Barite Sag Occurrence and Resolution during Angolan Completion Operations

摘要

Barite sag or solids sag is a phenomenon where by a fluid sheared at low flowrate does not have the viscous properties necessary to keep the solids within it in suspension. The solids can subsequently fall out of suspension, which can lead to an undesireable build-up of solids in the wellbore or density contrasts within the fluid. During drilling activities solids sag can be avoided by increasing flow rates, rotating the drill pipe, and viscosifying the drilling mud. However, during completions operations, none of these mitigating practices may be possible due to limitations of the setting tools or installed equipment deployed into the well. For open hole completions where sand screens are deployed in highly conditioned muds, solids sag has the potential to be a serious issue, and needs to be avoided. Since 2009, there have been at least four incidents of solids sag occurring during completions operations in the Greater Plutonio development in deepwater Angola, at relatively low fluid densities. Each event manifested itself in a different way, and caused different problems during and after installation of the completion equipment. This paper presents case studies of the solids sag events that occurred during completions operations, and how they either affected installation operations or well productivity. It then describes the work done to eliminate the solids sag, including fluids testing performed to identify it, changes to operational practices, short term modifications to mud rheology, and long term mud solutions.