Directional well profile: surface-hole section
Most directional wells are drilled from multiwell installations, platforms, or drillsites. Minimizing the cost or environmental footprint requires that wells be spaced as closely as possible.
Well spacing and slot allocation
It has been found that spacing on the order of 2 m (6 ft) can be achieved. At the start of the well, the overriding constraint on the well path is the presence of other wells. Careful planning is required to assign well slots to bottomhole locations in a manner that avoids the need for complex directional steering within the cluster of wells. At its worst, the opportunity to reach certain targets from the installation can be lost if not carefully planned from the outset.
Visualizing the relative positions of adjacent wells is important for correct decisions to be made about placing the well path to minimize the number of adjacent wells that must be shut in as a safety precaution against collisions. The steel in nearby wells requires that special downhole survey techniques be used to ensure accurate positioning. This section is generally planned with very low curvatures to minimize problems in excessive torque and casing wear resulting from high contact forces between drillstrings and the hole wall.
Many directional wells are drilled from surface pads and offshore locations. Close surface locations always have the potential for collisions near the surface. Planning proper surface-hole surveying strategies to prevent collisions is critical in well planning. Gyro surveys (single/multiple shots) are often used to eliminate problems associated with close wellbore spacing. Modern well-planning software has used the survey uncertainty model in the anticollision calculations.
Traveling-cylinder diagram (TCD)
Perhaps the most important technique in collision avoidance is the traveling-cylinder diagram (TCD). The TCD provides an effective means of portraying the actual position of the well being drilled relative to its planned course and to adjacent wells. It also allows complex, 3D interwell tolerances on the allowable position of the borehole trajectory to be presented in a simple and unambiguous form. Because the original hand-drawn version was developed in 1968, various algorithms have been devised to produce the TCD in the well-planning software package.
Among the three versions of the diagram commonly available, the normal-plane TCD is the most efficient tool. The normal-plane projection displays the intersection of wells with a plane constructed in space to be normal to the direction of the planned well at the point of interest. Because of its clear and simple presentation of a complex situation, the normal-plane TCD has recently been used at the wellsite to assist the simple go/no-go decision and the visualization of collision potential without making any interpretive judgments on well convergence or survey error values.    
- Lyons, E.P. and Mechem, O.E. 1968. Design and Implementation of Directional-drilling Programs THUMS Offshore Islands Development Wells East Wilmington Field. Presented at the Spring Meeting, Pacific Coast District, Production Div., API, Bakersfield, California,, 14-16 May. API-68-075.
- Thorogood, J.L. 1980. How BNOC Controls Directional Drilling. Petroleum Engineer Intl.(May): 26–44.
- Hodgson, H. 1983. Computerized Well Planning for Directional Wells. Presented at the SPE Annual Technical Conference and Exhibition, San Francisco, California, 5-8 October. SPE-12071-MS. http://dx.doi.org/10.2118/12071-MS.
- Hauck, M. 1989. Planning Platform Wells: The Below Ground Structure. Ocean Industry 24 (5): 36–40.
- Thorogood, J.L. and Sawaryn, S.J. 1991. The Traveling-Cylinder Diagram: A Practical Tool for Collision Avoidance. SPE Drill Eng 6 (1): 31-36. SPE-19989-PA. http://dx.doi.org/10.2118/19989-PA.
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