Directional drilling is accomplished by causing a mechanical or hydraulic deflection of the bit during drilling. Predicting BHA behavior prior to drilling and taking survey check-shots while drilling is common practice because understanding the directional tendency of a BHA is critical to successful well placement. It has been suspected that different modes of drilling cause a near instantaneous change in the size, shape and centerline position of the borehole and, thus, change BHA tendencies. This is not backed up by continuous MWD inclination data which shows a long transition between the slide/rotate zones.

Since examining a wellbore to accurately determine these effects is impractical, a logical approach would be to set up a directional drilling process where the wellbore could be dissected and examined. Precise measurements of the wellbore centerline, diameter or circumference can be made under laboratory conditions and compared to field sensor measurements to determine the accuracy of the field measurements in terms of wellbore position, tortuosity and rugosity.

Schlumberger developed a directional drilling test capability utilizing concrete blocks placed sequentially and drilled horizontally. The blocks can be positioned to accommodate changes in wellpath. They are sealed to allow mud flow during the drilling and then broken apart for extensive laser measurements and scans. The blocks are tagged to allow surveying to maintain a 3D coordinate system to millimeter accuracy.

Chevron and Schlumberger conducted a series of drilling tests to evaluate wellbores drilled with positive displacement motors (PDM’s). The blocks were drilled with slide/rotate sequences with up to 10 deg/100ft curve rates. Multiple MWD directional surveys were taken and compared to the actual position of the wellbore from the lab measurements.

This paper discusses the TVD accuracy of the well as a function of different survey spacing and its impact on horizontal well placement. Changes in touch points of the BHA within the varying hole diameter of the slide/rotate sections are also discussed. Metrics are proposed for managing wellbore quality in terms of tortuosity and rugosity along with their impact on the wider drilling community.