A General Approach to Production Forecasting of Long-Term Reservoir Performance

Abstract Arps’ empirical equations have gained widespread acceptance due to their explicit nature and the ease of applicaton of the equations to field data. The major challenge, however, is that the analyst needs to have a prior knowledge of the type of decline (whether exponential, hyperbolic or harmonic). This has long been a major concern because not only is the Arps’ decline exponent difficult to estimate, but also an erroneous estimate of the decline exponent does have a significant adverse effect on the predicted rates and estimated reserves. The traditional efforts to solving this problem encompass type curves analysis with several values of Arps’ decline exponents; a trial-and-error technique; rate derivative plots and assumption of a minimum decline rate. Unfortunately, type curve analysis is subjective and non-unique, trial-and-error method is time-consuming, and rate derivative data are almost always noisy. This paper proposes a general approach that can be used to forecast long-term reservoir performance without a prior knowledge of the type of decline. The proposed approach couples the rate-material balance time relation, which was previously developed by Palacio and Blasingame, and the rate-time exponential equation. In particular, the rate-material balance time relation is used to estimate the input parameters of the rate-time exponential equation. The theoretical basis for coupling the rate-material balance time relation and the rate-time exponential equation is presented. Models for estimating the ultimate recovery and productive life of the reservoir are also presented. The validity of the rate-material balance time relation for any types of rate decline renders the proposed approach general in its applicability; consequently, the proposed approach will enable engineers to apply a single unique method to predict long-term reservoir performance regardless of the type of decline. Simulated and field data are presented to demonstrate the utility of the proposed approach.