Geology of Shallow-Water Flow at Ursa: 1. Setting and Causes
Authors: Winker C.D. , Stancliffe R.J. 
Published: October 2008

Ursa field in the deep-water Gulf of Mexico is characterized by high overpressures close to the seafloor, resulting in very narrow drilling margins. These high near-surface pressures are attributed primarily to geologically recent (last 100 kyr) rapid sedimentation on the continental slope by the late Pleistocene Mississippi River. Overpressured sands occur primarily in a basin-floor fan complex known as the Blue unit, and secondarily in channel and levee sands of Ursa Canyon, part of a thick slope fan complex that overlies the Blue unit. Overpressures in both clayey and sandy deposits begin nearly at the seafloor and climb steadily with burial depth, typically within 60-75% of the fracture gradient, for near-surface drilling margins of only 0.2-0.5 pounds per gallon (ppg).

Introduction

In the annals of civil engineering, unconsolidated soils, excess pore pressures, and underwater environments have often presented major challenges to the installation of large structures. In the case of Ursa field in the deep-water Gulf of Mexico, engineers of the Ursa partnership had to confront all three in a combination that threatened to be a "show-stopper" for an important deep-water development project. This combination of problems is known in the drilling community by the somewhat misleading term "shallow water flow" (SWF). A more technically appropriate term would be something like "deep-water, near-surface, narrow-margin drilling," but SWF is the name that has persisted, and will be used in this discussion.

The basic conditions at Ursa are:a thick accumulation of fine-grained, low-permeability sediment that had been deposited very rapidly and recently in the geologic past;subsurface drainage inadequate for excess pore pressure to dissipate (even near the seafloor); andbecause of first two conditions, pressures nearly at the fracture gradient.

This translated not only into a narrow drilling margin, but also very low effective stress and thus low soil strength. The challenge was, therefore, in soil barely strong enough to support its own weight, (1) to drill closely spaced holes without fracturing or otherwise weakening the formation; (2) to hold those holes open long enough to run casing and pump cement; and (3) to enable the resulting structure to support the loads of risers and production hardware for as long as 20 years without intervention.

Purpose and Approach of This Paper

The basic principles of shallow water flow and its remedies have been covered in a number of earlier papers (Alberty et al., 1997; Alberty, 1999; Pelletier et al., 1999; Ostermeier et al., 2000, 2002;). Furthermore, near-surface drilling operations and SWF problems at Ursa have been described in great detail by Luke Eaton (1999a, 1999b), the drilling engineer at Ursa. The purpose of this paper and its companion (Winker and Stancliffe, 2007) is to present essentially the same engineering information in a more complete geological context. This paper describes the regional geological setting of the near-surface section, in order to explain why SWF at Ursa was especially severe.