This technique has the advantage of real time readings from down hole, but the disadvantage of the wireline and associated handling problems.

Although technically wireline steering tools are also MWD systems, the term MWD is commonly used in the industry to mean systems with non-wireline data transmission systems.

The disadvantage of the mud pulse MWD systems is their relatively slow data rate and hence update of the downhole measurements. In a number of applications, e.g. with deep kick-offs or high torque motors, the MWD data rate is insufficient for obtaining a consistent orientation, and a wireline system has to be used.

Magnetic and gyroscopic survey tools are used for wireline steering. Magnetic steering tools are influenced by magnetic interference, both from the drillstring and from adjacent wells, and the gyro steering tool has been developed to overcome this problem.

Wireline steering tools

Wireline steering tools can only be used when the drillstring is not being rotated. Wireline steering tools are applied when:
·kicking-off;
·side-tracking;
·making correction runs.
Wireline steering tools save substantial rig time and are cost effective in comparison with single-shot surveys, despite higher direct survey cost.
The time savings result from:

• they allow the reactive torque of the mud motor to be taken into account;
• they provide a continuous real time read-out; and so allow an early response on problems of assembly orientation, hole direction, tool failure, etc.

Wireline steering tool surveys taken in kick-off assemblies are usually sufficiently accurate for controlling and monitoring the kick-off interval, but for an accurate survey a magnetic or gyroscopic survey should be taken through the kick-off section.

Principle of operation

Wireline steering tools comprise a downhole probe, conductor wireline and surface equipment. The downhole probe can be a solid state magnetic tool, a conventional gyro tool or a North Seeking gyro tool. Magnetic tools are most frequently used, but have an application restriction due to magnetic interference, which does not exist for the gyroscopic tools.
For wireline steering in vertical wells gravity toolface can not be used. Magnetic toolface is also not applicable due the magnetic interference of the casing. For these events a gyroscopic survey using gyro toolface should be used.

Solid state magnetic wireline steering tools

The most common steering tools:

·DOT (Directional Orientation Tool) - Baker Hughes Inteq;

·SST (Sperry Steering Tool) - Sperry-Sun;

·EYE (Electronic Yaw Equipment) - Scientific Drilling International;

·Bob Fournet Company (BFC/Tensor) Steering Tool;
·Servco.

Quality assurance

Quality assurance for solid state magnetic wireline tools is identical to that of solid state single and multi-shot tools.

Pre-survey checklist

On arrival the survey engineer should obtain the following information from the well site drilling engineer:
Wellhead co-ordinates
Depth reference (e.g. rotary table or wellhead)
Kick-off depth and outline of proposed well plan
Target azimuth (proposal)
Tie in data
Survey depths and interval
BHA/drillpipe size, weight, ID, restrictions, connections
Maximum bottom hole temperature and mud properties
Depths of any severe dog legs or expected problems
Magnetic field strength, dip angle and declination (for magnetic wireline steering tools).

The Wellsite Engineer should ensure that the following checks have been carried out.
- Check that the proper equipment for the job is available, e.g.:

1.sufficient length of NMDCs if required;
2.crossover as required;
3.orientation sub;
4.full flow mule shoe, telescopic joint, etc.;
5.circulating head;
6.wireline unit, steering tool probe and surface equipment;
7.side entry sub with split kelly bushing (if required).
Before making up the bottom hole assembly check that the key seat (mule shoe seating) in the orientation sub is aligned with the high-side of the bent sub.

- Check that the mule shoe is full flow type.
- Space out the instrument with spacer bars to land sensor at the optimum position within the NMDCs if required.
- Circulate hole clean before running in the instrument, also after making a connection.
- Consider use of side entry sub to save wireline tripping time.

Quality control

Wireline steering tools are run by the Survey Engineer. For use of full length of NMDC or correction programme to reduce/correct for magnetic interference. Quality control of solid state magnetic wireline steering tools is governed by the correction programme.

Uncertainties of solid state magnetic wireline steering tools

Uncertainties of solid state magnetic wireline steering tools comprises tool uncertainties, geomagnetic uncertainties correction programme correction uncertainties and system uncertainties. System uncertainties are wireline depth, toolface dependent misalignment, and BHA deflection. Misalignment and BHA deflection are mainly in the vertical plane affecting mostly inclination readings. Toolface dependent misalignment can be corrected by using correction programme (rotational shot or toolface offset centre correction technique).

