When boreholes are deviated, the true vertical depth can be calculated by Strater. Several calculation methods are available. Each method creates a mathematical approximation of the true X, Y, and Z value along the borehole path. Each technique is used in different circumstances. Available options are Tangential, Average Tangential, Balanced Tangential, Radius of Curvature, and Minimum Curvature.
The tangential method is probably the most simple method for computing true vertical depth. This method uses only the inclination and direction measured at the lower end of the borehole. The borehole path is assumed to be a straight line throughout the course. This method has historically been used more than any other, but is the least accurate. Boreholes calculated with the tangential method often appear too shallow and the lateral displacement along the borehole is too large. In a typical deviated borehole, the true vertical depth can be wrong by more than 50 feet. The error is minimized if short intervals are used between points.
The average tangential method uses the inclination and direction measured at the top and bottom of the borehole and averages the two sets of measured angles. This averaged angle is used over the course of the borehole. This method is very simple and provides a more accurate calculation than the tangential method. The longer the distance between survey points, the greater the error in the true vertical depth.
The balanced tangential method uses the inclination and direction at the top and bottom of the borehole and averages the two sets of measured angles. Unlike the average tangential method, this method calculates the inclination at the top and bottom and combines them in the proper sine or cosine functions before averaging. This technique provides a smoother curve than either the tangential method or the average tangential method and more closely approximates the actual borehole trace between surveys points. The longer the distance between survey points, the greater the error in the true vertical depth.
The radius of curvature assumes that the borehole follows a smooth, spherical arc between survey points and passes through the measured angles at both ends. This method is one of the more accurate means of calculating the position when the survey spacing is large. This method is less sensitive to placement of the survey points. When the survey data is closely spaced or if a single inclination value is used (from a collars table, for example), this method is not recommended.
The minimum curvature method is the default method in Strater. This method assumes that the borehole follows the smoothest possible circular arc between points. This method is very similar to the Balanced Tangential Method, with each result multiplied by a ratio factor.
For additional information on these methods, refer to the following locations.
Crain's Petrophysical Handbook at http://www.spec2000.net/19-dip13.htm#b1
"Bulletin on Directional Drilling Survey Calculation Methods and Terminology," American Petroleum Institute (API) Bulletin D20, December 31, 1985.
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