Determination of Cutoffs and Implications in Integrated Reservoir Studies

Abstract The building of 3D models from seismic, geological, petrophysical, well, drilling, reservoir and production engineering data which are used in the study of optimal recovery of hydrocarbons from petroleum reservoirs has become commonplace. During the static modeling phase of the workflow, there is the need for the application of limits on the petrophysical properties to differentiate reservoir from non-reservoir rock. These limits are called cutoffs: limiting values of petrophysical properties in the static models. In this case study, multiple regression analysis was used to derive the best relationship of the form k = f(Ø, Vsh, Sw) which was then used to estimate optimum petrophysical cutoffs for the reservoir. Four static models of the reservoir were then built: the first model with only a Vsh cutoff of 0.42 to delineate reservoir rock from the non-reservoir rock. The other 3 utilized various Ø, Vsh and Sw cutoffs in their building. Four dynamic models of the reservoir were then built from the static models and were history matched to observed production and pressure data. Performance prediction was then carried out using the different history matched models. Finally, using the results of the forecast and other data, an economic analysis was carried out to determine the economic implications of the different cutoffs on the results of the studies. The results showed that the values of the Ø cutoff affected the values of the corresponding Vsh and Sw cutoffs as well as those of the HCIIPs, with the general trend being that a reduction in the Ø cutoff increased the values of the Vsh and Sw cutoffs. The economic analysis showed that petrophysical cutoffs used in the development of the static (and the dynamic) models sure had some impact on the results of the analysis.