Transition Time of Cement Slurries

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This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 90829, "Transition Time of Cement Slurries, Definitions and Misconceptions, Related to Annular Fluid Migration," by M.J. Rogers, SPE, R.L. Dillenbeck, SPE, and R.N. Eid, SPE, BJ Services Co., prepared for the 2004 SPE Annual Technical Conference and Exhibition, Houston, 26-29 September. Cement-slurry transition time is a term that has been used in the oil industry for many years. During this time, the term has been defined, redefined, and misused to cover a wide range of cementing topics. This has led to numerous misconceptions and confusion as to what “transition time” really means. For many years, this term has been tied to the term right-angle-set, which relates to the speed at which slurries undergoing continuous shear go from a pumpable to a non-pumpable state. The full-length paper establishes a definition for cement transition time and discusses the misconception of using only gel-strength development to control gas migration. Introduction Annular gas-migration control after cementing has been the subject of many studies and papers. The two things that all these studies have in common are that they all present valid conclusions and, although beneficial, they all have failed in field applications at one time or another. These failures illustrate that although we have learned a great deal about the causes and prevention of gas migration, there is still a lot to learn. The term transition time has been used to refer to the dynamic-set profile of cement slurries as exhibited on a pressurized consistometer. Slurries that had a short transition time were those that had a “right-angle-set” on a thickening-time chart. A right-angle-set is one where the viscosity of a slurry remains relatively low through most of the test and then increases rapidly in 20 to 45 minutes to more than 70 Beardon units of consistency (Bc). Once cement movement is stopped, static gel-strength (SGS) development may occur within several minutes to more than an hour, depending on well conditions and slurry design. Wellbore temperature and pressure as well as slurry density and additives are the main factors determining how SGS develops. The term right-angle-set (a dynamic analysis) should not be used when referring to slurries with short transition times (static-state test). Another misconception about transition time is the use of the initial compressive-strength determination derived from an ultrasonic cement analyzer. In some cases, the time from 50 to 500 psi has been used as a determining factor for how fast a slurry will transition into a solid. The time to 500 psi is important in reducing waiting-on-cement time, but it may have little to do with SGS development of cement slurries. Transition time also has been described as the period during which the slurry changes from a true hydraulic fluid to a highly viscous mass showing some solid characteristics under static-state conditions. It is generally accepted that at SGS development greater than 100 lbf/100 ft2, gas or fluid intrusion into the gelled cement can occur. This is considered the start of the transition time and is the point where full hydrostatic-pressure transmission is restricted. The transition time ends when the SGS increases to 500 lbf/100 ft2. At this point, the slurry has developed sufficient gel strength to prevent fluid or gas migration into the cement column. It has been proposed that the transition time between 100 and 500 lbf/100 ft2 be reduced to less than 40 minutes to minimize the time for fluid or gas intrusion into the cement.