On the threshold of flow in a tight natural rock

The behaviour of hydraulically 'tight' barrier rocks is a key determinant of the long-term integrity of potential underground storage sites for the waste products from low-carbon emission energy production technologies (including nuclear waste and CO2 captured from fossil fuels). Here we isolate the relationship between crack-induced permeability and porosity using an initially crack-free natural crystalline material. We vary secondary porosity from an initial value of zero, and demonstrate that the bulk permeability K varies with total connected porosity Phi above the percolation threshold Phi(c) as K = K-0(Phi - Phi(c))(n), where n = 3.8 +/- 0.4, i.e., similar to results obtained for higher porosity rocks, indicating universality of this scaling law. Close to the percolation threshold a modest change in total porosity from 1% to 5% or so results in a massive change in permeability of 7 orders of magnitude or more. The results are consistent with a continuum percolation model that reflects the microstructure of the pore/induced microcrack network in the natural material. Citation: Meredith, P. G., I. G. Main, O. C. Clint, and L. Li (2012), On the threshold of flow in a tight natural rock, Geophys. Res. Lett., 39, L04307, doi: 10.1029/2011GL050649.