The challenge in measuring low-permeability materials: a comparison of different approaches
Authors: Laurich Ben 
Published: November 2021

Abstract. The German repository site selection procedure calls for a radioactive waste
containment zone with a low-permeability host rock (kf<10-10 m s−1, StandAG §23, 5) and long-term sealing by
barrier materials (EndlSiAnfV, 2020; ESK, 2019). The potential host rocks,
clay and rock salt, as well as the considered barrier materials, bentonite and
compacted crushed salt, show permeability in the range of kf∼10-16 m s−1 (K∼10-21 m2). These low
values suggest that advective flow is as slow as diffusive mass
flux. Measuring such low permeability with adequate accuracy challenges
measurement setups and respective error evaluation. Methodologies. Several low-permeability measurements are carried out
by transient tests, e.g. by monitoring controlled fluid pressure changes in:
(1) pressure decay and (2) oscillating pulse tests. The first method (1)
deviates permeability from the time needed to compensate pressure differences
through the sample. The latter (2) monitors phase shift and amplitude
attenuation of controlled pressure pulses passing through the sample. Any
permeability measurement needs to be post-processed, e.g. for: (1)
material-intrinsic controls (saturation state, storativity, the fluids'
compressibility, etc.), (2) environmental controls (temperature, confining
pressures, etc.) and (3) theoretical considerations (Klinkenberg correction,
multi-phase wetting angles, etc.). Salts. A porosity-permeability relation was found down to K=10-19 m2 (e.g., Popp et al., 2007). Testing fluids were NaCl brine,
oils, He and N2 as a fluid. As a matter of current research, a
critical, low-permeability value might be associated with the so-called
“percolation threshold” that defines the minimal requirements for an
interconnected pore system (e.g., DAEF, 2016). Clays. A major challenge is the long duration of sample saturation
(up to several months) and pressure equilibration (often days), as well as
precise, temperature-compensated measuring and the determination of the
samples' storativity (e.g., Winhausen et al., 2021). Testing fluids are
commonly designed mixtures mimicking the rocks' pore waters. Geotechnical barrier materials. The permeability testing performed is
similar to that of salt and clays mentioned above. However, both barrier
materials, crushed salt and bentonite, have significant permeability early
after emplacement. This is beneficial, as it allows the outflow of unwanted
canister corrosion gases. Eventually, the permeability drops by orders of
magnitude and the barriers become tight seals in the long-term. Here,
identifying the gas entry/breakthrough pressure has been valuable (e.g.,
Rothfuchs et al., 2007). Figure 1 shows a preliminary sensitivity analysis as an example of pressure
decay measurements. It suggests that the pressure equilibration term (c), and
hence the test duration, is most sensitive to the calculation of low
permeability. However, the large variation of (representative) material and
environmental controls make permeability measurements complex. This workshop
aims to encourage discussions on uncertainty and sensitivity of the
influencing controls, such that it may lead to a “best-practice” guide for
permeability measurements.