Abstract

• Reserves estimation is heavily affected by technical uncertainty. The underground oil reserves estimation methods can be grouped in three groups. There are Analogy Method, Volumetric Method and Performance Method. Performance Method includes Material Balance Method, Decline Curve Analysis and Reservoir Simulation. In early stages of development, reserves estimates are restricted to the Analogy and Volumetric calculation. As production and pressure from a field become available, Decline Curves Analysis and Material Balance calculation, become predominant methods of calculating reserves. These methods greatly reduce the uncertainty in reserve estimates; however, during early depletion, caution should be exercised in using them. Decline Curves relationships are empirical, and rely on uniform, lengthy production periods. It is more suited to oil wells, which are usually produced against fixed bottom-hole pressure. Analysis on these methods, however provide quality assurance, for estimating hydrocarbon reserves. Different estimation methods may yield significantly different results, and reconciliation of the differences may be difficult. If there are wide differences, application of two or more methods may reveal the need for future investigation. Comparison of the reserves estimates with actual results on a post mortem basis will provide valuable learning point. The comparison must include quantitative assessment of basis and accuracy of estimate.