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Uncertainty Analysis in Capillary Controls Across Faults


An important fluid-rock interaction is the capillary control on hydrocarbon flow retardation by fault rocks formed by the processes of gouge or smear. These capillary controls impact both cross-fault flow between juxtaposed reservoirs, and up-fault flow for hydrocarbon migration at the reservoir and basin scale. A typical work-flow for evaluating capillary controls on faults cutting siliciclastic rocks is to estimate the clay content from algorithms such as shale gouge ratio or clay smear factor, convert these to capillary threshold pressure, and determine fault rock seal capacity. The clay content distribution across the fault is determined from the clay in the protolith and the fault throw. Uncertainties in this analysis may be introduced by the conversion of the clay content to flow properties, changes in the stratigraphic model, along-fault variation in fault geometry and throw, and fault rock heterogeneity. Introducing these uncertainties into the analysis, however, is often time consuming and tedious to address in 3D models. The stratigraphic distribution is generally modeled from local well data, seismic stratigraphic interpretation, and analogues, but the uncertainty in the clay estimates for the range of possible scenarios remains large. An alternative discussed here is running multiple realizations of base-case models of stratigraphic variation, applying different algorithms for predicting clay content and varying the throw. A range in the fluid properties such as density also defines a range in the predicted free-water level distributions. We define a methodology for the rapid evaluation of these important uncertainties by using multiple realizations based on triangle diagram analysis from single or multiple input wells. The analysis requires only well data and fault throw without the constraints of a 3D model. The methodology described results in more efficient calculation of the uncertainties and expected ranges in properties that influence capillary controls on flow.