Marlin Failure Analysis and Redesign: Part 2—Redesign
Authors: Ellis R. C. , Fritchie D. G. , Gibson D. H. , Gosch S. W. , Pattillo P. D. 
Published: December 2007

This second of three related papers addresses applying the failure analysis from the first paper to the remaining Marlin wells. The fact that all five initial Marlin penetrations were predrilled up to the completion stage limited mitigation options. Within the limitations posed by predrilling, well design concepts were developed and screened using agreed-upon risk-acceptance criteria for health, safety, and environment (HSE); do-ability; and operability. On the basis of the risk profile and cost associated with each option, a vacuum-insulated tubing (VIT)/fiber-optic completion concept was selected. This paper focuses on the VIT redesign process.

Following the review of the initial investigation of the tubing/ casing failure on the Marlin A-2 well,1 this paper addresses application of the failure analysis to the remaining Marlin wells. The fact that all five initial Marlin penetrations were predrilled up to the completion stage had the following consequences:

The remaining wells can be expected to be exposed to the same loads as the failed A-2 well. The remaining wells have the same vulnerability as the failed well because only two material differences in well design exist between Well A-2 and the subsequent A-3 through A-6 wells. The 10¾-in., 60.7-lbm/ft tieback casing in Well A-2 was grade P-110; on subsequent wells, it was grade N-80. Well A-2's intermediate-casing (tapered 13⅜-×10¾-in.) cement top was not designed to go into the preceding 16-in. liner, whereas on subsequent wells, the intermediate-casing (tapered 13⅝-×10¾-in.) cement top was designed to go inside the preceding 16-in. liner, with collapsible foam strapped to the outside of the uppermost 17 joints of the 10¾-in. section. The shallower cement top was intended to ensure isolation of a potent, hydrocarbon-bearing zone. Although calculated cement tops were not verified by bond logs, full returns were experienced on three of the remaining four wells. Apart from the redrill option, one does not have complete freedom to devise solutions for the remaining completions.

Within the limitations posed by predrilling, well design concepts were developed and screened with agreed risk-acceptance criteria for HSE, do-ability, and operability. Based on the risk profile and cost associated with each option, a VIT/fiber-optic completion concept was selected.

After the VIT well concept was selected, an extensive assurance plan was initiated that involved the physical testing and analysis of every component in the new well design. This paper summarizes the redesign process, including insights on:

The analysis of annular-fluid expansion (AFE), focusing on the sensitivity of the calculation to fluid and formation properties. The importance of thermally modeling various types of completion fluids. The performance of tieback casing connections in less-conventional combined-load regions, such as compression and external pressure. Full-scale qualification testing of VIT to obtain overall heat-transfer coefficients for both the tube body and the coupling.

The final paper in this series2 discusses the implementation and assurance of the solutions resulting from this discussion.