Estimation of recoverable reserves: the geologist’s job

Abstract

The accurate prediction of recoverable reserves in the North Sea has proved to be a difficult task for many operators. This paper summarizes the performance of a number of fields by tracking the way in which reserves have changed with time, compared to the volume of reserves thought to exist at the time that development approval was given. In many fields reserves have changed by between 20 and 70%, with the areal distribution also exhibiting considerable variation from that in the field development plan. The changes have been both positive and negative. In the latter case, the economic consequences of a significant reduction in reserves are obvious. However, increases in reserves also bring financial penalties in poorly sited and sized facilities, additional well requirements and inefficient use of funds.Although this work has shown that there are a number of reasons for poor estimation, the underlying cause is usually insufficient recognition of reservoir complexity and thus the use of simple or inappropriate models to predict reserves. It is also clear that there are geological factors that are common to fields that have exhibited large losses or increases in reserves. In the latter category are fields that are structurally simple, often deposited in submarine fan environments, where the volume of movable oil was underestimated. Fields suffering large drops in reserves are often structurally complex, highly compartmented and contain high permeability zones.It is proposed that geologists should take a much more active role in reserve estimation than has generally been the case in the past. In particular, at the appraisal stage, early recognition of complexity is vital, and understanding of the complexity should become one of the major goals of the appraisal wells. In addition, the geologist needs to become closely involved in the construction and operation of the simulation models that are used by most reservoir engineering groups to predict reserves. This is to ensure that geological features which affect drainage are correctly modelled in the simulators.