Steering tools
Orienting deflecting tools by the methods, is very time consuming. Furthermore the deflecting tool may not give the expected dog-leg under practical conditions so that the next survey may show some unexpected results. Much of the uncertainty is removed by using a kind of telemetry surveying method. The kind of tools specifically designed to orientate deflecting tools and monitor the well’s progress during a correction run are known as “steering tools”. A steering tool is a wireline telemetry surveying instrument which measures inclination and direction while drilling is in progress. The use of a wireline to send signals to surface means that the drillstring cannot be allowed to rotate. Steering tools can only be used when a mud motor is being used to make the correction run.
The downhole component of the steering tool is called a probe which continuously measures hole direction and the position of the toolface. This data is sent via the wireline to a surface unit which gives a numerical read-out and may also give a circular dial showing the orientation of the toolface with respect to the High side of the hole. This is of particular value to the directional driller because he can see how the toolface is changing (due to geological effects or reactive torque) as the well is being drilled. If the toolface must be changed by rotating the pipe the steering tool will give the new heading instantaneously. This makes the orienting procedure very much simpler and saves a lot of time. The directional driller can use the steering tool to make the well build or drop, turn to left or right depending on the orientation of the toolface shown on the surface dial. The steering tool allows the directional driller to see exactly what is happening downhole.
An orienting sub with an adjustable key is made up above the bent sub. The key is aligned with the scribe line of the bent sub. A non-magnetic drill collar is made up on top of the orienting sub. Once the BHA is run in the hole a circulating head with a wireline pack off is installed on top of drillstring. The steering tool with a “muleshoe stinger” on the end of it is lowered on a single conductor wireline until it engages the key in the orienting sub, thus aligning the probe with the toolface.
The probe remains in this position while the pumps operate the downhole motor and drilling proceeds. The probe continuously monitors the course of the hole and orientation of toolface as drilling continues. When a connection is made the probe must be pulled out while a new joint of pipe is made up. Once this has been done the probe is run back in on the wireline and drilling proceeds as before. In order to minimize the time wasted in tripping the probe, connections are only made at every 3 joints (i. e. the circulating head is installed on a stand of drillpipe, and so connections are made at 90′ intervals).
A slight modification to the standard steering tool is to run a side-entry sub. This allows the wireline to pass from the drillpipe into the annulus at some point below the rotary table. The purpose of this modification is to allow joints of pipe to be added without pulling the probe. However, care must be taken when making connections since the wireline must pass through openings in the drillpipe slips. (This may also cause problems if a kick occurs and BOPs must be closed on this line. If the BOPs do not seal, the wire will have to be cut). If the drill pipe becomes stuck at some point below the side entry sub a free point indicator cannot be run.
The advantages of using a steering tool as opposed to a photographic instrument for orienting and surveying may be summarised as follows:
• Saves rig time due to:
sending results to surface more quickly
fewer attempts required to get orientation correct
allows a correction run to be completed in shortest possible time.
• Better directional control of well path due to continuous monitoring
• Able to monitor the orientation of deflection tool during drilling.
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The main disadvantage is that due to the wireline, steering stools cannot be used in conventional rotary drilling — only with a mud motor.
The next logical step in the advancement of directional surveying was to have a steering tool which did not depend on wireline. Hence the MWD tools — measurement while drilling were developed and used for this purpose.