Geology of Shallow-Water Flow at Ursa: 2. Drilling Principles and Practice
Authors: Winker C.D. , Hannaford R.J. 
Published: October 2008

Abstract
Ursa field in the deep-water Gulf of Mexico is characterized by high overpressures close to the seafloor, resulting in very narrow drilling margins. During Ursa drilling operations, the two strategies for drilling through the SWF zone were:overbalanced drilling with a riser, which tends to fracture the formation, with loss of drilling mud and cement; andunderbalanced riserless drilling with seawater followed by killing with heavy mud; this allows water flow from the sands (i.e., shallow water flow, or SWF) during drilling, which tends to wash out the sands and contaminate the cement.
Both strategies frequently resulted in flow to the seafloor outside casing and/or loss of structural support for the shallow casing, resulting in casing buckling and premature abandonment. These problems culminated in loss of the first batch set of development wells at the intended TLP site. Foundation failure leading to casing failure can result from:cement failure;loss of soil strength; andsubsidence of the overburden due to excessive sand washouts.
Ultimately, SWF was controlled at a second batch set site through the employment of additional casing strings, improved drilling practices and technology, and relocation to an area of thicker overburden and less sand.

Introduction
In the first paper (Winker and Stancliffe, 2007) we presented the geological setting in which shallow water flow (SWF) typically occurs in the Gulf of Mexico. We also established some of the parameters of the geological conditions faced by drillers, engineers, and geoscientists attempting to drill and cement a batch set of production wells prior to installation of the tension-leg platform (TLP):An interval of 600-900 ft of overpressured sands, within the recently deposited (&#x98;20 kyr) Blue unit, is overlain by a silty clay overburden approximately 1000 ft thick.The overpressure gradient is about 60-75% of the fracture pressure gradient above of hydrostatic. Within 3000 ft of the seafloor, the drilling margin is only 0.2-0.5 pounds per gallon (ppg), in contrast to the minimum safety margin of 0.5 ppg typically used to design casing programs.Different sands in the Blue unit are on different pressure gradients, indicating that inter-sand pressures in the Blue unit have not equilibrated.Details of vertical sand distribution within the Blue unit vary between well sites. The fine-scale stratigraphy of these sands is virtually impossible to predict, even with high-resolution seismic data, because acoustic properties of sand and mud are insufficiently differentiated at these burial depths.

Shallow Water Flow: Basic Principles

Well Construction.
A deep-water well can be thought of as an upside-down building constructed remotely, underwater, at extremely high pressures. To extend this analogy, the essential components of a properly executed well are excavation, foundation, and plumbing (Figure 1). To understand the challenges presented by SWF, it is useful to consider the basic criteria of a deep-water well properly constructed through the near-surface section. Excavation (Figure 1A). The well bore should be vertical and in-gauge, without washouts, tight spots, or cave-ins. Furthermore it should be drilled without significant alteration of the formation.