Viscous-Pill Design Methodology Leads to Increased Cement Plug Success Rates; Application and Case Studies from Southern Algeria

Abstract

Abstract 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%. Introduction Plugging oil wells is a very common operation. Every well must at some time be abandoned and this usually involves setting cement plugs at several depths. Renewed interest in recovering more oil from well established fields has lead to an increase in sidetracking from existing wells and this commonly requires a cement plug to be set to kick off from. Problems in drilling a new well are also sometimes addressed by using cement plugs, for example to cure lost circulation. Whilst certain plug cementing tasks are viewed as being straightforward, setting cement plugs off-bottom is an operation that is beset with difficulties. Off-bottom plug cementing operations have long1,2 been accepted as a task in which several attempts will be made in order to set a successful plug. An industry average figure for setting cement plugs is frequently quoted as 2.4 attempts per successful plug3,4. However, the failure rate often varies considerably according to local conditions. In Southern Algeria, where the focus of this paper lies, prior to the program adopted by the authors, the average success rate for setting off-bottom plugs was 25%, leading to an average extra cost per plug of $75,000 plus 4 days of lost rig time. Causes of failure have been investigated by several authors3–7 and include the following:Poor mud removalPoorly designed slurry propertiesIncorrect slurry volumePoor temperature estimationPoor job execution and placementInstability of the interfaces and fluid swapping Note that the last of the above problems is only prevalent for off-bottom plugs.