If you've spent any time looking into directional measurement instruments, and particularly gyros then you'll know that there are two main types of gyro tool: Integrating (or surface-referenced) and north-seeking.

If you've not read too far into it then the key fact to take away is that integrating gyros accumulate errors over time; the longer you run them the less accurate they are. These are commonly used for surveying because here they can be run very quickly and over that time maintain high accuracy.

North-Seeking gyros on the other hand measure the spin of the earth itself to get an absolute measurement at each survey point, regardless of the time since the last measurement. These are therefore better suited to measurement-while-drilling applications. The main disadvantage of these is that to gain an accurate reading they need to be perfectly still for several minutes, which usually means shutting off flow to the motor and often this will need to be preceded by wiper trips etc, making the whole process quite a long operation.

Running gyros on coiled tubing brings many mutual benefits to these drilling processes.

Coiled tubing is great for work-over of existing wells, being able to work in smaller spaces, and with greater ability to control well pressures. Here, the benefits of gyros abound. The surrounding metalwork from the casing and tubing of adjacent wells wreaks havoc with magnetic instruments, and at the same time you've got all the additional safety precautions needed in terms of collision avoidance when drilling amongst live well bores. A high accuracy drilling gyro will provide the confidence required for carrying out these procedures.

On top of that you have the ability to reliably run cable down to the drilling BHA for high-speed real-time data. When monitoring the gyro data this gives the user even more control and confidence with the ability to quickly analyse and quality control the survey data and verify that you are heading in the right direction.

With the right BHA you can even go from casing exit all the way to horizontal without the need to pull out and change equipment, really unlocking the speed potential of coiled tubing drilling.

Author: Benjamin Brooking

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