Curved Well Conductors and Offshore Platform Hydrocarbon Development
Authors: Cox B.E. , Bruha W.A. 
Published: December 2007

This paper discusses drilling offshore directional wells with conventional equipment through curved conductor pipes. Curves and graphs are presented to compare the lateral-reach characteristics of directional wells drilled through curved and vertical pipes. Well trajectory, directional drilling data, and tubular information illustrate the success achieved by drilling wells through curved conductors.

Introduction

The 1970 Federal Offshore Louisiana Lease Sale required the development of shallow-depth hydrocarbons from sites in 150 to 400 ft of water. Because the lateral reach of a conventional vertical conductor was limited in depths ranging from 3,500 to 6,000 ft, several platforms would be required to handle these requirements. The potential for extended lateral reach prompted a review of potential for extended lateral reach prompted a review of unconventional methods to improve platform well reach. (The term, conductor, used in this paper refers to the part of the well tubulars often called drive pipe.)

Fig. 1 illustrates the reach improvement obtained when the rate of building drift angle increased from 2 degrees to 4 degrees per 100 ft (55 degrees maximum). If an angle builds faster below the per 100 ft (55 degrees maximum). If an angle builds faster below the conductor, the coverage of vertical conductors is improved noticeably, as evidenced by the area factor values in Fig. 1. Initiating well deflection above the water instead of below the conductor also improves the reach significantly.

In discussing the problem with drilling engineers, we found that a well could be drilled around a curve of 6 degrees per 100 ft if the curvature was smooth and drillstring weight below the curvature minimized. This rapid increase in drift angle through the 150- to 400-ft water zone of an offshore platform can improve the mud-line drift angle 9 degrees to 24 degrees, respectively. When an angle improvement above the mud line is combined with that below the mud line, the curved conductor becomes a very useful tool. This method was evaluated with a successful field-test well in 1970.

Well Planning Considerations

Generally, curved conductors are most advantageous at water depths of 150 ft when developing hydrocarbon reservoirs shallower than a true vertical depth of 7,000 ft. These conductors have applications not only in conventional rig development drilling, but also in exploratory drilling from platforms with conditions that normally could require the more expensive mobile rigs.

The improved reach characteristics of curved conductors has a significant impact on the choice of platform sites. Locations may be selected at some distance from the center of hydrocarbon accumulation to avoid surface and near-surface obstructions such as salt plugs, shallow faults, sea-bed reef deposits, gas seeps, shallow gas pockets, mud slides, debris, and shipping fairways. Final pockets, mud slides, debris, and shipping fairways. Final choice of platform location also is influential when considering the most efficient reservoir development and optimization of the drilling and completion techniques.

The design of an individual well trajectory is influenced by reservoir size, required maximum lateral well displacement, and drilling or completion considerations. Drilling and completion efficiency generally is reduced with an increasing wellbore angle and by tong sections with varying hole angle. The curve conductor can minimize these problems by reducing the length of wellbore with varying angle and the maximum drift angle required to reach a target.

JPT

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