Drillpipe (DP) work strings are used for continuous operations during the drilling phase and also for various cementing operations, such as running casing, liners, casing with inner string, squeeze jobs, and balanced-plug cementing. There are reported cases of residual cement adhering to the inside walls of the drillstrings after the cementing operation is completed, and the work string is pulled out of the hole (POOH). The residual cement ultimately hardens, and this hardened scale cracks and flakes off in various sizes because of vibrations and pipe flexing. There is a possibility that the larger sizes of scale may catch in the bottomhole assembly (BHA), plugging the measuring while drilling (MWD) tools, downhole motor, and even the bit. Cement scale may also plug work strings used in cementing plug and squeeze operations. In offshore deepwater operations, pulling out a plugged BHA or work string could cost USD millions in lost time, depending on the depth of the wellbore. During primary cementing, where the DP is used to run the casing, the cement scale from the reused DP could plug the float equipment, which may cause the premature release of the wiper plugs, packoff at tight restriction, or affect the hanger setting tools and may result in a major job failure.

A primary cause for residual cement on the inside of DP is zero velocity at the wall-fluid interface caused by a no-slip condition. A 3D displacement simulator was used to perform post-job analysis for two balanced-plug jobs on a deepwater Gulf of Mexico (GOM) well to predict the residual cement. The simulator solves momentum conservation equations for the velocity field, and a convective-diffusion equation for the concentration field. The deepwater GOM well had a work string consisting of a tapered string of 6.625-in. DP (0 to 10,500 ft) followed by 5-in. DP (10,500 to 23,800 ft) and 3.5-in. DP (23,800 to 24,800 ft). The selection, provision, and proper preparation of the DPs is solely the responsibility of the operator. The effects of wiper darts and fluid volumes on the thickness of the residual cement layer were also simulated. This type of 3D displacement simulator can be extremely valuable as a prejob cementing design tool and for post-job analyses.

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