Analysis of Drilling Fluid Losses, Mitigation and Recovery while Transiting from Depleted to Overpressured Zones

Identification of the extent of potential drilling challenges that could be encountered while transitioning from depleted or normally pressured zones to overpressured zones has never been straightforward. Front-End Engineering activities such as geological evaluation/modelling, drilling fluid (mud) designs, casing shoe setting depth selection criteria (formation isolation intervals), etc. comprise the minimum design inputs required to ensure safe and cost-efficient drilling through steep pressure ramps. However, these have not resulted in guaranteed success while drilling, as was seen in several wells studied in the same field. This paper discusses the case of an overpressured well in the Niger Delta where severe mud losses of over 120bbls/hr were encountered while drilling through an overpressured inter-reservoir shale below a depleted sand. Several attempts to cure the losses and strengthen the wellbore yielded varying degrees of success especially when measured against time in static and dynamic conditions. Further analysis of the petrophysical data indicated that the upper part of the inter-reservoir shale, just beneath the depleted zone, sits in a near-hydrostatic pressure regime with lower fracture gradient while the lower part of that shale layer is significantly overpressured with, consequently, higher fracture gradient. Petrophysical data, pore pressure prediction models and wellbore integrity considerations are integrated in a novel way to yield far-reaching insights into the impact of inter-reservoir shales on well design. This, together with loss circulation treatment options, is determined to result in improved characterization of hole stability risks and overall drillability of the well.