Evidence for the Heterogeneity of the Pore Structure of Rocks from Comparing the Results of Various Techniques for Measuring Hydraulic Properties

Abstract

We applied three oscillatory methods, the previously presented axial pore-pressure and pore-flow methods, and the laboratory application of the radial oscillatory pore-flow method, and performed steady-state flow-through experiments (Darcy tests), for comparison, in experiments on samples of Westerly granite and Wilkeson sandstone. The granite and the sandstone exhibit pore spaces dominated by micro-fractures and by the granular-medium character with a connected porosity of about 1 and 10 %, respectively. Permeability determined by the axial pore-pressure method shows the closest agreement with the results of the Darcy tests. Apparent porosity and drained modulus derived from specific storage capacity deviate from measured connected porosity and reference values, respectively. The observed deviations of the hydraulic properties between methods suggest that they bear information about the structure of the pore space. Only for the sandstone, anisotropy in hydraulic properties appears to contribute to differences between the results of the various methods. We argue that oscillatory testing provides three indicators for heterogeneity, period dependence, the relation between apparent and connected porosity, and the relation between amplitude ratio and apparent penetration depth, calculated from the simple scaling law for homogeneous materials. These indicators consistently classify the samples of Wilkeson sandstone as hydraulically homogeneous and those of Westerly granite as heterogeneous.

Pore Pressure in Montmorillonite During Frictional Sliding

“…Permeability of Westerly granite has been measured in the past >50 years (e.g., Brace et al, 1968;Schepp & Renner, 2021). Schepp and Renner (2021) showed a range from ∼4 × 10 −19 to ∼4 × 10 −17 m 2 at confining pressures up to 110 MPa using several dynamic methods. Brace et al (1968) showed a range from ∼4 × 10 −19 at confining pressure of 10 MPa to 4 × 10 −21 at confining pressure of 400 MPa.…”

“…First, while the surface of the consolidating layer in the Terzaghi model is exposed to air pressure, the surface of our montmorillonite sample was covered by a filter made of Westerly granite. Permeability of Westerly granite has been measured in the past >50 years (e.g., Brace et al., 1968; Schepp & Renner, 2021). Schepp and Renner (2021) showed a range from ∼4 × 10 −19 to ∼4 × 10 −17 m 2 at confining pressures up to 110 MPa using several dynamic methods.…”

Digital Rock Physics: A Geological Driven Workflow for the Segmentation of Anisotropic Ruhr Sandstone

“…Nevertheless, in order to use the threedimensional information provided by XRCT, it is necessary to utilize additional sources of information. An already established source of information is laboratory measurements that can provide rock properties such as permeability or porosity at ambient or elevated pressure and temperature conditions (e.g., Ahrens et al, 2018;Schepp and Renner, 2021). However, such laboratory measurements are resource and cost intensive, which is why the application of DRP is particularly interesting for small sample quantities.…”