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The velocity of cement during placement has been recognized for many years as a major factor in removal of mud and the success of cement jobs. Field results, together with recent laboratory studies, show that a very low displacement rate can be used to advantage where other techniques are impractical.
Most drilling muds are sensitive to cement and, when contacted, react instantly to forma coagulated mass of high gel strength. This mass has unique properties which are unlike those of either the mud or cement. When bypassed and left behind, it can cause unsuccessful cement jobs. The slow-pump-rate procedure results in a minimum of contamination, since the mass at the cement-mud interface remains unbroken as it moves up the hole. The system actually provides a piston-like displacement by the action of floating the mud from the hole and can effectively remove mud from large washouts and irregularities in the annulus.
The slow-pump-rate procedure for cementing service, trademarked Sloflo, has proved highly successful in liner cementing in the Gulf Coast area and in primary long-string cementing in areas where turbulent flow techniques create formation breakdown problems.
Primary cement jobs are done to secure pipe in the hole, seal off undesirable zones, isolate zones and prevent fluid migration behind the pipe. To accomplish these purposes, it is essential that a good bond be obtained between the pipe and cement and between the formation and cement. Studies of job failures have shown that poor bonding quite often is due to inefficient removal of drilling mud and to mixing of mud and cement during placement. Since the effect of turbulent flow of cement on mud removal was first reported in 1948,this placement technique has gained wide acceptance and has undoubtedly greatly increased the success ratio of primary cement jobs.
In many cases, however, mechanical limitations or well conditions make turbulent flow impractical. For example, when large washouts or irregularities exist in the annulus, cement slurry can easily be in turbulent flow in the smaller annular sections, but revert to laminar flow in the washouts. This results in mixing of mud and cement, the possibility of channeling, by-passing of the mud by the cement, and subsequent job failure.
