Unconventional Method of Conductor Installation to Solve Shallow Water Flow Problems
Authors: Choe Jonggeun , Juvkam-Wold Hans C. 
Published: December 2007

In the Gulf of Mexico, in water depths from about 2,000 to 4,000 ft, a number of wells have experienced serious problems with high flow rate shallow water flows from over-pressured aquifers, typically at depths from 400 to 2,000 ft below mud line. The resulting erosion of the wellbore can cause serious borehole enlargement. Continued erosion by the water flows can result in the conductor (or structural) casing losing its support, thereby sliding downwards, causing the next casing or any inner casings to buckle, sometimes resulting in well failure that requires abandonment. One of the potential solutions to shallow water flow problems is to drive the conductor casing(s) to about 2,000 ft below mud line.

This paper presents evaluation of the technical feasibility of implementing the above method using wave equation analysis. It also includes evaluation of remolded miniature vane shear soil strength in the Gulf of Mexico for depths down to 2,000 ft below mud line. Cost analysis comparison to conventional drilling procedures is given.

The initial objective of driving the 30-inch conductor to a depth of 2,000 ft in one pass is technically feasible but time and cost would be unacceptably high with current technology. In most cases it is possible to drive a 30-inch conductor to a depth of 800 ft below mud line at a cost below half a million dollars. This amount of penetration should provide sufficient support to hang all subsequent casing strings, and to prevent downhole buckling resulting from settling or subsidence.

Introduction

The presence of over-pressured shallow water flows in the Gulf of Mexico (GOM) has resulted in numerous drilling and cementing problems. Shallow water flows encountered in deep water wells are sometimes difficult or impossible to stop because of the narrow margin between pore pressures and fracture pressures. The resulting erosion of the wellbore can cause serious borehole enlargement. Continued erosion by the water flows can result in the 30-inch casing losing its support, thereby sliding downwards, causing the 20-inch and any inner casings to buckle, sometimes resulting in well failure that requires abandonment.

In the Gulf of Mexico, in water depths from about 2,000 to 4,000 feet, a number of wells have experienced serious problems with high rate saltwater flows. Four mechanisms have been identified as causes of shallow water flows: over-pressured aquifers, induced fractures, induced storage, and transmission of geopressure through cement channels. Flows from over-pressured aquifers are typically at depths from 400 to 2,000 feet below the mud line (BML). Flows from depths outside these limits have also been reported. The flows can occur up the annulus between the 20-inch and 30-inch casings, or outside the 30-inch. Several multimillion-dollar wells have been lost due to this problem. In addition, it is feared that such flows may result in the failure of tension leg platforms because of soil erosion around the sea floor template, resulting in losses that could reach hundreds of millions of dollars.

A number of potential solutions to the shallow water flows have been proposed, and some have been tried. One proposed solution is to drive the conductor casing to 2,000 ft BML. The driving mechanism proposed originally uses a downhole mud motor and bit to remove cuttings from inside the conductor while it is being driven into place by a drilling jar. Commercial bottom-driving hammers can also be used even without a mud motor and bit. The method proposed essentially eliminates the annulus where the water flows would normally occur. An added benefit of this solution would be the control of shallow gas. The current status of other solutions such as the development of shallow water flow database, development of a permeability impairment mud system, and development of cement slurry designs is available in Ref. 2. There are other studies for successful cementing through shallow water flow zones.

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