Schlumberger

Case Study: Isolation Scanner Service Determines Cement Condition to Guide Casing Retrieval

Comprehensive evaluation quantifies cement condition in Gulf of Mexico deepwater well to increase cut-and-pull operational efficiency

Challenge: Accurately characterize the completion environment of multiple cemented and uncemented concentric casing strings to optimize the retrieval program.

Solution: Comprehensively evaluate the cement condition and casing wear in a single run of Isolation Scanner cement evaluation service.

Result: Characterized casing eccentering and mud solids settling using Isolation Scanner evaluation to help determine the force required to recover the casing.

Retrieving noncemented casing

An operator in the Gulf of Mexico needed to retrieve noncemented 7-in 38-lbm/ft casing inside a 9 5/8-in 43.5-lbm/ft outer string. Conducting an efficient cut-and-pull operation required thorough understanding of the completion environment to determine the force needed to recover the casing, especially if “barite sag” had occurred. Mud sags when the original drilling fluid in the noncemented annulus degrades over time. The solids settle to the lower portion of the hole or to changes in deviation (doglegs). This settled drilling fluid effect essentially cements the pipe in place.

Comprehensively evaluating cement and casing

Isolation Scanner service combines the classic pulse-echo technique of ultrasonic tools with flexural wave attenuation to definitively evaluate material adjacent to the outside of the casing. The unique third-interface echo (TIE) that arises from the flexural measurement is used to assess casing positioning within the borehole. Isolation Scanner service’s evaluation also pinpoints casing corrosion, erosion, and drilling-induced casing wear to comprehensively evaluate the cased hole environment in a single logging run.

Quantifying casing conditions

The interval from X,000 ft to X,300 ft was logged with Isolation Scanner service. TIEs from the cement/formation interface accurately characterize the cemented zone and the outer casing, confirming cementation of the lower annular zone. The 7-in casing is shown to be poorly centralized, even with mechanical centralizers on each joint. As evident from the galaxy patterns on the cement map image, the 7-in casing is in almost continuous contact with the outer casing on the 180° side of the 3D images of the flexural waveforms.

The high impedance measured for the annular space is consistent with the presence of heavy Class G cement or highly packed mud solids. The measured impedance is greater than 6.5 Mrayl, which is equivalent to more than 3,500-psi [24-MPa] compressive strength for the cement. Significant mud sag is detected across an otherwise free-pipe zone in the upper interval at a change in wellbore deviation. From these data and images, the contact area can be calculated to determine whether the estimated required pulling force for casing recovery is safely within the capacity of the drilling rig. The operator used this information to optimize the casing retrieval program and eliminate unproductive casing cuts.


Download: Determine Cement Condition Before Casing Retrieval (3.35 MB PDF)

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Cement Evaluation Diagnoses Mud Sag Before Casing Pulled

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