Setting Free the Bear: The Challenges and Lessons of the Ursa A-10 Deepwater Extended-Reach Well

Abstract

Summary This paper details the case history of the highly challenging extended-reach deepwater A-10 well, drilled in the Ursa (“Bear” in Latin) prospect in the Gulf of Mexico (GOM). This 30,000-ft well, drilled from the Ursa tension-leg platform (TLP) at a vertical depth of 18,000 ft and a horizontal displacement (HD) of 20,000 ft, targeted the Yellow sand in the Ursa-Princess section of the greater Mars-Ursa basin. During the drilling of the original hole (OH), two subsequent sidetracks, and two mechanical bypasses, a number of significant hole problems materialized that caused extensive nonproductive time (NPT) and an associated cost overrun. These problems were clearly associated with the drilling of a complex well that combined a high-deviation and extended-reach wellbore with a very narrow and pressure-depleted drilling window, characteristic of the GOM's challenging geopressured environment. In all, at least five independent borehole-failure mechanisms were encountered while drilling the OH and its successive sidetracks/bypasses, which were exacerbated by an additional complicating factor: Lost circulation in natural fractures, ultimately responsible for the loss of the Ursa A-10OH Lost circulation in induced fractures, with associated heavy mud losses Borehole fatigue, caused by stress cycling on weak formations caused by annular-pressure fluctuations Borehole instability caused by too-low downhole hydrostatic pressure, responsible for the loss of Ursa A-10 Sidetrack 1 Borehole instability caused by an in-situ fractured formation that proved hard to stabilize on wells Ursa A-10 Sidetrack 1 Bypasses 1 and 2, and ultimately forced the well to be completed in shallower Magenta sands Complicating factor: barite sag of synthetic-based mud in high-deviation wellbores, which led to exacerbation and complication of the previous failure mechanisms An extensive lookback study was carried out on the Ursa A-10 well, leading to the development of several important lessons learned and best practices [e.g., for hole cleaning, equivalent-circulating-density (ECD) management, sag control], and to the development of new systems (including novel, sag-resistant synthetic-based-mud formulations). A succinct overview of the Ursa A-10 case history and a comprehensive summary of its learnings are provided here to help the future drilling of extended-reach wells in geopressured, low-margin deepwater environments.