Reducing geothermal exploration risks by predicting the properties of potential deep geothermal reservoirs from surface and shallow borehole data

The economic viability of deep hydrothermal heat production strongly depends on achievable production rates. For economic operation, flow rates on the order of 60-100 l/s are required. Such rates exceed those of producing hydrocarbon wells by up to two orders of magnitude setting high demands on the hydraulic conductivity. Geothermal exploration of unproven reservoirs from which no production data is available consequently bears a high reservoir risk. Therefore, the overarching goal of this project is the reduction of the discovery risk through a cost-effective characterization of the reservoir properties.

Reservoir properties were investigated in (1) two shallow boreholes and (2) surface outcrops. Well data bridge the mesoscale between lab measurements on small rock volumes and deep-borehole data, and after upscaling serve as representative reservoir parameters for subsequent reservoir modeling. Wells and outcrops are located in geological units that extend beneath the Vienna Basin and are regarded as analogues for the subsurface targets with respect to lithostratigraphy, tectonic position, and deformation history.

(1) The boreholes EVN RO-1 (64.5 m) and EVN RO-2 (25.0 m) penetrate fractured dolomites and dolomite breccias. Geophysical logging (DFEL, Full Wave Sonic, Spectral Gamma, Acoustic Image Log) and permeability measurements (packer-, slug- and pumping tests) were carried out in both boreholes. Logs indicate porosities of ~8 %, while packer tests reveal highly variable permeabilities spanning more than two orders of magnitude (0.13-385 mD), mainly due to variable fracture density and clay content.

(2) Outcrops were investigated through geological profiles, sampling, and petrophysical analyses of parameters relevant to geothermal reservoirs, including porosity (0.9-9.8 % with an average of 4.02 %; measured at 400 psi confining pressure), permeability (plug-derived values 0.002-130 mD with an average of 6.23 mD at 400 psi), thermal conductivity (mean values 4.21 and 5.45 W/mK for dry/water-saturated dolomite), heat capacity (average 2.38 and 2.98 J/m³K dry/water-saturated), and fracture density. Power-law porosity-permeability relationships (K = a·ϕ^b) were derived for datasets representing different sample volumes (plugs, slug tests, and packer tests). All correlations exhibit high scatter, low exponents (b = 0.28-1.36), and weak correlation coefficients (<0.1), likely reflecting a substantial proportion of closed or isolated pores that contribute little to permeability. Porosity/permeability measurements at confining pressures corresponding to those at ca. 3000-3500 m depth show that porosity/permeability in this depth is about 90 % (permeability) and 50 % (porosity) lower than the data measured under atmospheric conditions or low confining pressure. Thermal conductivity and heat capacity are expected to increase at higher confining pressures at depth due to the sensitivity of porosity to pressure.

In sum, borehole and lab data characterize the investigated dolomite formation as a highly inhomogeneous fractured reservoir. Results also show that lab and well data are required to account for the scale dependence of permeability and predict petrophysical reservoir properties that are adequately extrapolated to pressures at reservoir depth. Future work will use the data in conceptual reservoir models for predicting the most likely reservoir performance.