9. Predicting Permeability in the Target Reservoirs on the Rock Springs Uplift, Southwest Wyoming

Estimates of permeability in carbonate rocks from porosity alone are highly uncertain but can be improved when pore geometry information is incorporated. We developed a permeability model for a 400-ft-thick carbonate reservoir on the Rock Springs Uplift, Wyoming, with the objective of increasing the accuracy of flow simulation during CO₂ sequestration. Core data was used to identify hydraulic flow units within the reservoir and to further distinguish them through lithofacies analysis of thin sections. We used both the Flow Zone Indicator (FZI) and Winland’s R ₃₅ method to identify the flow units. FZI and pore throat radius values were obtained from the log-of-permeability-versus-porosity crossplot of the core sample measurements. For the rock types composing the Madison Limestone on the Rock Springs Uplift, both the FZI and R₃₅ methods proved to be effective techniques for rock-type classification. We found that acoustically derived porosity estimates within the Madison Limestone stratigraphic interval correlate well with those derived from the FZI. Sonic velocity in carbonates is a function not only of total porosity but also of the predominant pore type that determines the permeability of the rock. Hence our permeability estimation used both the density porosity and calibrated sonic porosity from conventional wireline logs. In the Madison Limestone, vug development within dolomitized sparitic carbonates has resulted in layered structures of super-permeable zones sandwiched between non-vuggy, less permeable micritic dolostones. Among the various vuggy zones of the Madison stratigraphic interval, permeability was found to vary by two to three orders of magnitude.

We also estimated the permeability in a 670-ft-thick sandstone unit within the Weber Sandstone on the Rock Springs Uplift with the objective of increasing the accuracy of our CO₂ flow simulation program. We used core data to identify the porosity-permeability relationship for the cored depth interval. On the basis of this relationship and well log data, we constructed a continuous vertical permeability profile. Seismically derived porosity values along the Weber horizon were used to model spatial permeability variations away from the RSU #1 well. The resulting statistical estimators of the permeability distribution led us to classify the Weber Sandstone as highly heterogeneous and variably permeable strata.