SummaryUndulating wells are one of the alternatives to horizontal wells in field developments for extremely low-vertical-permeability formations. Undulating wells offer higher reservoir exposure, and they do not depend on vertical permeability as much as horizontal wells do. This makes an undulating well trajectory sometimes more attractive than horizontal wells.In this paper, we will present two different approaches to predicting undulating-well performance: (1) an analytical model that uses a segmented vertical-well model with a slanted skin factor to account for the effect of undulation and (2) a more-comprehensive model that evaluates undulating-well performance in a parallelepiped reservoir on the basis of a line-source solution. The analytical model is simple and works well under certain formation- and well-structure conditions. The line-source model is more flexible on permeability assumptions and well location. We compared the results of the two approaches to identify the conditions in which the analytical model can be applied.Field examples are used in the paper to illustrate optimizing undulating-well performance by the model. The Cosmopolitan field, Cook Inlet Alaska, and the Oooguruk field, North Slope, Alaska, are planned for development with undulating horizontal wells. Both reservoirs are challenged with relatively low permeability, low kV/kH ratios, and moderate-viscosity oil. Undulating horizontal wells are needed to overcome these issues and deliver commercial production rates. The performance model developed in this study demonstrated the additional value of an undulating well over a conventional horizontal well, which provided key support in the well-design and field-development plans.
