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.

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

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractRecent 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-forpurpose 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.

Providing competent hydraulic isolation between multiple reservoir sections in horizontal wellbores represents a difficult industry challenge. This paper will discuss some of the practices and tools used for cementing production liners in critical horizontal wells. Issues with bore hole conditioning, ovality, effective solids removal, and hole collapse are more problematic in horizontal wells. Historically issues occurred while running and cementing highly deviated liners. This was true before and after the cement placement including the premature setting of cement, an inability to run the liner to bottom, difficulties in setting the hanger assembly, pressure in the tubing-casing annulus, and cement channeling. Proper pre-job preparedness including pilot testing of the fluid systems, mud conditioning practices, centralization, cement placement simulations, and well site execution procedures for cementing these liners will be discussed. The practices to be presented have been executed for horizontal as well as deep vertical gas well applications. Scorecards were developed to measure and assess the processes used. This work explains how this effort as well as the cementing service delivery practices have led to successful jobs. The recommended practices have a positive business impact by minimizing or mitigating operational complications, saving rig time, and reducing the need for remedial work due to poor primary cementing operations. The paper highlights how the combination of enhanced cementing practices in conjunction with new cementing tools and technologies made important contributions to delivering effective zonal isolation in a highly challenging wellbore environment.