Comparative petrophysical characterization, BIB-SEM imaging and permeability models of tight carbonates: Case study Upper Jurassic (Malm ß), N Bavaria

Abstract

Tight carbonate rocks are important potential although unconventional geothermal and hydrocarbon underground storage reservoirs and prospective CO 2 -EGS sites. We study these rocks by using the Upper Jurassic “Malm ß” in Southern Germany as an outcrop analog example to understand bulk properties in relation to microstructure and to test a variety of models for permeability prediction. Bulk petrophysical methods (Archimedes isopropanol immersion method, Helium pycnometry, mercury injection capillary pressure, gamma density core logging, gas permeability measurements) combined with microstructural investigations (BIB-SEM) are applied. Samples are macroscopically undeformed limestones collected from a tectonically overprinted quarry wall near the town of Simmelsdorf (38 km NE of Nürnberg city). Bulk porosities are below 5% and argon permeabilities are on average 1.4E-17m 2 . The presence of stylolites in some of the samples has neither a significant effect on porosity nor permeability. Pore sizes are in the submicron range and the diagenetically lithified stiff limestones are characterized by a relatively high Young’s Modulus averaging 73±5 MPa. Moreover, no trends in properties were observable towards the faults, indicating that faulting was post-diagenetic and that the matrix permeabilities were too low for intensive post-diagenetic fluid-rock interaction. Petrophysical properties are very close to those measured in stratigraphic equivalent rocks of the South German Molasse Basin, illustrating the widespread homogeneity of these rocks and justifying the quarry scale (500 x 580 m) as a reasonable reservoir analog for these rocks in the subsurface. The application of various permeability prediction models showed that the Capillary Tube model and the Saki Model are very well suited for predicting permeabilities from BIB-SEM and mercury injection capillary pressure results, respectively. We thereby found that the matrix permeability is primarily controlled by the pore throat diameters rather than the effective porosity.

Review of discrete fracture network characterization for geothermal energy extraction

“…In a DFN framework for geothermal energy production, all the rock discontinuities need to be represented in the 3D domain even though not all of them are hydraulically active. In fact, inactive fractures can represent either boundaries that can influence the flow, or surfaces of mechanical weakness that play a role in the development of EGS systems of igneous and metamorphic origin (Caulk and Tomac, 2017;Förster et al, 2018;Freitag et al, 2022). Reliable models of 3D networks of fractures are fundamental in EGS systems to predict the new framework of rock discontinuities after the injecting of fluids at high pressure.…”

“…These geomechanical parameters must be determined on both intact and fractured samples. An intensively fractured geothermal reservoir is assumed being characterized by geomechanical parameters closer to the fissured samples selected from the core cuts (Villeneuve et al, 2018;Rosberg and Erlström, 2021;Freitag et al, 2022).…”

“…Indeed, more elevated angles between the direction of the σ 1 and the fractures provide lower values of mechanical apertures due to interplays of dilatational and shear forces (Bisdom et al, 2016;. This information on the mechanical aperture need to inform DFN models (Romano et al, 2020;Rosberg and Erlström, 2021;Freitag et al, 2022). Recent and promising research focuses on determining the Young's modulus value directly from the reservoir studying the acoustic impedance from ATV logs; this approach is either alternative or integrative to geomechanical testing in the laboratory of cored samples (Raef et al, 2015;Roshan et al, 2023).…”