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 three-dimensional (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.

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