Use of geostatistical simulation in reservoir thermodynamics assessment and interpretation at the Durkov hydrogeothermal structure, Slovakia

Geothermal energy in Slovakia is under a systematic research since 70's of the last Century. Out of 27 identified prospective geothermal fields within a territory of the Western Carpathians, the Durkov hydrogeothermal structure in the Kosice Depression is repeatedly accented as of the most enormous potential for a heat (and in some, rather wishful, plans for power) production. Unlike a conventional evaluation of thermal potential and temperature, this study aims at description of the reservoir under an exergy concept; quantifying reservoir enthalpy, exergy and specific exergy index based on geostatistical modelling prior classifying a system according to a thermodynamic quality. Geostatistical simulations have become very popular in different areas of spatial modelling for spatial simulation of properties, geometries or heterogeneities. The geostatistical approach is supportive to quantification and interpretation necessary to understand for reservoir engineering and construction of reservoir prediction and response models. Turning band method of simulation was used to create multiple realisations of the convection indicators within the reservoir body due to evident non-stationary behaviour, mainly in vertical direction, which made it impossible to use Gaussian type model. Non-conditional simulations were tested conditioned by universal kriging using modelled variograms of global trend residuals with respective drift functions for each studied variable. The risk volumetric curves were derived from final numerical models and probable reservoir volumes above different boundary conditions (cut-offs) were calculated and visualised. Based on numerical models, reservoir enthalpy greater than 800 kJ · kg^-1 has been identified in deeper parts of a system with very small proportion of specific exergy index above 0.2. Thermodynamic data, and models based on them, show the resource is rather suitable for a large-scale heat (district heating) production - supply. (authors)