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

Abstract

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. This 30,000 ft well, drilled from the Ursa TLP at a vertical depth of 18,000 ft and a horizontal displacement 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, a subsequent sidetrack and two mechanical bypasses, a number of significant hole problems materialized which caused extensive non-productive time 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 Gulf of Mexico's challenging geopressured environment. In all, at least five independent borehole failure mechanisms were encountered while drilling the original hole 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-10 original hole. Lost circulation in induced fractures, with associated heavy mud losses. Borehole fatigue, caused by stress cycling on weak formations due to 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 & 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 (for hole cleaning, ECD management, sag control etc.), and 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 in order to help the future drilling of extended reach wells in geopressured, low-margin deepwater environments.

Subsurface Containment Assurance Program - Key Element Overview and Best Practice Examples

“…The multiple borehole instability and lost circulation issues identified for the Ursa A-10 well by Gradishar et al (2013) illustrate the diversity of potential failure mechanisms that might be encountered in deepwater environments. Shear failure (i.e., faulting) of a fractured shale occurred by invasion of higher-pressure synthetic drilling mud into the existing fractures, decreasing the effective stress and triggering faulting.…”

“…As a result, formation (pore) pressures can exceed hydrostatic values which, in combination with small values of least-principal stress S 3 (corresponding to S hmin in a normal-faulting tectonic regime), define a narrow drilling window (given at the specified depth by S 3 minus pore pressure) (e.g., Heppard et al, 1998;Smith et al, 1999;Shadravan and Amani, 2012). Narrow drilling windows are especially troublesome when drilling deviated and long-reach wells in deepwater environments, where changes in the magnitudes and orientations of the local stress state near salt bodies, shales, and other subsurface geological heterogeneities can potentially lead to wellbore enlargement and lost circulation, as documented, for example, at Shell's Ursa field (Gradishar et al, 2013).…”