Study Investigates Casing Wear in the Culzean Field

Abstract

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 206221, “Casing Wear: Prediction, Monitoring, Analysis, and Management in the Culzean Field,” by Florian Aichinger, SPE, Loic Brillaud, SPE, and Ben Nobbs, SPE, H&P Technologies, et al. The paper has not been peer reviewed. _ This paper presents predicted vs. measured wear for six wells in the Culzean field, a high-pressure/high-temperature (HP/HT) gas condensate field in the central North Sea. The focus of the complete paper is on the monitoring and analyzing process, particularly how to use offset data to improve accuracy of casing-wear prediction. Field and Well Background Wells in the Culzean field are quite deep, with a measured depth (MD) of 15,000–16,500 ft, and are J-shaped with a slight sail angle (inclination 30–40°). These features result in significant tension loads at the buildup and, potentially, at unplanned curvatures in the vertical section. Combined with possibly high rotating hours, these factors create a considerable risk of high casing wear. Additionally, a challenging pressure regime is present. The steep pressure-transition zone (PTZ) overlying the target reservoir, and the expected predrilled narrow mud-weight window between the pore pressure and formation at the top of the Pentland reservoir of 0.060 ppg, called for a new well-design approach. To address this challenge, the production casing was designed as a 10-in. production liner and a 10¾×10-in. tieback production string. The well design and operations plan required drilling both 12¼- and 8½-in. sections without the 10¾×10-in. tieback production string installed. Consequently, the 14×13⅝-in. intermediate casing could be exposed to higher pressures than usual. Extensive work was performed to enhance casing-wear prediction and to determine the likelihood of failure if a well-control risk materialized. The 12¼-in. section and 10-in. production liner shoe were placed as deeply as possible into the PTZ in the Upper Valhall/Tuxen formation. An appreciable risk was the possibility of a kick while drilling the 12¼-in. section from the Pentland reservoir. The most extreme well-control event the 14×13⅝-in. intermediate casing could experience, however, would have been a well-full-of-gas event while drilling the 8½-in. section. The assessment of the risk associated with such an event was addressed by careful attention to an assessment of casing wear and remaining casing wall thickness and the probability of failure of the 14×13⅝-in. casing, assuming that the casing was worn. The failure-probability assessment of the 14×13⅝-in. intermediate casing was performed using a reliability-based design approach using survival design principles and probability-of-failure estimation. Methodology: Wear Computation and Measurement Analysis of casing wear between a drillstring and the inner surface of casing requires application of an appropriate casing-wear model. A stiff-string model, as used in this work, is advantageous because it can consider pipe clearance, pipe stiffness, and axial and radial casing contact-point location. The simulation uses an adaptable 3D mesh, allowing the calculation of wear in all radial orientations for every chosen MD. The multifinger-caliper-log (MFCL) interpretation methodology uses inner-diameter measurements to calculate an ellipse that best fits the casing shape. The methodology does not require a base log and increases accuracy compared with the traditional method of comparing the maximum inner diameter of MFCL logs taken before and after drilling. The methodology also can be applied to sonic logs as a second information source apart from the direct wall-thickness measurement if the latter is not available. A detailed overview of this process is included in the complete paper.