This paper presents methods and laboratory equipment that enable a more accurate assessment of foamed cement used to provide zonal isolation in oil and gas wells. The rotating cup and rotor viscometer commonly used to evaluate foamed cement rheology can introduce a high degree of error. Such errors can lead to the conclusion that low-quality ("quality" defined as the percent of foam in the slurry) cement foams have lower viscosities than their base fluids; such conclusions have been reported by other research organizations.The following attributes of the "bob and sleeve" conventional rotational viscometer are contrasted to those of a threedimensional (3D) design, referred to as the Fann Yield Stress Adapter (FYSA).• Bob/sleeve can exhibit particle or bubble separation. The FYSA provides 3D mixing to maintain a homogeneous mixture of bubbles or particles. • Bob/sleeve produces "wall slip," a common problem with two-phase fluids. The FYSA minimizes wall slip.• Low rev/min of the bob/sleeve may not create sufficient stress to ensure velocity profile distribution in the bob/sleeve cup, resulting in significant errors. The FYSA provides 3D volume averaging shear-stress/shear-rate regimes.A key feature of the FYSA viscometer is that it directly measures the yield point (YP) of a fluid; therefore, it does not rely on statistical regression of shear stress vs. shear rate data. It also measures shear stress vs. shear rate for a variety of complex fluids.The rheology of foamed cement has a direct impact on the hydraulic properties of the slurry. The advantages of applying foamed cement are • Increased wall shear stress for better displacement of drilling mud from the annulus and removal of filter cake from the wellbore walls • Improved fluid-loss control • Prevention of gas migration and water influx during the critical gel-strength period • Reduced slurry density for protection of weak formations This paper presents rheology-testing results using the FYSA and contrasts FYSA results with those obtained using the bob/sleeve method. IntroductionIn the completion of oil and gas wells, cementing operations are employed to seal the annulus and provide zonal isolation, establishing structural integrity for the wellbore. One of the most critical factors to success in any primary cementing operation is effectively displacing the drilling fluid. In today's oil and gas industry, operators are drilling into more difficult formations and drilling highly deviated, multilateral, and high-pressure, high-temperature (HPHT) wells, which make it harder to achieve effective mud displacement.However, because of its good displacement capabilities, foamed cement can be used in many of such challenging situations to help achieve effective mud removal. In studying its displacement capabilities, foamed cement rheology has been recognized to play a huge role in achieving good mud displacement. Therefore, accurately understanding and measuring the properties and rheologies of foamed cement becomes important.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractCement integrity preservation during completion, stimulation, production, and even, during well abandonment is of critical importance for an operator from long-term economic, productivity, and safety perspectives. Traditionally, compressive strengths have been considered indicators of cement integrity. However, numerous squeeze cementing jobs regularly performed on completed wells are testament to the poor correlation between compressive strengths and cement integrity. Additional mechanical properties such as tensile and flexural strengths, elastic modulus, and Poisson's ratio are being taken into account with increasing frequency for maximizing the cement sheath performance during the life of the well. Unfortunately, all such measurements are performed on samples that have been cured, either under wellbore conditions (for example, pressure and temperature), or laboratory conditions (for example, atmospheric pressure) but tested at atmospheric pressure and temperature. Such properties may at best be useful for comparing different formulations in the selection process but do not provide information about the cement properties under downhole conditions.Using ultrasonic shear wave and compression wave combination measurements, dynamic mechanical properties, such as elastic modulus, bulk modulus, and Poisson's ratio and compressive strength, are measured under pressure and temperature. These measurements are compared with mechanical properties obtained from load vs. displacement under static conditions and acoustic compression and shear wave measurements at atmospheric pressure and temperature. Correlations are made for several slurries. The results are presented. The results also will present cases where the measurements made using this method demonstrated unique advantages over the conventional load vs. displacement techniques.

