Aquifer analogue study to determine the effects of fracture networks regarding the deposition of CO2
Country / Region: Germany / Northern Black Forest
Begin of project: December 1, 2011
End of project: November 30, 2015
Status of project: November 30, 2015
Fig. 1: Cross section of Baden-Württemberg
Source: LGRB
Regarding the potential storage of CO2 in the deeper underground in Germany, the suitable storage formations principally are sandstones with sufficient thicknesses. In Central Europe such porous and fissured sandstones often occur in Rotliegend and Buntsandstein formations.
To investigate the storage properties of these sandstones, core samples were used to determine their lithological and mineralogicical/geochemical parameters. The natural fracturing as another important factor for the storage potential can only be specified in outcrops. Hence, for the presented aquifer analogue study fractures were measured in outcrops of the Middle Buntsandstein in the northern Black Forest near the city of Gaggenau. Subsequently these parameters were geostatistically evaluated.
Fig. 2: Core taken from the research borehole Kraichgau 1002
Source: BGR
This approach helps to improve the interpretation of the previously obtained drill core data. In some cases the base of the Buntsandstein was encountered at about 670 m below ground level (cp. Figure 1).
The cores were taken from boreholes in areas with natural CO2 occurrence which is often related to magmatic processes in the upper Earth’s mantle. The degassing of CO2 primarily takes place along jointed zones which can be described only insufficiently by means of the borehole data. On the other hand, the cores provide a good possibility to directly study the fluid rock interactions. The colour variations visible in Figure 2 could be associated with bleaching processes taking place under reducing conditions.
Fig. 3: Quarry of the Badischer Bausandstein near Rastatt
Source: BGR
Detailed field measurements of the joints were carried out in outcrops in the northern Black Forest (Figure 3), where rocks of the Middle and Lower Buntsandstein are exposed at the surface and can be examined as substitutes for the storage complex Buntsandstein which is located in deeper positions e.g. in the Oberrheingraben.
Two different measuring procedures were adopted during the geologic field work. On the one hand the fissure and fracture orientations were determined by measurements with a geological compass. Concurrently parameters like fracture length and distance were also determined.
Fig. 4: Stereographic projection
Source: BGR
In the course of this the so called scanline technique was applied. This method uses a tape measure which is tightened in an arbitrary angle across an outcrop wall. Joints are measured along this straight line. For the fractures which are intersecting this line the above mentioned parameters can be obtained.
The orientations of the fracture planes can be visualized as poles in a stereographic lower hemisphere projection. At the same time the point density can be displayed (Figure 4) to be able to identify preferred spatial directions (cluster centres).
Fig. 5: Laser scanner in field operation
Source: BGR
To adjust the statistical under-representation of fractures intersecting the scanline at a low angle it is necessary to introduce a weighting factor.
As a second measurement method, a 3D laser scanner of type ILRIS 3D manufactured by Optech Inc was deployed together with four plastic spheres as referencing objects to receive exact survey data of the selected outcrops (Figure 5). All the generated point data were summarised in an geometrical model (Figure 6).
Fig. 6: Laser scan 3D surface
Source: BGR
By means of these available data it is possible to complement and verify the performed manual measurements by virtual measurements in the 3D model.
In a further step the statistically processed data can be used to construct a geological 3D model. In this context the software SKUA-GOCAD™ with the Fracture Modeling Modul (FracMV™) provided by Paradigm® will be applied.
Fig. 7: Possible percolation paths
Source: BGR
The 3D model shall be utilized to characterize the transport properties of fractured rocks by determining values for porosity and permeability. Decisive for the migration of fluids along fractures is if the fracture density is sufficient to create a linked fracture network which reaches the so called percolation threshold. Figure 7 shows possible percolation paths with an assumed vertical hydraulic gradient.
Link:
Literature:
- HOUBEN, G., WEITKAMP, A. & KAUFHOLD, S. (2024): The roughness of fracture surfaces and its scale dependence – a methodological study based on natural fractures in sandstones from Southern Germany. - Environ. Earth Sci. 83, 388. doi: 10.1007/s12665-024-11699-8
- RUPF, I. & NITSCH, E. (2008): Das Geologische Landesmodell von Baden-Württemberg: Datengrundlagen, technische Umsetzung und erste geologische Ergebnisse - LGRB-Informationen 21. Regierungspräsidium Freiburg - Abt. 9 (Hrsg.); Freiburg i. Br.
Promotion / document number:
Topic ENERGY.2011.5.2-1: Understanding the long-term fate of geologically stored CO2; Contract Number:281196