opportunity to construct realistic thermal models and thus to shed light on thermal conditions in the upper crust. Our numerical simulation study, a Swiss contribution to the German KTB drilling project, aims to understand the steady-state thermal and hydraulic field in the surroundings of the KTB. The simulations consider state-of-the-art petrophysical aspects relevant to deep, pressurized, high-temperature structures and were performed on discretized 2-D/3-D finite-element meshes that contain topography, geological structures and hydrogeological features. Our analysis of the KTB temperature field suggests three zones of particular geothermal settings: a low-heat-flow zone in the uppermost layers with a transition to high heat flow at 500 m depth; the underlying region accessed by the borehole with its characteristic uniform gradient; and the mid-lower crust that must be responsible for the high-heat-flow regime at the KTB site. The two first zones are treated in the present paper. A 3-D thermo-hydraulic model was set up in order to evaluate the first 2000 m, including the uppermost 500 m low-heat-flow zone. This model incorporates the complex geological information from the KTB pilot hole and topography-driven fluid flow. The lateral boundaries of the model were carefully chosen by analysing the flow pattern within a large, regional 3-D domain. The drilled section is analysed by a 2-D model using the available structural information. Due to dominating refraction effects, a careful temperature gradient analysis has to be carried out for such steeply dipping, anisotropic structures. Both models indicate a thermal regime dominated by diffusive heat transfer. Hydraulic flow seems to be important only for the uppermost (similar to 400 m) part of the drilled depth section; our simulations do not support significant fluid circulation at greater depths. In the drilled section the rather uniform gradient and the pronounced vertical heat-flow variations can now be explained. Finally, the potential and the limitation of the analysis of heat hows and temperature gradients are demonstrated. Heat-flow interpretations are conclusive only for nearly horizontally layered, isotropic geological units. In steeply dipping and anisotropic formations the heat-flow field is perturbed over a large distance (>1 km) around the point of interest. In such geological units only the temperature gradient interpretation can provide reliable information on the surrounding material.