No abstract

This paper presents methods and laboratory equipment that enable a more accurate assessment of foamed cement used to provide zonal isolation in oil and gas wells. The rotating cup and rotor viscometer commonly used to evaluate foamed cement rheology can introduce a high degree of error. Such errors can lead to the conclusion that low-quality ("quality" defined as the percent of foam in the slurry) cement foams have lower viscosities than their base fluids; such conclusions have been reported by other research organizations.The following attributes of the "bob and sleeve" conventional rotational viscometer are contrasted to those of a threedimensional (3D) design, referred to as the Fann Yield Stress Adapter (FYSA).• Bob/sleeve can exhibit particle or bubble separation. The FYSA provides 3D mixing to maintain a homogeneous mixture of bubbles or particles. • Bob/sleeve produces "wall slip," a common problem with two-phase fluids. The FYSA minimizes wall slip.• Low rev/min of the bob/sleeve may not create sufficient stress to ensure velocity profile distribution in the bob/sleeve cup, resulting in significant errors. The FYSA provides 3D volume averaging shear-stress/shear-rate regimes.A key feature of the FYSA viscometer is that it directly measures the yield point (YP) of a fluid; therefore, it does not rely on statistical regression of shear stress vs. shear rate data. It also measures shear stress vs. shear rate for a variety of complex fluids.The rheology of foamed cement has a direct impact on the hydraulic properties of the slurry. The advantages of applying foamed cement are • Increased wall shear stress for better displacement of drilling mud from the annulus and removal of filter cake from the wellbore walls • Improved fluid-loss control • Prevention of gas migration and water influx during the critical gel-strength period • Reduced slurry density for protection of weak formations This paper presents rheology-testing results using the FYSA and contrasts FYSA results with those obtained using the bob/sleeve method.

Summary Cement integrity preservation during completion, stimulation, production, and even well abandonment is of critical importance for an operator from long-term economic, productivity, and safety perspectives. Traditionally, compressive strengths have been considered indicators of cement integrity. However, numerous squeeze cementing jobs regularly performed on completed wells are testament to the poor correlation between compressive strengths and cement integrity. Additional mechanical properties such as tensile and flexural strengths, elastic modulus, and Poisson's ratio are being taken into account with increasing frequency for maximizing the cement sheath performance during the life of the well. Unfortunately, all such measurements are performed on samples that have been cured, either under wellbore conditions (for example, pressure and temperature) or laboratory conditions (for example, atmospheric pressure), but tested at atmospheric pressure and temperature. Such properties may at best be useful for comparing different formulations in the selection process, but do not provide information about cement properties under downhole conditions. Using a combination of ultrasonic shear wave and compression wave measurements, dynamic mechanical properties such as elastic modulus, bulk modulus, Poisson's ratio, and compressive strength are measured under pressure and temperature. These measurements are compared with mechanical properties obtained from load vs. displacement tests under static conditions and acoustic compression and shear wave measurements at atmospheric pressure and temperature. Correlations are calculated for several slurries, and the results are presented. These results include cases where the measurements made using this method demonstrated unique advantages over the conventional load vs. displacement techniques. Introduction Cement slurry design and testing is used to provide a cement system that can withstand well operations. The laboratory-measured values provide input data for the engineering analysis needed to evaluate cement-sheath integrity. It is a common practice to cure cement formulations under downhole conditions, particularly at downhole temperatures, either under pressure or at atmospheric pressure, and at the end of the cure period, allow the samples to come to ambient conditions prior to testing for mechanical properties. However, the mechanical properties measured on such samples do not reflect the properties of the cement formulations at wellbore conditions. Moreover, the depressurization and cooling to ambient conditions before performing cement property measurements may have the following unavoidable consequences:introduction of microdefects into the system, andelimination of any effects (which are likely to be significant) of curing conditions (namely elevated pressure and temperature) on the measured mechanical properties. As a result, the engineering analysis based on mechanical properties measured at ambient conditions will not be a true representation of cement performance in a wellbore. The challenge of measuring mechanical properties under downhole conditions is not trivial due to the lack of suitable instrumentation that can cure and maintain the cement under downhole conditions while testing for mechanical properties. The ultrasonic cement analyzer (UCA) is the only commercially available instrument that measures compressive strengths at least at downhole temperatures and pressures that are prevalent in a wellbore (Rao et al. 1982). This method is based on correlating the transit time of compression waves through cement and correlating the wave velocity with compressive strengths by crushing identically cured samples at ambient conditions using a mechanical load. Ultrasonic shear waves have been used for many years to measure dynamic mechanical properties, as well as to detect voids and cracks in concrete and rock samples under nondestructive conditions (Krautkramer and Krautkramer 1977; Leslie and Cheesman 1949). They have also been used for studying early-stage cement paste properties (Voigt and Shah 2004; Fam and Santamarina 1996). In oilfield applications, they have been used as a component of acoustic logging tools for many years. The relationships between the velocities of compressive and shear waves and the material properties of a homogeneous, isotropic, elastic solid are shown in Eqs. 1 through 4. Shear waves do not propagate in liquids and gases, and therefore shear wave velocities in a fluid medium are zero.