1TU Munich, Geothermie-Allianz Bayern; Geothermal Technologies; 2TU Munich, Geothermal Technologies
Pore pressure prediction is a key factor for a safe and economic drilling strategy. Blowouts, kicks, stuck pipe, wellbore instability and loss of circulation are just some issues that may occur while encountering unexpected fluid pressure anomalies during drilling. Since pore pressure prediction is commonly applied to make decisions on safe range of drilling mud weights and casing points, it is vital to have an accurate estimation of the expected pressure magnitudes in the subsurface with a realistic level of uncertainty.
The North Alpine Foreland Basin in SE Germany is a hot spot for hydrothermal use of deep geothermal energy in Europe. Several new projects are planned in the greater Munich area and southeastern part of the basin to further exploit the prolific Upper Jurassic carbonate aquifer. However, to the South and East of Munich drilling of deep geothermal wells is usually challenged by an overpressured zone in the lower part of the Cenozoic basin fill and Upper Cretaceous (Drews et al., 2018; Müller et al., 1988). Estimation, prediction and monitoring of pore pressure magnitudes is therefore key to safely and efficiently reach the Upper Jurassic target. Previous studies have shown that pore pressure can be estimated using seismic velocities from vertical seismic profiles or sonic log data (Drews et al., 2018), but often these data are neither available from offset wells nor measured while drilling. In these cases, other data sources have to be used.
This paper addresses resistivity-based pore pressure estimations using Eaton’s method (1975) on the basis of a shale normal compaction trend (NCT) derived by the Waxman-Smits equation (1968). The Waxman-Smits model takes into account formation salinity, temperature, clay content and cation-exchange capacity (CEC). Here, the Waxman-Smits method was first applied to obtain the depth-dependent NCT using measured resistivity logs of ~80 hydrocarbon and deep geothermal wells in the North Alpine Foreland Basin, SE Germany. The pore pressures calculated from Eaton’s method have further been calibrated to available drill stem tests, wireline formation tests, pressure integrity tests, kick pressures, gas influx data while drilling and actual drilling mud weights. Moreover, our resistivity-based prediction model has been compared to previous pore pressure predictions from sonic logs and vertical seismic profiles (Drews et al., 2018).
The study shows an excellent agreement of our resistivity-based model with pore pressures inferred from drilling mud weights and measured pressures. It can therefore be used as an individual method or in complement with velocity-based predictions and has also potential as a pore pressure monitoring application during drilling operations in overpressured formations.
