Setting heavy cement plugs on top of lighter wellbore fluids is a major problem in Southern Algeria, for both drilling and work-over wells. When setting cement plugs off-bottom, for kick-off or abandonment, an average of 3 attempts are necessary before achieving a competent cement plug. The extra cost of the failed plug operations is around $75,000, plus 4 days of lost rig time. An in-depth analysis of field practices for cement plugs demonstrated that the root cause of the failures is related to poor or non-existent design of the viscous pill rheology. In effect, until recently, the viscous pill was made the same way, regardless of the cement properties and of the cement-viscous pill interface stability.

There is a natural tendency for a heavy fluid to fall through a lighter fluid placed below it and it is only by resisting this tendency that cement plugs, set off-bottom, are able to remain static until the cement hardens. The larger the difference in fluid densities or the larger the size of hole, the harder it becomes to stabilize a heavy plug.

Commonly, a viscous-pill is set between the cement slurry and drilling mud, in order to improve stability. Recent work, both theoretical and experimental, has determined a reliable estimate of the yield stresses required of the fluids to stabilize an interface separating heavy and light fluids, under any conditions of hole size, deviation and density difference between the fluids. When a viscous pill is used two interfaces: that between the cement slurry and viscous pill and that between the viscous pill and mud, could be unstable. The estimates have been turned into a computer model that can be used to design the physical properties of both viscous pill and cement slurry, so that mechanical stability is ensured.

Application of this model to cement plugging operations in Southern Algeria has given rheology and density design targets, which have been applied to both cement slurry and viscous pill. Three case studies are presented in which the top of cement was found exactly as expected, on the first plug set. Over a number of recent cement plug jobs, the success rate has improved from ~25% to close to 100%.

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