Multizone Isolation with Lightweight Cement Slurry in a Cased-And-Perforated Well in a Mature Offshore Field, Brunei

Abstract

Effective zonal isolation in multilayered and depleted reservoir sands remains a persistent challenge in mature fields in Brunei, particularly for cased hole completions where the prevention of interzonal crossflow of reservoir fluid is critical. In the subject well, a conventional 14.0-lbm/gal Class G cement slurry required for a 7-in. production liner exceeded the lowest fracture gradient of the well. This condition created a high likelihood of losses during cementation and placed the zonal isolation requirement for a multilayered sand interval at risk. A multidisciplinary collaboration among fluid, well engineering, and subsurface teams assessed observations from offset wells and associated cementing risks. Based on this assessment, a lightweight cement slurry design was selected to reduce the cementing equivalent circulating density (ECD), maintain an effective balance between reservoir pore pressure and fracture gradient, and preserve the rheological hierarchy with the drilling mud and spacer systems. An 11.2-lbm/gal glass-microsphere Class G cement slurry system was designed and applied to mitigate loss risk during liner cementation and provide effective zonal isolation in the target intervals. Another challenge addressed included the validation of cement quality behind the liner. The presence of hollow glass microspheres required the use of an ultrasonic cement evaluation tool rather than conventional sonic tools. Other success factors included top-of-cement (TOC) design optimization to isolate the shallowest hydrocarbon-bearing zone, pump-rate optimization, reinforcement of cementing execution best practices, and a slurry design tailored for high-flow-potential sands. The first application of a lightweight cement slurry for multilayer zonal isolation in a production liner in Brunei is described in this paper, which includes the initial design-stage workflow, laboratory qualification, and execution best practices. The outcome established a new technical standard for future wells with similar depletion-related cementing challenges.