Institute of Geophysics of the CAS, v. v. i.

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Studies on pore space geometry and its relation to physical properties and microstructure

Rocks near the Earths surface contain void spaces the size, shape and orientation of which affect many important rock physical properties such as permeability and strength including their anisotropy. We investigate the processes leading to formation and alteration of the void space by means of microstructural and petrophysical analysis.

The microstructural analysis is used to describe qualitatively and quantitatively the microscopic voids (e.g. different types of microcracks and micropores associated with grain boundaries) and to asses the relationship between these void types and the rock primary (pre-deformational) microstructure and the thereon superposed secondary processes such as fracturing or alteration.

The petrophysical analysis serves for quantification of physical properties related to the rock primary microstructure as well as to its modifications by the secondary processes and in turn the measured physical properties allow indirect inferences on some of the rock microstructural properties.

The results of our studies bear implications for both applied and fundamental research fields:

  • assesment of rock massif suitability for
    • deep geothermal energy production
    • CO2 sequestration
    • heat storage
    • waste storage
    • water production
    • fossil fuel production
  • seismology: the effect of microcracks on seismic properties of rocks

  • deep Earth systems: the role of microcracks in earthquake zones and during metamorphism

 

In a recent study (Staněk et al., 2013) we focused on the orientation of microcracks in a fractured granite and its relation to the granite primary microstructure and to the anisotropy of P-wave velocity, magnetic susceptibility and permeability. We sampled a granite with macroscopic magmatic structure (schlieren) and fractured by subhorizontal exfoliation fractures and steep cooling-related fractures (Fig. 1).

Figure 1. (a) Photograph of sampled granite outcrop featuring flat exfoliation fractures and steep cooling-related fractures, (b) closeup photograph of the outcrop showing the exfoliation fractures and the magmatic structure (schlieren) of the sampled granite.

We extracted a block from in between two exfoliation fractures spaced approximately 10 cm (Fig. 2).

Figure 2. (a) Photograph of the sampled block and (b) detailed gray shade photograph of the block YZ face perpendicular to both the reference exfoliation fractures (horizontal) and the magmatic structure (left-top-left to right-bottom-right).

We prepared three mutually perpendicular petrographic thin sections impregnated by fluorescent epoxy (Fig. 3) and by digital tracing we analysed the orientation of grain boundaries and cracks therein (Fig. 4) and we inferred their 3D orientation (Fig. 5).

Figure 3. Micrographs of the YZ thin section in (a) plane-polarized, (b) cross-polarized and (c) fluorescent light showing (a, b) the magmatic microstructure and (c) the crack network.

 

Figure 4. (a) Scheme of orientation of the three mutually perpendicular thin sections with respect to the mesostructures and (b, c, d, e, f) rose diagrams of orientation distributions of the traced (b) grain boundaries, (c) intergranular cracks, (d) cracks in quartz, (e) in mica and (f) in feldspar.

 

Figure 5. Stereograms showing (a) the orientation of mesostructures and (b) the 3D orientations of the traced features based on the 2D results shown in Fig. 4.

We found that the grain boundaries and cracks except in quartz have similar orientation as the granite magmatic structure represented by preferred orientation of long axes of the grains (approximately top-left to bottom-right in the figures). In contrast, the highest proportion of cracks in quartz is oblique to the magmatic structure and subparallel to the exfoliation fractures limiting the sampled block in terrain (horizontal in the figures).

From the same sampled block we prepared a set of oriented specimens: a sphere with 5 cm diameter for multidirectional analysis of P-wave velocity (VP), plugs for analysis of anisotropy of magnetic susceptibility (AMS) and plugs for permeability measurements.

We found that the direction of the lowest VP was perpendicular to the preferred orientation of the grain boundaries and of most of the cracks, whereas the highest VPs were measured in directions parallel to these microstructural features (Fig. 6a). Taking advantage of the VP measurements at different levels of confining pressures we found that both cracks parallel to the magmatic structure and cracks parallel to the fractures comply markedly at depth less than 500 m. This is demonstrated by the dominant increase of VP in directions perpendicular to the cracks due to augmentation of the confining pressure from ambient to 10 MPa (Fig. 6b).

Figure 6. Stereograms showing orientation distribution of (a) VP at ambient pressure and (b) of change in VP between measurements at ambient pressure and at 10 MPa; legend units are m s-1, S - schlieren, F - fracture, white and black dots - directions of maximum and minimum values.

The AMS analysis revealed that the biotite grains are aligned with the magmatic structure in good correlation with the orientation of cracks in micas which generally form along the mica cleavage planes parallel to the mica basal plane. This is demonstrated by the AMS ellipsoid minimum principal axis (k3) oriented perpendicular to the magmatic structure (Fig. 7), taking into account that a biotite single grain AMS ellipsoid minimum principal axis is perpendicular to the grain basal plane.

Figure 7. Stereogram showing orientation of the AMS ellipsoid principal axes.

Finally, the permeability measurements along and perpendicular to both the magmatic structure and the fractures revealed that the direction of intersection of these two structures features the highest permeability, whereas low permeabilities were measured perpendicular to them and intermediate permeabilities along one or the other (Fig. 8a, b). Furthermore, the permeability and VP values measured along these directions correlate (Fig. 8c) which means that the void space is interconnected.

Figure 8. (a) scheme of directions of permeability measurements, (b) table showing values of permeability and of VP measured along the directions and (c) VP vs permeability plot.

This study lead us to the following conclusions:

  • the magmatic structure represents mechanical anisotropy controling orientation of future cracks via mica and feldspar mineral cleavages
  • in cleavge-free quartz grains the cracks form parallel to the macroscopic fractures
  • the cracks comply at depth less than 500 m
  • permeability of the rock is the highest along the direction of intersection of the fractures and the magmatic structure
  • the cracks detected by the VP experiment form an interconnected network