A comparison of plug-derived and probe-derived permeability in cross-bedded sandstones of the Virgelle Member,Alberta,Canada: Te influence of flow directions on probe permeametry

摘要

In this article we compare permeability measured on radially con- fined cylindrical plugs to permeabilities measured using a millimeter-scale probe tip on unconfined end-faces of the same plugs. Our investigation focuses on directional attributes of probe permeametry data as a means to understand the differences be- tween plug-scale and probe-scale permeability magnitudes. Hori- zontal and vertical plug and probe flow tests were carried out on fine- to medium-grained, three-dimensional (3-D) cross-bedded sandstones collected from estuarine channel successions within the Cretaceous-age Virgelle Member at Writing-on-Stone Provincial Park, southern Alberta, Canada. The sandstones are heterogeneous in that they contain thin, discontinuous micaceous/carbonaceous laminae and apparent bedding-parallel grain-scale fabric. The resulting probe-derived permeabilities (0.4-3.5 d) are mostly higher than corresponding plug permeabilities (0.5-1.6 d), and mean probe-derived permeability anisotropy is significantly lower than that derived from plug-scale measurements, that is, plug kv/kH is less than probe-derived kv/kH. The results can be under- stood as a consequence of the model flow geometry of probe flow tests (Goggin et al., 1988), namely, that flow is preferentially di- rected parallel to the sample surface, radially outward from the probe tip. On end-faces of vertical plugs the bedding-parallel fabric fur- ther enhances flow parallel to the sample surface, and probe measurements yield reliable estimates of horizontal permeability, having magnitudes comparable to the permeability of equivalenthorizontal plugs. For probe measurements on end-faces of hori- zontal plugs, flow paths appear to be variably confined parallel to bedding as a function of grain-scale and laminae-scale fabric, sys- tematically yielding an overestimate of horizontal permeability (kpRH > kPLW. Interpretation of probe data in this orientation, however, is very uncertain, due to partitioning of flow paths par- allel to and at angle to bedding. Significantly, almost all probe permeameter core and outcrop studies are based on measurements of this type.