Prediction of Oil Reservoir Performance And Original-Oil-in-Place Applying Schilthuis And Hurst-Van Everdingen Modified Water Influx Models

This paper predicts the future performance of an oil reservoir with no initial gas cap, being produced by a strong underlying aquifer using the Schilthuis steady state and Hurst-Van Everdingen modified water influx models. The aim of this analysis is to highlight the discrepancies in the capabilities of the Schilthuis steady state water influx model and the Hurst-Van Everdingen modified model to effectively give the reservoir engineer a thorough understanding of the effects of aquifer influx into a reservoir on the cumulative oil production and estimation of oil-in-place. This was achieved by carrying out a simulation analysis using the Schilthius steady state and the Hurst-Van Everdingen unsteady state models in the MBAL package to predict changes in the following reservoir parameters for a 20-year period. For the production period being analysed, the oil recovery factor was given as 26.76%. The difference in recoverable reserves estimated using the Schilthuis steady state and Hurst-Van Everdingen modified water influx models was 0.406738 MMSTB. This implies that in the year 2020, using the Schilthuis steady state model to estimate the water influx into the reservoir, would not be able to account for 0.406738 million stock tank barrels of oil that had been recovered from the reservoir. This is attributed to the unrealistic assumptions of the Schilthuis steady state model that the pressure of the aquifer is constant as the dynamic nature of the reservoir-aquifer system will suggest a change in pressure with time as production of oil continues in an oil reservoir. Therefore, the Hurst-Van Everdingen modified model has been proven to be a more effective tool for the reservoir engineer because it takes into consideration the dependence of pressure changes in a reservoir-aquifer system with time.