Permeability measurement and prediction in samples from the Wolfcamp Fm. (Delaware Basin), West Texas

We measured steady-state liquid (dodecane) permeability in siltstone, dolomitized carbonate, and calcareous mudstone core plug samples from the Wolfcamp Fm. in the Delaware Basin, West Texas, at multiple effective stress conditions (6.9 MPa to 58.6 MPa). We interpret in situ permeability of 1.25x10^-18 m², 2.04x10^-18 m², and 3.06x10^-19 m², respectively, in these samples. We applied the critical path analysis and percolation theory in our mercury intrusion capillary pressure and nuclear magnetic resonance measurements to predict permeability in these samples. The permeability prediction matches the measured value very well for the dolomitized carbonate sample and within a factor of 5 for the siltstone sample. The predicted permeability for the calcareous mudstone sample, however, is 60x smaller; we attribute the narrow pore size distribution present for this significant discrepancy in our prediction. The scanning electron microscopy images of our samples revealed nanometer- to micrometer-scale interparticle pores in the matrix consisting of clays, quartz, and carbonate grains. The higher permeability samples exhibit significant micrometer-scale intraparticle pores within dolomitized microfossils. A better understanding of permeability will lead to new approaches to determine the best completion and production strategies and, more importantly, to predict ultimate production from unconventional shale reservoirs accurately.