Gyroscopic orientation tools

Where there is magnetic interference from adjacent wells or fish or in the event of a sidetrack from a vertical casing, a magnetic steering tool cannot be used for orienting a drillstring during a kick-off. In these situations, gyro single-shots can be used. The gyro cannot be left in the drillstring while drilling, so the string is oriented with the gyro, including an estimate for the reactive torque from the downhole motor. After drilling a single, a further single-shot survey is made to confirm the orientation and direction of the well.

Initially, film-type gyro single-shots were run on wireline, however these have a limitation of about 15 minutes survey time, beyond which the accuracy can degrade. This gives a practical depth limitation of about 2000 ft AHD.

Surface read-out gyro tools have tended to replace the film-type tools. The surface read-out gyro tools are run on conductor wireline and can give provisional data for the azimuth, inclination and toolface in real time. The provisional data will change when the drift curve is closed at the end of the survey.

North Seeking gyro tools can also be used for this application. The North Seeking gyro tools have the advantage that they do not require orienting to a foresight and hence requires less time to take a survey. However, as with the film-type instruments, the conventional surface read-out gyro and North seeking gyro cannot be left in the drillstring while drilling. The exception to this is the North Seeking Finder from Scientific Drilling International, which in its ruggedised form, can be used as a wireline steering tool.

Finder GWD tool (SDI)

Currently the Finder GWD, (Gyro-While-Drilling) is the only gyro tool that can remain in the drillstring while drilling in non-rotary mode only. It is a ruggedised version of the Scientific Drilling International (SDI) 'Finder' North Seeking gyro. The Finder GWD can be used for kick-offs and sidetracking, especially where magnetic interference is likely to affect magnetic tools. Applications include sidetracking past fish or through a window milled in casing.

Initialisation

Prior to the start of the job, the survey engineer will make up the Finder in the appropriate running gear for the well to be surveyed. A locking (latch down) stinger and sliding sinker bar should be used in all Finder steering jobs. It is recommended to centralise the tool if the drillstring inner diameter allows.

If the well temperature is less than 72°C the Finder can be run inside a 1.75" OD pressure barrel. If the well temperature is greater than 72°C the Finder must be run inside a thermoshield. This will limit downhole time due to the gyro temperature build-up inside the thermoshield.

Quality assurance

A field calibration should be performed to check the spin and input axis mass unbalance and accelerometer scale factor offsets. The differences between office and field offsets obtained should be within the acceptable values specified. If the differences are not within this range, it is recommended that a back-up tool is run. If this is not possible, the Finder should be returned to base for checking as soon as possible after the job.

Pre-survey checklist

In addition to the pre-survey checklist given above, the Wellsite Drilling Engineer should supply the survey engineer with the latitude and the depth where inclination is approximately 15°.

Gyrocompassing mode

Below 15° inclination the Finder GWD tool should be used in gyrocompassing mode taking toolface data with pumps on. Accurate inclination and azimuth readings can only be taken with the pumps down.

Continuous mode

Above 15° inclination the Finder GWD tool should be used in continuous mode. After drilling a single the tool is pulled up to the previous depth and run down in continuous mode taking surveys. At the start of the continuous mode a North seek initialisation should be performed.

Quality control

During surveys quality control checks should be performed. These control checks should be specified on the quality control sheet and should be checked by the well site drilling engineer. Whenever rig operations and time allow, a post-job field calibration is done as soon as possible after the survey is completed, before leaving the location. The pre- and post-job calibration differences should be checked by the well site drilling engineer against the acceptable values.

Uncertainties of Finder GWD tool

The uncertainties of the Finder GWD tool comprises tool uncertainties and system error.

Steering the mud motor

Wireline steering tools are nearly always used with a bent sub and mud motor. The scribe line on the bent sub (which indicates the direction the bit will try to drill) on the probe is aligned by a mule shoe orienting sub. From this the toolface offset is determined.

A magnetic or gyroscopic steering tool can read the orientation of the scribe line. For magnetic steering tools this direction can be read:
·relative to the high-side of the hole (toolface angle) (inclination > 5°);
·relative to Magnetic or True North (azimuth) (inclination < 5°).

For the Finder GWD tool the orientation of the scribe line can be read:
·relative to the high-side of the hole (toolface angle) (inclination > 3°);
·relative to True North (azimuth) (inclination < 3°).

Wireline inside drillpipe

There are three ways of getting the wireline inside the drillstring while being able to circulate. They involve the use of:
·a circulating head;
·a side entry sub;
·a wet connector.

A side entry sub is always used with a lockdown type mule shoe. A circulating head can be used with or without a locking device. Locking devices should always be used at high flow rates.

