Shallow Water Flow Technology Update
Authors: Medley George H. 
Published: October 2008

One of the most imposing problems encountered in the deepwater drilling environment identified by the DeepStar consortium Drilling and Completions committee is shallow water and/or gas flows. Shallow flows are most common in water depths exceeding 2,000 ft. As water depth increases, fracture pressures in shallow sands 300–2,000 ft below the mud line (BLM) are more likely to be near the formation pore pressure than in wells drilled in shallower water depths. DeepStar has conducted several studies to address this issue, which have resulted in several new techniques and systems to help combat the problem. Fluid systems that reduce the permeability of the formations in question and cement slurries that are effective in sealing the annulus to shallow water flows were developed in early phases of the DeepStar project. Techniques of setting casing seats below shallow water sands were also investigated. Several of these systems and techniques have been employed in the field with varying degrees of success. Additional applications are on the verge of employment in a field environment. A review of these efforts is warranted to determine whether or not additional work is required, and if so, what tactic should be pursued.

Introduction

Shallow water flow (SWF) refers to flow of water from an over-pressured formation near the top of the hole. While these flows may occur in any water depth, they are more prevalent in water depths greater than 2,000 feet. They typically occur at depths below the mud line (BML) of 300–2,000 feet1. Control of a SWF is made difficult because the pore pressure may be very close to the fracture pressure in these shallow sands. The difference between pore pressure and fracture pressure decreases as water depth increases. This concept has long been recognized and documented in the industry, and was probably best illustrated by Bourgoyne and Holden2 (Figure 1). The actual flow of water from the over-pressured zone can be caused by anyone of several conditions3,4. Hydrostatic pressure induced by drilling fluid at a casing shoe can fracture a shallow formation, resulting in loss of hydrostatic head and a flow path for the water. Shallow, normally pressured permeable sands can become charged when they are drilled overbalanced and may flow back when circulation stops. Geopressured shallow sands may be drilled with mud weights that are not high enough to prevent flow, or they may flow through cement channels behind surface or conductor casing and actually broach to the surface. The most significant problems with SWFs are those associated with the erosion of the well structural support when the flow reaches the sea floor behind one or more filings of casing. Figure 2 shows the type of trench that has reportedly been formed by SWFs in the past3. Most of the deepwater drilling fraternity is all too aware of the catastrophic loss suffered by major operating companies in the past year because of this type of failure on a much larger scale. The potential loss of tens or hundreds of millions of dollars is the primary reason that solving the problem of SWFs is a top priority of the DeepStar Drilling and Completions committee.