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Directionally Drilled Wells

A directionally drilled, or deviated, well is defined as a well drilled at an angle less than 90 deg to the horizontal (Fig. 3-9). Wells are normally deviated intentionally in response to a predetermined plan; however, straight holes often deviate from the vertical due to bit rotation and natural deviation tendencies related to rock types and structure.

The technique of controlled directional drilling began in the late 1920s on the U.S. Pacific coast (LeRoy et al. 1977, Chilingarian and Vorabutr 1981). Through the use of controlled directional drilling, a wellbore is deviated along a preplanned course to intersect a subsurface target horizon(s) at a specific location (Fig. 3-10). Our primary interest in directionally drilled wells in this textbook centers around their application to subsurface mapping.


Figure 3-9 Diagrammatic cross sections of (a) a simple ramp or L-shaped well, (b) a more complicated S-shaped well, and (c) a horizontal well. (Figures [a] and [b] published by permission of Tenneco Oil Company; [c] published by permission of J. Brewton.)


Figure 3-10 Block diagram showing the vertical and horizontal plan views of a well directionally drilled to a predetermined subsurface target. (Published by permission of Eastman Christensen.)

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