Pressure-dependent permeability Sample Clauses

Pressure-dependent permeability. These unimaged small sizes of pores (< 20 nm; type V) are likely to provide a network of connected flow paths between the large, unconnected pores. Using data from bulk measurements of permeability, compressibility and acoustic velocity measurements, Xxxxxxxx et al. (2017) found a similarly porosity system for gas in Whitby shales. Permeability measurements of seven Whitby mudstone samples were performed under the permeability–effective pressure at a constant value of pore pressure, whereby increasing effective pressure for the first time causes a non-recoverable reduction in permeability of up to two orders of magnitude, after which the permeability–effective pressure relationship settles to a reproducible pattern (Figure 5). The permanence of this observed initial permeability reduction was tested by completely depressurizing one sample after it had been pressure cycled. The depressurization did not fully restore permeability to its original value, implying that permanent physical changes occurred during the initial stage of pressure cycling. These suggest the pores that form connected networks must be (a) very small to account for the low absolute permeability of these rocks, (b) of very low aspect ratio (i.e. be crack-like) to account for the high compressibility of these pores, and hence a high sensitivity of permeability to effective pressure, and (c) have their platy form strongly oriented parallel to bedding to account for the marked permeability anisotropy of shales. All of the permeability data for the seven samples range between 3 × 10-21 and 2 × 10-19 m2 (3 and 188 nD).