TY  - GEN
ID  - heidok28286
TI  - TLS supported volumetric and DFN modeling of a fault zone in the Lower Buntsandstein, SW Germany
Y1  - 2020///
AV  - public
UR  - https://archiv.ub.uni-heidelberg.de/volltextserver/28286/
N2  - Fluid flow is governed by primary and secondary porosity of rocks but also by their permeability. Often the values of primary porosities and permeabilities are not sufficient to allow fluids to flow from potential geothermal or hydrocarbon reservoirs. To ensure an efficient productivity, fractured reservoirs come into focus as they might provide an economically viable fluid flow. Subsurface fractured reservoirs are difficult to investigate, outcrop analogues like the one investigated help in a better understanding. The studied outcrop represents a Lower Triassic braided river succession within an arid alluvial plain, affected by the main fault of the western Rhine Graben (southwestern Germany).
The research thesis was carried out with the help of terrestrial laser scanning (TLS) to generate a digital outcrop model (DOM), used to digitize data and serve as basis for the subsequent modeling in two steps. These are (i) the volumetric modeling of the investigated fault zone within the Triassic Lower Buntsandstein, and (ii) subsequent modeling of the discrete fracture network (DFN).
Volumetric modeling comprises three main points: (i) the application of a fault zone facies concept, (ii) stair-stepped fault gridding, and (iii) splitting the fault zone into two geobodies, well established in structural terminology, the damage zone ?DZ? and the fault core ?FC?. For the subsequent DFN calculations a thorough fracture data parametrization was carried out providing six defined fracture sets, the fracture shape, the log-normal aperture distribution, the log-normal length distribution, the P32 intensity, and fracture truncation percentages at bed boundaries (DZ only). DFN upscaling was then conducted with the ?Oda? and ?Oda Corrected? methods for the fracture permeability calculations.
The resulting volumetric model comprises 13 fault zone facies types. Their distribution within the DZ follows the encountered beds? morphology. Within the FC three facies distribution cases were modeled. Seven different DFN configurations were calculated, consisting of 162 fracture sets in total. Fracture permeability amounts between 190 and 720 D within the DZ and 14,130 to 55,189 D within the FC, while the fracture porosity shows values of about 0.4 % for the DZ and 2.38 % for the FC.
The study shows that volumetric fault zone modeling requires a simultaneous fault facies analysis and grid construction. Because stair-stepped fault grids facilitate a high complexity but lack cell size flexibility, a thoroughly considered choice of the cell size, dependent on the smallest geological objects present, is crucial. Characterization and processing of fracture aperture constitutes the most important part of the parametrization, as different methods can lead to distinct differences in the modeled final fracture permeabilities, spanning multiple orders of magnitude, even for exactly the same values of mechanical aperture. Inclusion of fracture connectivity lowers the resultant horizontal fracture permeability by 26 to 38 %, while truncation of fractures on bed boundaries can overestimate permeability values. Although the FC shows a significantly higher fracture permeability than the DZ it is affected by extreme fracture permeability cutoffs due to the fault cores? specific architecture, resulting in a conduitbarrier system. Fracture porosities are more insensitive to parameter changes, because of its dependence on the mechanical aperture only.
The presented multi-approach thesis highlights the challenges, limitations, and great possibilities of fault zone models, to help in a better understanding of the impact fault zones might have on geothermal and hydrocarbon reservoirs, and thereby support exploration.
CY  - Heidelberg
A1  - Miernik, Georg Josef
ER  -