Survey Calculation Methods

A1

12.4.4. Drilling Directional Wells Kicking Off The Well

Jetting is the term used to describe the deviation of a well using bit hydraulics to erode the formation in a particular direction. A special jetting bit may be used or a conventional tricone bit run with two undersized and one oversized (or blanked) jet nozzles. Usually the bit is run on a typical build up assembly (bit, full gauge near bit stabiliser, orienting sub, non-magnetic and steel-drill collars as required) and once on bottom the blind nozzle, representing the

‘tool face’, is oriented in the desired direction. Maximum circulation is then established and the washing action begun. Some of string weight is slackened on the bit and the weight indicator will give an indication of drilling off if the formation is soft enough to be washed out.

In formations where the degree of compaction makes jetting ineffective, deviation is started with a downhole motor. This has become the most commonly adopted method of kick off.

With downhole motors, bent and orienting subs (or combined bent/orienting sub) are required. With the deflection assembly in the hole, there is a correction to apply to the desired tool face setting or proposal direction. This correction is due to the reactive torque developed by downhole motors. Reactive torque is dependent on motor power, weight on bit, formation, hole inclination and drilling assembly design and length. The actual value of reactive torque must be assessed as drilling proceeds as it is unique to the conditions prevailing.

During the kick off, the advantages and/or disadvantages of the different methods of orientation are highlighted. With single shot orientation, reactive torque can only be estimated based on the experience of the Directional Driller in the area of operation. Since the survey tool is at least one joint above the bit, the first assessment of actual reactive torque can be made only after the second joint has been drilled.

Steering tools provide the most accurate measurement of tool face position. A continuous read-out on surface enables adjustment of the weight on bit/rate of penetration in order to maintain a constant tool face. MWD tools provide the same information with the advantage of not require a wireline and the consequent rigging up and trip time. On the other hand, steering tools provide extremely high data rates that may be of critical importance when drilling with very high rates of penetration.

Build Up Section

After the desired direction has been reached, the kick off assembly may be replaced with a rotary build up assembly. However, if jetting has been the method of initial control, drilling can continue with the same BHA in the rotary mode without requiring a trip. Selection of the appropriate build up assembly is dependent upon the angle achieved during initial kick off and maximum angle required.

The decision of when and if to replace the kick off assembly depends on several factors such as hole size, weight on bit and rate of penetration, response of the kick off assembly, residual bit life and final planned inclination. Controlling the BUR is imperative if fatigue to drill pipe and drill collars is to be avoided.

This can be accomplished by varying the drilling parameters (weight on bit, rotary speed and pump pressure) or changing the BHA. In this case careful assessment must be made to consider whether the amount of time lost in tripping out of hole to change the assembly, would be gained later with a better rate of penetration or by preventing difficulties.

The alternative is to accept the current performance and make adjustments at the next bit trip.

Tangent Section (Hold On Section)

When the desired inclination has been reached, the kick off or build up assembly is replaced with a stiff bottom hole assembly that will maintain the inclination and direction.

Small variation in behaviour of a BHA can be obtained by adjusting the weight on bit and rotary speed.

Providing it is necessary, the earlier a correction to inclination or direction can be made the better it is. As the bit get closer to the target, longer corrections are required to get the well back on the target. Advanced planning should be continuously done during operations to ensure that, should a trip become necessary at short notice, any change to the BHA may be made at the same time.

Drop Off Section

Drilling a directional well it may be necessary to allow the drift angle to straighten back to vertical or near vertical.

Drop off assemblies should be used starting with the least successful. The reason being that the higher the inclination, the greater the pendulum effect and the same rate of drop might be achieved with the least successful assembly at 50° and the most successful assembly at 30°. Therefore, as the inclination is reduced, stronger dropping tendency assemblies may be run to maintain the rate of drop required.

Only where the maximum negative side force is required, at low inclinations and in hard formations, should pendulum assemblies be run (i.e. assemblies without a near bit).

Care Of Stabilisers

The bottom 120 (40m) of a drilling assembly is the critical portion for controlling a directional well. The stabilisers used in this area should be full gauge to ¹/16" under unless undergauge stabilisers are required to hold or drop angle.

Stabilisers shall be gauged each trip: undersized tools should be moved up higher in drill collar assembly or replaced with full gauge tools.

All stabilisers shall be magnafluxed at the end of each well phase.

As a general rule, do not drill out casing shoe with a ‘packed hole assembly’. However, the decision whether or not to use stabilisers to drill casing shoe shall be evaluated case by case.

Figure 12.I - Build up Assembles

Near Bit Stabilizer Near Bit Stabilizer 30' Non Mag.

Near Bit Stabilizers Near Bit Stabilizers 30' Non Mag.

Drill Collar

30' Non Mag.

Drill Collar

String Stabilizers

10' Drill Collar 10' Drill Collar

Maximum