Factors Contributing to Cement Sheath Deposition in Casing Under Highly Deviated Well Conditions

Abstract

Summary Deposition of a cement sheath inside 9-5/8-in. intermediate casings cemented in highly deviated North Sea wells often required reaming before drilling operations could be resumed. Attempts to remove or prevent solids deposition by means of additional wiper plugs, flushes, or flow-rate variation were ineffective and led to plugs, flushes, or flow-rate variation were ineffective and led to a laboratory investigation of the factors contributing to the cement sheath formation in highly deviated wells. The laboratory study was conducted in a large-scale cement-displacement test facility previously used in similar investigations to simulate field conditions in scale-model proportions with actual field operation pumping equipment, materials, and techniques. This paper describes the identification of cement sheath deposition through case histories, test facilities, conditions, and results and indicates a satisfactory resolution for this problem. Introduction Previous studies of horizontal/deviated well cementing have focused on Previous studies of horizontal/deviated well cementing have focused on effective displacement of drilling fluid and solids from the annulus to obtain a continuous cement sheath. This study focuses on cement slurry settling within the casing string before cement slurry enters the annulus. The problems are quite similar in nature; both are characterized by loss of solids through settling from the fluid of interest during dynamic (i.e., flowing) conditions. The other investigations emphasized identification of drilling-fluid properties that control solids settling during placement/displacement. This study, however, centers on the facts placement/displacement. This study, however, centers on the facts that (1) solids are settling from the cement slurry rather than from the drilling fluid, and (2) settling is occurring in the casing string. This second point proved somewhat perplexing and was ultimately a major factor in the decision to conduct a full-scale laboratory investigation. When actions to alleviate settling were applied unsuccessfully in the field, it became apparent that the problem could be understood better by examination under controlled, laboratory-type conditions. The problem was thought to be a cement slurry or spacer-related phenomenon aggravated by the wellbore deviation angle. An examination of the cement sheath problem as identified off-shore Norway, initial attempts to alleviate the problem, and details of laboratory investigation findings illustrate the development of an effective strategy for preventing cement sheath deposition under deviated well conditions. Background Field Case Histories. In 1983, the cement sheath problem was identified on the Valhall platform offshore Norway. Well deviation angles from the Valhall platform averaged from 24 to 74; most were between 40 and 60. The first recorded cement sheath (Well A-8, Table 1) occurred at 1,900 ft and required 6.5 hours of reaming to remove. Other wells also would require reaming over the next few months, with ream times ranging from 26 to 39 hours (Wells A-9, A-10, and A-11). Initial attempts to alleviate the cement sheath problem focused on the cementing tool, which used a wiper plug design suspected of cleaning the casing inadequately, plug design suspected of cleaning the casing inadequately, Shifting to a five-wiper blade plug was no more successful at alleviating the cement sheath problem than previously used techniques, however, even when as many as five plugs were used to increase wiping within the casing (Well A-11). At this point, it became clear that mechanical remedies would not solve the cement sheath problem effectively. problem effectively. When the switch to a more durable wiper plug failed to solve the cement sheath problem, the focus was shifted to changing the cement composition. Because the cement slurry clearly did not suspend solids adequately, the fluid-loss additive was changed to increase the yield point (YP) of the slurry (Well A-12). This simple change in the cement slurry composition brought about the desired change: no cement sheath and therefore no reaming. Only one of the next eight wells cemented (Wells A-13 through A- 19) required reaming.

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“…Table 1 establishes a guideline for drilling fluid yield points as a function of hole inclination to ensure proper cuttings removal (Schlumberger, 2003). This result, as well as other industry guidelines and best practices, are supported by extensive laboratory studies (Sabins, 1993). If excessive settling or cuttings buildup is observed, then additional wiper trips may be necessary.…”