Consider two possible procedures for drilling from A to B – both of these trajectories start and end with the same attitude and have the same measured depth difference.
Suppose we were at 50 degrees inclination at A and built to 60 degrees inclination at B. In the case on the left the curve comes first followed by a straight and vice versa on the right. Clearly the wellpaths are different but the surveys would be identical since the measured depths are the same, the azimuth has not changed and the inclinations have risen by 10 degrees. Because a single arc is applied, there is a potential for significant TVD error to accumulate along the wellpath if the changes in geometry are not sufficiently observed.
For this reason it is often recommended that when building angle faster than 3 degrees per 100 ft (or 30 m) it is best to survey every pipe joint rather than every stand to ensure adequate observations to truly represent the well path. In the next chapter it will be seen that a rate gyro can observe at very short intervals indeed with no additional survey time and in general these are better at determining TVD than MWD. Normally however, gyro surveys are interpolated at 10 or 20 ft intervals or equivalent just for ease of handling and processing the data.
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