Outcrop analogue studies of rocks from the Northwest German Basin for geothermal exploration and exploitation

Abstract

Rock heterogeneities in terms of layering and fault zones are common phenomena in sedimentary basins such as the Northwest German Basin (NWGB). At geothermal projects, these heterogeneous rock properties affect many issues associated with exploration, drilling, and reservoir stimulation. This thesis investigates how high resolution data from outcrop analogues can be used to improve predictions of both normal fault structure and rock mechanical conditions at greater depths. To better understand normal fault structure and associated fracture systems in sedimentary rocks of the NWGB, 58 outcrop-scale normal fault zones were analysed in detail. The focus was on fracture orientations, densities, apertures and lengths, separately for fault damage zones and host rocks, as well as structural indices. Pronounced differences between carbonate and clastic rocks were found, and mainly in carbonate rocks clear damage zones with increased fracture densities occur. While the maximum aperture is similar for both rock types, the percentage of fractures with large apertures is much higher in the damage zones. In carbonate rocks, damage zone fractures may differ significantly in orientation from that in the host rocks. In clastic rocks, fractures show a similar orientation in both fault damage zones and host rocks. Structural indices indicate that normal fault zones in carbonate rocks are more damage-zone dominated and have more profound effects on enhancing permeability in fluid reservoirs than those in clastic rocks. Based on measured Young’s moduli and fracture density distributions, effective stiffnesses Ee within normal faults are calculated and yield a significantly smaller stiffness decrease for clastic-rock damage zones compared with carbonate rocks. To improve knowledge about properties of typical NWGB rocks, physical (P-wave velocities, porosity, and bulk and grain density) and geomechanical parameters (Uniaxial compressive strength (UCS), Young’s modulus, destruction work and indirect tensile strength; each perpendicular and parallel to layering) were determined for 35 outcrop samples taken from quarries and 14 equivalent core samples. A subgroup of these samples, consisting of one volcanic rock sample, three sandstone and three carbonate samples, was used for triaxial tests. Because core material is rare, this thesis aims at predicting in situ rock properties from outcrop analogue samples. Properties of samples from depths are compared with equivalent outcrop samples – that is, same stratigraphic age and comparable sedimentary facies. Equivalence is confirmed using thin section analyses. Empirical relations of UCS with all physical and geomechanical parameters were determined with regression analyses, lithologically separated for outcrop and core samples. Most relations have high coefficients of determination; properties of core samples lie within 90% prediction bands of empirical relations for outcrop samples. Similarly, linearized Mohr-Coulomb failure criteria, expressed in both principal stresses as well in shear and normal stresses were determined from triaxial test sequences. A comparison with core samples shows that it is possible to apply principal stress failure criteria for clastic and volcanic rocks, but less so for carbonates. Expressed in shear and normal stresses, however, applicability is good for all rock types. Transferability of empirical relations to rocks at depths is expected. The most important aspects regarding applicability of obtained criteria are porosity and textural comparability of outcrop equivalents with core samples. Using FRACOD, fracture propagation in heterogeneous rocks at stimulation treatments was analysed for numerical models involving layered and fractured scenarios characteristic for NWGB. Results of both fault-related fracture systems and geomechanical properties are used as input parameters. Contrasts in Young’s modulus and Poisson’s ratio between alternating layers were found to have less effect on the fracture trajectory than contrasts in fracture toughness. Scenarios involving a set of parallel pre-existing fractures reveal a complex interaction with an induced hydrofracture. Presented results of this thesis can be of manifold use: they will help to explore fault-related geothermal reservoirs with high natural permeabilities; laboratory measurements provide approaches as to how to predict mechanical properties at greater depths from outcrop samples, as well as input data for future numerical modelling of geothermal problems; and numerical modelling of hydrofracture propagation in heterogeneous rocks gives insight on relevant parameters affecting fracture path which helps adapting the stimulation strategy to reservoir conditions.