Hydraulic behavior of fault zones in pump tests of geothermal wells: a parametric analysis using numerical simulations for the Upper Jurassic aquifer of the North Alpine Foreland Basin

Abstract

Fault zones in the Upper Jurassic aquifer of the North Alpine Foreland Basin are generally regions with possibly increased hydraulic properties. They are consequently often part of the geothermal exploration concepts in this area and a primary target for the drilling operation. Data from this aquifer, gathered in pump tests, however, show that only four out of 41 successful wells exhibit hydraulic proof for the presence of such a fault zone in terms of a bi-/linear flow regime. Besides technical effects, also the contrast in hydraulic properties itself, between fault zone and surrounding host rock, can prevent the detection of a fault zone in pump test data. This means a certain threshold has to be surpassed until its effects become clearly visible. A simplified realistic numerical model was constructed and calibrated with pressure data from an exploration site in the south of Munich. This model was then used to observe the presence of linear and bilinear flows in dependence on the Malm aquifers parameter space. Sampling the possible hydraulic property combinations with the help of an HPC (high-performance computing) cluster and automating the detection of the corresponding main flow type allowed to quantify the areas in parameter space where the fault zone-related flow regimes of interest are present. Through the investigation of more than 30,000 combinations between fault zone permeability, matrix permeability, fault zone storage, matrix storage and fault zone thickness, it was found that, in the parameter space of the Malm aquifer, a bilinear flow can be observed for the first time only if the matrix permeability is lower than 2.0 × 10 −13 m 2 , and a linear flow for matrix permeability values below 6.0 × 10 −14 m 2. Additionally, it was shown that fault zones, which have better hydraulic properties than the surrounding matrix, can indeed be hidden in pumping tests due to the parameter setting.

Porosity–permeability relationship derived from Upper Jurassic carbonate rock cores to assess the regional hydraulic matrix properties of the Malm reservoir in the South German Molasse Basin

“…Therefore, petrophysical and hydraulic properties of the aquifer rock are presumed to be variable throughout this carbonate platform setting as well. A detailed evaluation of pumping tests in the Munich area showed that, contrary to the expectation of a rather linear or bilinear flow, a significantly more frequent radial flow could be observed (Konrad et al 2019). Most of the investigated geothermal boreholes (37 of 41) show a homogeneous flow behavior, which was observed by the interpretation of pressure derivatives and indicates radial flow conditions.…”

“…The Malm reservoir in the Bavarian Molasse Basin is generally defined as fracture-and karst-dominated porous aquifer (Fritzer 2012, Konrad et al 2019. The hydraulic properties of the reservoir can therefore to a certain extent be interpreted as dual porosity and preferential flow.…”

“…They have an impact on a variety of different rock properties like rock density, thermal conductivity as well as geomechanical parameters (Stober and Bucher 2013). For thermal, hydraulic, and mechanical modeling (THM) of a reservoir, the distribution and variety of these parameters for each type of rock are required as an important input parameter range (Brehme et al 2016;Cacace and Jacquey 2017;Cherubini et al 2013;Jacquey et al 2018;Konrad et al 2019). For a better prediction of mining models and productivity prognosis in the Molasse Basin, it is therefore necessary to investigate these parameters and their influence on local aquifer conditions in detail.…”

Global Sensitivity Analysis to Optimize Basin-Scale Conductive Model Calibration – A Case Study From the Upper Rhine Graben

“…Physical process simulations are used in a wide range of geothermal studies -from the investigation of global and regional temperature fields, to local reservoir studies (Cacace et al, 2013;Kolditz & Clauser, 1998;Konrad et al, 2019;Randolph & Saar, 2011). As in many geoscientific applications, these simulations are based on models with associated uncertainties in the selection of the physical model (conductive, hydrothermal, mechanical, etc.…”

Temperature log simulations in high-enthalpy boreholes

“…Geothermal explorations depend strongly on reservoir conditions which are evaluated by increasingly sophisticated reservoir simulators (Cacace et al 2010;O'Sullivan and O'Sullivan 2016;Konrad et al 2019). Also, data acquisition, mostly of seismic data, has reached a high degree of complexity.…”