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.

A modified cellulose-based polymer has been tested according to API recommended practices in various cement slurries and was identified to have multiple beneficial functions in addition to be environmentally friendly. Test results demonstrate that this single additive controls fluid loss better than commonly used fluid loss additives at temperatures up to 170 °F while also controlling free fluid and performing as an extender. In addition, it was found to work as a foam stabilizer and gas control agent in cement slurries, which was not observed for any other cellulose-based polymers tested. The modified cellulose-based polymer also exhibits less slurry viscosification, which facilitates surface mixing and pumping of the corresponding cement slurries. This paper will describe test procedures, discuss test results, and demonstrate that a single modified cellulose-based polymer can replace several additives in a cement system to adjust the required cement slurry performances for optimum placement and properties in the wellbore. These findings are accomplished by a case history showing the successful application of the modified cellulose-based polymer for a deepwater cementing operation in the South China Sea. The here presented multifunctional biopolymer simplifies cement slurry design and operations contributing to higher quality cement jobs.

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The study presents an innovative cement spacer fluid based on microemulsion technology and an operationally simple cement design using a water-based multifunctional polymer. A case history is described where their combination was successfully applied on a deepwater exploration well in the South China Sea. Laboratory testing, modeling, and engineering design that preceded the field operation are outlined. The spacer's performance to clean the mud from contact surfaces was verified with the goniometer method. Mud/spacer and spacer/cement tests for optimum compatibility were conducted and a fluid friction pressure chart for the mud-spacer-cement train at different displacement rates was generated. The results show that the designed spacer is highly effective in displacing the mud and converting an oil-wet surface to a water-wet surface, and therefore to provide a clean and water-wet surface to which cement can strongly bond. A water-based multifunctional polymer in the designed cement slurry was tested to validate its ability to adjust slurry properties for deepwater challenges. The cement slurry was easy to mix at surface, stable under downhole conditions, and had a sufficient short transition time at low temperature, preventing water and gas intrusion. Furthermore the evaluated multifunctional polymer was found to work as a stabilizer and extender as well as provide very good fluid loss, free fluid, and gas control. As a consequence, the multifunctional polymer reduces the total number and amount of required chemicals, thereby simplifying logistics and operations for deepwater wells. The presented spacer and cementing technologies contribute to successful zonal isolation of deepwater wells and so minimize risks as well as expensive rig and nonproductive times due to remedial work.

Modified surfactants, in the form of ethoxylate-coated resins (ECR), serve as the foundation for fluid loss additive systems that are functional in a wide range of applications and environments. ECR's, when combined with polyvinyl alcohol, produce fluid loss additives that:are minimally retardive,can be placed in turbulent flow, andpossess anti-gas properties. Blending ECR with hydroxyethylcellulose yields a multi-purpose fluid loss agent with higher efficiency, greater salt tolerance, and improved thermal stability as compared to conventional fluid loss additive systems. Presented are the laboratory data substantiating the use of surfactant enhanced fluid loss additives, as well as supportive case histories demonstrating the utility of these systems in field cementing operations. Introduction Fluid loss control agents are commonly employed in field cementing operations to lower the rate of cement filtrate loss under dynamic (and static) conditions. In the presence of differential pressure, filtrate loss to permeable strata can dramatically alter the physical properties of oil well cements. Thickening time, rheology, and mud displacement efficiency are all impacted by the changes in the water-cement ratio brought on by the loss of cement filtrate. As the liquid phase of the cement passes into the formation, a deposited layer of solids (filter cake) is formed on the formation face. Fluid loss additives function primarily by promoting the deposition of a low permeability filter cake, thereby limiting the rate of filtrate loss to permeable strata. The uncontrolled loss of filtrate during the cementing operation can result in:job failure due to dehydration of the cement into an unpumpable state;high equivalent circulating densities;an increased likelihood of annular gas migration; orunsuccessful squeeze operations. After the cement has been placed, the continued loss of filtrate increases the solid-water ratio which impedes the transfer of hydrostatic pressure to the formation. This inability to transmit a full hydrostatic pressure prior to strength development is a primary cause of annular gas flow. P. 321^