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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Pressure events that occur after surface casing cementation, such as casing integrity testing, formation integrity testing, etc. all impose stress upon the recently set cement sheath. The magnitude of stress will depend on the pressure state, casing thickness, cement sheath thickness and mechanical parameters of the cement and formation. Should pressure testing take place during the early stages of cement curing, the tangential stress imposed by the pressure event can exceed the tensile strength of the cement, thereby inducing cement sheath failure. It is well documented that cement is much stronger in compression than in tension. In most wellbore pressure scenarios, cement fails in tension. The proportionality between the compressive strength and the tensile strength of set cement is generally assumed to be an 8:1 to 10:1 ratio. During typical pressure testing events, the cement will have a compressive strength ranging from the 500 psi required for the commencement of drilling operations to upwards of 2000 psi depending on the cement curing time. Accordingly, conventional wisdom would hold that the tensile strength of the cement would be in the range of 50 to 200 psi at the time of casing pressure testing. However, accurate prediction of the degree of pressure induced cement sheath stress requires more than a general correlation to derive cement tensile strength. This paper characterizes the early-state physical properties and mechanical behavior of accelerated API Class A, G, H and ASTM Type I cement designs during the twelve hours following placement. The confined/unconfined compressive strengths, ultrasonic compressive strengths, tensile strengths, Young's Modulus and Poisson's Ratio of four commonly used "tail" cements as a function of time are presented. Predictions of induced stress in typical casing/hole size combinations as functions of pressure are also included. The results from this testing provides guidance as to when pressure testing of the casing/formation can take place without inducing damage to the set cement sheath. Introduction Surface casing used in well construction serves to isolate unconsolidated formations and fresh water aquifers found at shallow depths. The surface casing prevents contamination of groundwater by drilling fluids used in the well and produced fluids such as oil, gas or brine. Additionally, the surface casing is the structural foundation of the well, as it is often the first casing string to which the blowout preventers are connected[1]. Surface casing cement performance requirements such as compressive strength at time of drill out or at a particular time interval, free fluid, etc. are stipulated by the responsible state or federal regulatory authority. Typical surface casing cement designs incorporate economical volume extended slurries mixed at 11.5 - 13.5 lbm/gal ("lead"cement) followed by a "tail" cement mixed at 14.8 - 16.5 lbm/gal (depending on cement type) that is placed in the lower section of the casing-wellbore annulus. Accelerators, such as calcium chloride, are often used to reduce the slurry thickening time and enhance early compressive strength development, thereby minimizing waiting on cement (WOC) time. Once the cement is in place and set, maintaining annular isolation will depend upon the mechanical behavior of the cement and formation and the stress conditions under which the cement sheath is placed. Post-cementing stress imposed upon the cement sheath is most often the result of a change in the pressure environment due to obligatory casing pressure testing, formation integrity testing or a change in wellbore fluid density. Recent efforts within the well cementing industry have focused attention on the long-term competency of the cement sheath under the stress conditions expected during the life of the well[2–7]. For the most part these investigations have centered on stress induced events found deeper in the wellbore or during the productive phase of the well history. The early state physical properties and mechanical parameters of cement employed in surface casing applications have not been reported in the literature.

The oil and gas industry, by default, has been pretty conservative when it relates to innovation and drastic changes in mind-set. Mainly focused on the costly drilling and completion steps, some of the "smaller" services have been ignored. As such, we have decided to take a deeper look at nano and micro sensored technologies in other industries and potentially replicate some of this innovation, allowing the industry to take "a step" closer to smarter zonal isolation. In general, the industry is quite aware of well integrity issues that we face. Be it immediate (whilst drilling/completing), within the life of production or even during the abandonment phase. There are many statistics proving that on a global scale, there is well integrity and sustained casing pressure issues on about 30-60% of all drilled wells. And we can confirm that a majority of these are directly related to well-cementing, creating an immense impact(s), that can negatively influence overall HSE, loss of potential reserves and bottom line dollar-amount. The ability to take a close look at well cementing has only proven feasible in a laboratory environment, beyond that, the knowledge and prediction of the actual state of the zonal isolation has proven difficult, confusing or costly. Regardless of the improved best practices, enhanced logging tools or state-of-the-art technological advances in chemicals/systems – we still seem to have that unanswered "gap" – on what actually happened, when it happened and how to avoid it in the future. This paper describes the background, the thought process and the potential advantage of the proposed well monitoring ideology and current R&D efforts to improve the cement isolation measurements and real time monitoring of its properties and integrity during the well life and after its abandonment, by sensoring it and communicating back to surface.