Circulating head

The basic system comprises a swivel and a hydraulic pack-off. The circulating head is mounted on the drillpipe which has been hung off the hook. The top of the circulating head is provided with a sealing arrangement (pack-off) through which the cable is fed into the drillpipe. If another stand or single has to be added, the tool probe has to be pulled out of hole to remove the wire from the drillpipe. The kelly is not connected when the system is in use.

In case of a top drive the hose of the top drive or of the standpipe manifold is attached to the circulating head. The drillpipe is hung-off in the slips.

To save rig time try to use a double or a triple drillpipe joint as a working stand. However, the use of drillpipe as a working stand precludes the use of high torque mud motors.

Side entry sub

Under normal circumstances with a circulating head, the probe will be tripped in the hole at the beginning of the drilling operation and will remain seated downhole until it is tripped out on wireline after drilling the working stand.

Side entry subs are used to avoid pulling the steering tool when adding pipe. Their use thus saves rig time. With a side entry sub arrangement, the cable is clamped on to the outside of the drillpipe and enters the drillstring via a side entry sub which is provided with a sealing arrangement (stuffing box).

There are several disadvantages associated with the use of side entry subs:

• pressure control problems; problems can occur when closing the drillpipe BOP rams (because the cable lies outside the drillpipe). Also annulus-drillpipe communication can occur if there is stuffing box packing failure.
• wireline damage; to reduce the possibility of wireline damage, it is recommended that free wireline is only run inside the casing otherwise it will be damaged by the casing shoe.

Side entry sub seal integrity and running speed

The wireline seal is expandable on a run-by-run basis. The seal is degraded by friction and heat as the wireline passes through it when the probe is run into the hole and retrieved. Seal wear is normal and expected. Seal wear occurs faster, however, when the wireline is run at high speeds, generating more heat. To preserve seal integrity and avoid round trips, a new seal should be inserted into the seal spacer before each run; and running speeds should not exceed 30-50 m/min (100 to 150 ft/min) especially when running in. A slower speed is advisable if the wireline is rough, dirty or frayed.

Safety precautions when using side entry subs

The wireline operator of the side entry sub system must work closely with the Driller since a wireline tension of 50-150 kg (100-300 pounds) is being used. An intercom is vital. The wireline is liable to damage at the rotary table and if severed will lash up, endangering workers on the rig floor.

• operate this equipment with great care;
• use a split kelly bushing. This permits normal operation of the kelly while allowing the wireline to pass through the rotary bushing to the upper sleeve without obstruction and greatly reduces the danger of severing the wireline at the rotary;
• if a split kelly bushing is not available use a circulating head;
• do not try to run the wireline side entry sub system with a standard kelly and kelly bushing;
• use a wireline unit with a constant tension facility and ensure that the operator remains alert throughout the job;
• ensure that the wireline does not get pinched between the drillpipe and the kelly bushing;
• after making up the side entry sub to the drillstring (with wireline threaded through) and seating the steering tool, pick up kelly and circulate briefly to check seal integrity.

Wireline wet connector and running procedure

In slim hole drilling, the small clearances make the use of a side entry sub for a wireline steering tool difficult. In some applications, the wireline on the outside of the drillstring is unacceptable from a well control point of view, where the wire could cause problems with BOPs making an effective seal on the pipe.

To overcome this, a rotating wet connector can be used. The system uses a similar rotating wet connector as used in coiled tubing directional drilling operations. It allows the wireline to be pulled for adding drillpipe and orienting, without pulling the steering tool, with the kelly still in place. The disadvantage is that the wireline requires cutting.

The running procedure for use of a wet connector is as follows:

1. Run the BHA and drillstring to the depth required for the start of oriented drilling.
2. Run in and seat the steering tool.
3. Cut and make up the wireline head. Hang off in the bottom of the wet connector which rests in a drillstring landing sub.
4. Thread the wireline through the pack-off attached above the swivel on the gooseneck or non-rotating upper connection of the swivel. Make up the loose end of the wireline to the overshot that connects with the top of the wet connector.
5. Connect the overshot to the wet connector. Check the wireline continuity and the function of the steering tool.
6. Disconnect the overshot and continue running the drillpipe in the hole.
7. Prior to starting drilling, the overshot can be run in and connected to the wet connector.
   Whenever pipe rotation is needed, or drillpipe has to be added, the overshot can be pulled into the kelly. When the overshot is connected, the operation is similar to a conventional steering tool.