Stockhausen effect can cause a substantial TVD error that is demonstrated in [1]. There are two major approaches to compensate for that. One is to utilize continuous inclination and static surveys to recalculate high def trajectory. Another approach is based on utilization of slide sheets and static surveys [2]. Our study shows that TVD recalculated with slide sheet introduces error which can be higher than the original one.
Here is an example: uncorrected trajectory TVD error is approximately 2.4 meters at TD of 3200 m, whereas high-def trajectory calculated with static surveys and slide sheets introduces more than 8 meters deviation from the true position:
Root-Cause
This result looks counterintuitive: we add trajectory steering information to compensate TVD error due to steering, but it increases that TVD error. To understand the reason of the issue we have to review TVD correction technique.
To calculate high-def trajectory with slide sheets we have to know average rotary build-rate and slide DLS. For that purpose we can use theoretical BHA performance modeling and empirical back-calculation slide sheet with static surveys, or some combination of both algorithms. However, all those algorithms introduce systematic errors that accumulate along trajectory. Problems of the theoretical BHA performance modeling are obvious. But why are we unable to calculate BHA performance accurately with empirical static measurements? The reason is the depth shift between MWD surveys and slide sheet due to BHA forward vision effect: MWD detects inclination change from slide in advance. The effect is well-known to DD and has been reproduced in control environment [3].
The amplitude of the effect depends on BHA design, stiffness, trajectory, and local DLS; typical shift is 1.5-4.5 m.
Another issue with slide sheet depth is the different drill pipe stretch in rotary drilling and slide drilling: due to the different tensile load for those two drilling modes, the relative drill pipe stretch can exceed 10 meters or more.
This relative stretch causes a depth error when we determine the start and stop depths of the slide. Usually, directional drillers try to compensate for this error, however, there is no generally accepted practice, and these practices are influenced by human factors.
All that make the depth shift modeling practically impossible.
Consequences
The depth shift between surveys and slide sheet causes overestimating/underestimating of rotor build-rate and slide DLS: slide interval overlaps with rotor one and vise versa. This error of BHA tendency estimation is systematic and accumulates from survey to survey. Our sensitivity study demonstrates that even small depth shift leads to substantial final TVD error. Figure 4 shows the dependency of the TVD absolute error on the depth shift amplitude for one trajectory:
Discussion
We are aware about certain practical evidences which demonstrate that the slide sheet TVD technique may work correctly: horizontal wells corrected by the discussed technique are well agreed with geological data.
However, our study that the average depths shift between slide sheet and MWD surveys will be the same for similar well profile. As result, systematic TVD errors of the neighbor wells will be correlated. Relative TVD error will be close to zero: the wells agree between each other. But absolute TVD error can still be significant, that leads to misinterpretation of the geological model.
1. Continuous Direction and Inclination Measurements Lead to an Improvement in Wellbore Positioning, Ed Stockhausen, et al, IADC/SPE 79917, 2003
2. Introduction to wellbore positioning, V06.02.2017, p. 57-58
3. Directional Drilling Tests in Concrete Blocks Yield Precise Measurements of Borehole Position and Quality, Ed Stockhausen, et al, IADC/SPE 151248, 2012