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.

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Efforts to produce better annular isolation for oil and gas wells resulted in the development of methods to quantify the induced stresses that may occur within the cement sheath of a well. Along with the ability to calculate induced stresses in a cemented annulus, is the realization that typically, if there are sufficiently induced stresses to cause a mechanical failure of the set cement, failure will likely be of a tensile nature. While the ability to predict the compressive and the tensile stresses likely to be induced in a cemented annulus is certainly a step forward for the ability to provide better fit for purpose cementing designs, it also tends to highlight a larger challenge. Specifically, even though the American Petroleum Institute (API) and the International Organization for Standardization (ISO) established recommended methods for the testing of oil and gas well cement compressive strengths, no similar standards currently exist for the testing of the tensile strength of oil and gas well cements. Without the ability to test an oil or gas well cement tensile strength in an accurate and repeatable manner, it becomes very difficult for design engineers to utilize induced stress data to determine if a given cement system possesses sufficient tensile strength to resist the induced tensile stresses. This lack of standardized tensile strength testing has led the oil and gas industry to adopt various tensile strength test methods that were originally developed for the construction concrete industry. The authors have used many of these different tensile strength test methods and devices in their own work. They discovered that very often, a cement system can yield widely different tensile strengths when tested with different procedures. In this paper, the authors review the basic construction concrete tensile tests most commonly used in the oil and gas cementing industry, and then analyze the tensile strength results obtained with the different testing methodologies. Correlations are developed between the tests that allow design engineers to better compare cement systems when the subject slurries have been tested with different tensile strength test methods. Introduction Given no current API test procedure for the determination of cement tensile strength, the authors, like most others involved in the pursuit of cements with enhanced mechanical properties, relied on test methods outlined by the American Society for Testing and Materials (ASTM), which has already developed tensile strength tests for construction concrete. In the course of evaluating various cement compositions and additives for the enhancement of ultimate tensile strengths, the authors had the opportunity to view tensile strength test data from a third - party lab on a particular organic cement additive that was similar to a product tested in their own laboratory. The representative of the company that produced the organic additive was very excited with the data obtained from the third-party lab, because it showed significantly higher tensile strengths than for similar materials tested by the authors. At first the authors were somewhat confused by the tensile strength data from the third-party lab, because it tended to show much higher tensile strengths than their own test results. Even though the physical properties of the two materials were slightly different, and perhaps could have accounted for the differing results, the authors decided to look further into the tensile strength test methodology used in the two laboratories, in an effort to determine if anything obvious could account for the widely varying test results.

Recent advances in oil and gas cementing technology allow for the modeling and prediction of both compressive and tensile stresses upon an annular cement sheath, throughout the life of a well. Given the knowledge of the type and magnitude of stresses likely to be encountered in a specific location in a wells annulus gives designers target parameters for designing the mechanical properties necessary in the set cement to be able to sustain those stresses without failing. Such a mechanical failure in a cement sheath can cause a loss of annular isolation. However, the authors feel the ability to model these stresses is only one-half of the information necessary to design cement systems for long-term zonal isolation. While some good work has been done on certain lower density cement systems in an attempt to develop fit-for-purpose designs with improved tensile and flexural strengths, the authors have found that some wells requiring higher density cement systems, also need cements with "enhanced" mechanical properties. Towards this end, the authors have conducted mechanical properties research of several relatively common cement additives. These included organic materials as well as non-organic materials. For this study, these materials were added to oilfield cements with water contents averaging from 50 to 66 % by weight of cement (bwoc). Besides the more common unconfined compressive strength tests, the samples are also subjected to tensile and/or flexural strength testing. While the API has long ago established procedures for running unconfined compressive strength tests, there are currently no API standards in place covering the testing methodology for tensile and/or flexural strengths of oilfield cements. Accordingly, the authors present not only the mechanical properties achieved with the use of the various materials tested, but also the methodology used to achieve their data. In an effort to more closely scrutinize the effect each individual material has on the mechanical properties of the set cement, each additive is examined independently. Armed with this information, design engineers should be equipped to propose cement systems that produce effective long-term zonal isolation at the induced annular stresses of their own wells. Introduction In the process of oil and gas well drilling various types of cement systems are being placed into the annular space between the casing and the formation. The purpose of this cement is to structurally support the casing string and prevent casing corrosion, as well as to create a competent hydraulic seal for long-term zonal isolation during the entire operational life of the well. As mentioned by Ravi1, the cement should meet a wide range of short-term criteria such as free water, thickening time, filtrate loss, gelling, strength development, shrinkage, etc., as well as certain long-term requirements like resistance to chemical attack, thermal stability and mechanical integrity of the cement sheath. In today's oil and gas fields, it is common to find design engineers who understand that changes throughout the life of a well can significantly impact induced stresses on the annular cement sheath responsible for maintaining annular isolation. Changes in wellbore stresses can affect the mechanical integrity of the cement sheath and can be caused by a variety of different factors such as:production rate changesdepleting reservoirsformation compactionworkoversstimulation treatmentspressure and temperature changessecondary and tertiary recovery methods

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