Complex chemical reactions are occurring within a cement matrix as it becomes a solid. During the initial phase of the cement hydration, the cement exhibits polymer characteristics that exhibit a shear yield point that has been described as "static gel strength" (SGS). Due to this behavior, the cement slurry develops SGS after it has been pumped downhole. The start of gel strength development signals the point at which the cement slurry begins to change from a true hydraulic fluid that transmits full hydrostatic pressure to a solid material that has measurable compressive strength. The cement gel strength is important for two reasons:

  • The static gel strength development determines the shut down safety factor on the job. If the cement slurry is stopped prior to placement, then the static gel strength allows the calculation of the pressure required to restart circulation.

  • The static gel strength affects the hydrostatic pressure distribution and the flow of gas or water into the cement filled annulus, known as fluid or gas migration.

This paper describes the discovery that as some slurries develop static gel strength, the attenuation of a high frequency acoustic signal transmitted through the slurry decreases. This change in amplitude correlates with the actual static gel strength of the slurry.

An acoustic method and system for determining the static gel strength of a cement slurry sample has been developed that provides nearly continuous, accurate, non-mechanical measurements of the static gel strength of cement slurry samples. The measurements are made at wellbore temperatures and pressures up to 400°F, 20000 psig.

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