Modelling of Re-Working Carbonate Sediments under Strong Tidal Current Setting

Abstract

A numerical model has been developed which successfully simulated not only carbonate precipitation but also re-sedimentation of carbonate material under a strong tidal current environment. Carbonate reservoirs in oil fields, offshore Abu Dhabi, were deposited under such environments, so that there is difficulty to predict the reservoir heterogeneity in the exploitation stage where no more appraisal wells would be available. A numerical process model is the only solution for predicting reservoir heterogeneity under such conditions; however, most of the carbonate models that have been developed in recent years take account only of the generation of carbonate material under specified depositional environments. Sedimentation by tidal currents, which is similar to clastic sedimentation, was also neglected in the models because of its lesser importance for carbonate reservoir characterization. The present model estimates in detail the distribution of tidal current velocity in the field by using a commercial flow model with multi-layered systems. For a simplification, only the bed load transportation was taken into account for transportation of carbonate sediments. It was possible to formulate the effect by bed load transportation as a function of water depth, grain size and current velocity at each point for a given grid system. For simulation of a longer period of ancient sedimentation, the ‘Mean Field Method’ and ‘Error Tolerance Level’ are introduced for effective calculation of the equivalent result using longer time conditions than daily tidal cycles. The model developed was tested against the present-day geologic situation from the Bahamas where many of the input parameters can be observed. Then it is applied to the real oil field, offshore Abu Dhabi. We could reproduce the geologic sequences that are conceptually similar to the observations in both cases. The model thus has the potential to predict ancient reservoir quality at the petroleum exploitation stage.