Modeling Imbibition Capillary Pressure Curves

Abstract Although spontaneous imbibition capillary pressures are relevant in all reservoirs having undergone or undergoing gravity stable water displacement, they are not commonly measured nor considered for reservoir modelling. Imbibition is particularly important when establishing initial hydrocarbons in place for reservoirs already having residual hydrocarbon columns prior to production commencement. As part of a saturation-height study, an empirical model relating imbibition capillary pressure curves to their drainage precursors was created. Drainage saturation-height functions could then be used to create imbibition equivalents based upon the saturation history of the reservoir rock. Calibration was carried out using laboratory measurements of both drainage and imbibition capillary pressure curves. This "imbibition from drainage" (IFD) model was verified against log derived water saturations from wells in the initial study area. The match was excellent in those wells considered to have reliable petrophysical evaluations. The IFD model was even able to describe imbibition capillary pressure curves for sections of reservoir in drainage transition zones prior to the commencement of water imbibition. The model has since been further verified in other hydrocarbon accumulations found in different basins. The IFD model developed represents the first publication of a technique for creating meaningful spontaneous imbibition capillary pressure curves and imbibition saturation-height functions through reservoirs with significant capillary transition zones. The technique should be widely used to describe water saturations in reservoirs with residual hydrocarbon columns, providing better estimates of initial hydrocarbons in place than with more commonly used drainage data. Iteration of the model also allows determination of both original and current day Free Water Level locations in systems having residual hydrocarbon columns from either leakage prior to production or from gravity controlled water sweep during production.