Low Perm – Development of a calibration standard for evaluating permeability measurements of tight rocks

Permeability of tight rocks is an important subject where fluid-rock interaction is concerned. It is of particular importance in the site selection process for a deep geological repository for high-level radioactive waste. According to the German site selection act the containment-providing rock zone shall isolate the waste and prevent or limit transport of radionuclides from the repository into the biosphere for 1 Ma. Consequently, the permeability of the host rock needs to be very low.

However, permeability measurements of tight rocks are a great challenge for rock physics laboratories. There is a wide variety of measurement methods and analytical equipment. Therefore, the data from different laboratories are generally hard to compare and it is difficult to build a trustworthy, comprehensive and reproducible permeability data base for possible host rocks of a disposal site for nuclear waste. This is of particular importance because trust in the applied data base is one key factor for a successful site selection process. Against this background it is desirable to:

• develop a calibration standard with uniform and consistent rock physical properties
• test this standard at various laboratories which apply different permeability testing methods and experimental setups.

Such a round robin test with a well-defined, homogeneous and stable standard would allow a reliable comparison of measurement results of the applied methods and support comparability of permeability measurements of tight materials.

The research project Low Perm, which we present here, is geared towards both these objectives and aims to improve the objective comparability of measured permeability values of tight rocks.

In a first step a calibration standard on corundum (Al₂O₃) basis is currently developed. Corundum is chemically resistant, has high mechanical strength, high temperature stability and low thermal expansion. The aim is to produce a ceramic with a porosity of roughly 30 % with pore diameters averaging between 10 and 100 nm and – most importantly – a permeability of 10^-18 to 10^-20 m². For this purpose, the Al₂O₃ granulate was first uniaxially formed into a cylindrical shape and then isostatically compacted at pressures between 100 and 300 MPa. Finally, the samples were sintered at temperatures between 1300° and 1600°C.

Quality control of the test specimens, i.e. determination of porosity and permeability, is done by a combination of He pycnometry, sorption measurements as well as BIB-SEM imaging, radial diffusion and gas permeability measurements. So far, specimens sintered at 1350°C and 300 MPa are closest to the desired permeability with 9.438 x 10^-18 m² (at a porosity of 38 %). A specimen at 1400°C / 300 MPa is currently investigated. It is expected that the permeability of this specimen is close to 10^-19 m², which in turn should make it a suitable calibration standard.

The developed specimens will be sent to five laboratories (yet to determine) for a round robin test. Each of them will carry out permeability measurements of five samples. Each laboratory will apply its own permeability testing methodology and equipment. This will be done to evaluate the permeability testing concepts of the participants. The results of the benchmark will eventually be discussed jointly, evaluated and published under open access.