Transition time of cement slurries is a term that has been used throughout 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 directly tied to the term right-angle-set, which relates to the speed in which slurries undergoing continuous shear go from a pumpable to a non-pumpable state. Once pumping is stopped, the profile of how cement transition from a liquid, to a gel, to a set cement changes. These changes can directly affect the performance of cement slurries to control fluid migration. With the advent of the Ultrasonic Cement Analyzer (UCA), the term "transition time" of cement slurries was redefined. UCA's have developed into an essential piece of equipment. Not only can they achieve compressive strength information, but the apparatus can also provide a continuous measurement of how cement sets in static state. This information has shortened wait on cement (WOC) time, and provides an excellent profile on how fast cement develops strength. However, the transducers in a standard UCA only provide information after the cement develops a compressive strength set. With improvement in computerization and transducers, a more sensitive evaluation of gel strength development can be studied. Another definition for transition time is the use of a static gel strength (SGS) analyzer to measure the time from which cement goes from 100 lbf/100 sq. ft (48 Pa) to 500 lbf/100 sq.ft (240 Pa). It has become an industry standard that once cement slurries reach an SGS of 500 lbf/100 sq. ft (240 Pa)., gas or other fluids cannot be transmitted through the cement. The faster that you achieve this optimum SGS, the less likely that the cement will transmit gas. This paper will establish a definition for cement transition time and discuss the misconception of only using gel strength development to control gas migration. Test data that exhibits gas tight slurries with long transition and those with short transition that allowed gas influx will be shown. Also discussed in the paper will be the advantages of cements with a short transition in controlling high-pressure water flows. Introduction The control of annular gas migration after cementing has been the subject of many studies and papers1–6. These include practical approaches, theoretical approaches, mathematical modeling and physical modeling, each concentrating on one or two specific causes of gas migration. The one thing that all these studies have in common is the fact that they all present valid conclusions, and although beneficial, have all 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. However, before we can progress, we need to make sure that we understand and are using the preferred nomenclature.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe investigation of cement integrity over life of well conditions continues to be a high priority within the well cementing industry. Increasing awareness of problems associated with cement sheath failure and subsequent loss of zonal isolation or sustained casing pressure have demanded that set cement material behavior and the coupled behavior of casing, cement and formation be more fully understood in order to make rational engineering decisions. Recent advances in wellbore stress modeling can now provide a probabilistic determination of the suitability of a particular cement design for the expected range of induced well stresses. This paper describes the cement evaluation and wellboremodeling methodologies specifically developed to predict the magnitude of tensile or compressive forces created by changing wellbore or reservoir conditions.

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe investigation of cement integrity over life of well conditions continues to be a high priority within the well cementing industry. Increasing awareness of problems associated with cement sheath failure and subsequent loss of zonal isolation or sustained casing pressure have demanded that set cement material behavior and the coupled behavior of casing, cement and formation be more fully understood in order to make rational engineering decisions. Recent advances in wellbore stress modeling can now provide a probabilistic determination of the suitability of a particular cement design for the expected range of induced well stresses. This paper describes the cement evaluation and wellboremodeling methodologies specifically developed to predict the magnitude of tensile or compressive forces created by changing wellbore or reservoir conditions.

Casing is cemented into the wellbore to provide zonal isolation of producing zones, protect fresh water zones from contamination, prevent casing collapse caused by moving salts or sloughing clays and isolate the casing from corrosive brines. In other words, casing is cemented to create annular isolation. Historically, the major physical property of the cement used to determine whether these results would be attained for the life of the well, was the unconfined compressive strength of the set cement. Recent studies have shown that there are other "strengths or mechanical properties" that are even more important to consider. These strengths include tensile and flexural. This paper discusses why tensile and flexural strengths must be considered in slurry design, along with the interrelationship of compressive, tensile and flexural stresses that occur in wells, and provides Appalachian Basin field examples, where reduced, unconfined compressive strength cement, with enhanced tensile and flexural strengths has been successfully used. Introduction The main purpose of any primary cementing job is to provide zonal isolation and hold the casing in place. There are many factors that influence the cement's ability to achieve these objectives. Hole conditioning, flow regime and pipe reciprocation are a few of the mechanical techniques which can be employed. These and others have been investigated and reported in various studies1–4. Physical and chemical properties of the unset cement slurry also have a role in the success of the primary job. Christian, et al5 and Beirute, et al6 showed how cement dehydration and its control effect the primary cementing. Sutton, et al7 demonstrated the relationship between gel-strength transition time and gas migration. The American Petroleum Institute (API) has established standards for cement slurry properties8. Historically, the only physical property of set cement that was tested was the unconfined compressive strength. Originally, it was felt that the set cement required a compressive strength at least equal to that of the producing formation9. In 1957 Craft10 published a study of west Texas and east New Mexico producing formations. He found that compressive strengths ranged from 8,215 psi to 22,500 psi. However, most set cements will only exhibit an ultimate compressive strength in the range of 5,000 to 9,000 psi. Since primary cementing jobs had been reasonably successful, the comparative compressive strength theory was dispelled. However, it was still generally felt that more was better. It had also been demonstrated that the tensile strength of the setting or set cement is of primary importance. Tensile strength relates directly to the ability of the cement sheath to hold tubulars in place. Farris11 showed that as little as 8 psi tensile strength is adequate to accomplish this requirement. Since the ratio of compressive strength to tensile strength in most cements ranges from 8 to 12, a compressive strength in the range of 100 psi is all that is needed to hold many casing strings in place.