11. When to run Gyros

Running gyro surveys is nearly always a benefit to survey accuracy and provide verification of the MWD surveys, but clearly the benefit has to be worth the cost. There are certain circumstances however, where running gyros are the only option for a safe an adequately accurate survey. Please note that in most of these scenarios apart from the highlighted below, the assumption is that the gyro used is of sufficient accuracy to exceed the accuracy of the MWD. That is not always the case depending on the type of gyro and the expected performance of the gyro must be ascertained by suitable QC to ensure adequate accuracy.

When magnetic interference from nearby steel preclude the use of MWD.
These circumstances include the following;

• Measuring inside casing
• Measuring close to casing shoe
• Measuring close to adjacent wells
• Measuring close to surface or shallow beneath the rig
• Measuring close to a fish or when side-tracking close to original casing.
  

This would naturally, include conductor surveys after conductors have been driven. This often neglected practice ensures that the collision risk assessment is based on the actual as built positions of the conductors and not an assumption that they landed vertically and parallel. It is not unknown for driven conductors to cross two rows of slots from their original surface position so the slot/ target allocation often has to be reviewed in the light of the conductor survey. When the casing of the nearest well is 50ft or more away it is usually considered to have negligible effect on MWD azimuth accuracy but the effect rises rapidly with proximity so gyros are often prescribed when separation from casing drops to 30ft or so.

1. When TVD accuracy is required less than 3/1000 on step out. This is very difficult to achieve with MWD in open hole and whilst the accelerometers may be just as accurate as the gyro sensors in the vertical plane, the hole quality and the measurement environment cannot deliver this level of accuracy with confidence.
2. When the MWD is surveyed every 90 ft but with dogleg severities exceeding 6o/100ft the MWD survey interval will not adequately represent the wellpath. Here the gyro provides a higher resolution survey and can be requested at very small intervals although 25 ft is common.
3. When the target dimensions are less than 2% of the step out (1% if IFR is employed). This size of target will not leave sufficient room for the directional driller to steer successfully without the reduction in uncertainty afforded by a high accuracy gyro survey for at least part of the well bore.
4. Anywhere, where the separation factor requirements cannot be met using MWD alone.
5. In side-tracks where the original hole contains a fish, or casing and the accuracy requirements demand an adequate survey during the side-track section close to the original hole.
6. When drilling close to leaselines, geo hazards, fault blocks or other ‘hard line’ boundaries where MWD uncertainty wastes too much pay.