The following is a simple method of correcting for the TVD on directional wells. This calculation will give the approximate TVD interval corresponding to the measured interval and is generally accurate enough for any pressure calculations. At the next survey, the TVD should be corrected to correspond to the directional Driller's calculated true vertical depth:
where TVD2 = new true vertical depth, ft TVD1 = last true vertical depth, ft CL = course length — number of feet since last survey cos = cosine
Example: TVD (last survey) = 8500 ft Deviation angle = 40 degrees Course length = 30 ft
Solution: TVD2 = cos 40 x 30 ft + 8500 ft TVD2 = 0.766 x 30 ft + 8500 ft TVD2 = 22.98 ft + 8500 ft TVD2 = 8522.98 ft
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