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

Florian Konrad,Alexandros Savvatis,Kai Zosseder,Florian Wellmann

doi:10.1186/s40517-019-0137-4

Florian Konrad, Alexandros Savvatis + Show 2 more

Open Access

https://doi.org/10.1186/s40517-019-0137-4

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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 m2, and a linear flow for matrix permeability values below 6.0 × 10−14 m2. 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.

Highlights

IntroductionCarbonatic rocks of the Upper Jurassic form a mid-enthalpy karstified and fractured porous reservoir
One of the most important aquifers for deep geothermal energy exploration in Europe is located in southern Germany in the North Alpine Foreland Basin
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 m2, and a linear flow for matrix permeability values below 6.0 × 10−14 m2

Summary

Introduction

Carbonatic rocks of the Upper Jurassic form a mid-enthalpy karstified and fractured porous reservoir They can be up to 600 m thick and mainly consist of small-pored white limestones as well as fine- to coarse-grained dolomites (Agemar et al 2014; Mraz 2019; Wolfgramm et al 2007). These reservoir rocks compose the ground surface at the northern border of the foreland basin (Frankish and Swabian Alb) and are inclined to the south due to lithospheric bending caused by the Alpine orogenesis. The inclination, and the possibility for high temperatures together with favorable hydraulic properties, potentially high-permeable fault zones and karstification, are the reason for the importance of the Upper Jurassic aquifer ( called Malm aquifer) in the context of geothermal energy exploration (Agemar et al 2014; Birner 2013)

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