Modelling density-driven mixing rates in petroleum reservoirs on geological time-scales, with application to the detection of barriers in the Forties Field (UKCS)

Abstract

Abstract This paper discusses how to deduce the strength of intra-field barriers in reservoirs which have significant present-day differences in reservoir fluid density. The method combines a knowledge of the geological time available since filling, with estimates of the rate of density-driven mixing. If the observed density differences are inconsistent with a barrier-free reservoir model, we show how limits can be set on the strength of the barrier needed to account for any observed lack of mixing. A case history from the Forties Field is presented, illustrating these principles. It is shown that a transmissibility barrier of at least 1% must be included to account for the present-day lack in density equilibration between Forties and SE Forties. The results are validated against pressure decline data since production start-up. Two methods of estimating density-driven mixing rates are given. One is based on an analytical formula applicable to a rectangular geometry, and the other uses an unmodified black oil simulator. One significant result is the weak dependence of mixing rates on vertical permeability over a wide range of values, for many typical reservoir configurations. This implies that in many geological circumstances, subsurface density differences (coexisting at the same depth) can be used to infer the presence of intra-reservoir barriers.