Control of the bit
• Conduct of the luid
• Conduct of the cutings
• Drill string lexibility
• Tasks performed down the hole
• A suitable drill string.
The driller must drive and control the working of the bit from his position at the collar of the hole.
In most cases the power to cause the bit to dig originates at the collar of the hole, and power must be transmited down the hole to the bit, where the work is being done.
Power transmission down the drill hole may be achieved by:
• cable, as on a cable-tool rig
• axial movement of pipe and rods
• rotary movement of pipe
• luid low.
In every case the function of the drill string is to convey the power to the bit, so the drill string must be conigured to do this as eiciently as possible.
Control of the bit
Simply transmiting power to the bit is not enough. Power must be delivered in the right way and in the best form to cause the bit to dig eiciently in the desired direction.
Cable systems: Cable systems are controlled by varying the:
1. cable motion, that is, by altering the:
— stroke length
— stroking rate
— speeds of lifting and dropping during the stroke.
2. shape and weight of the drilling tools.
Maximum eiciency depends on the tools falling truly vertically and as free as possible. Drilling tools must rotate steadily about the centre line of the hole.
Rod and pipe systems: When compared with cable systems, pipe systems provide more control over the bit, but often not as much as the driller thinks. In rod and pipe systems he can control the:
• thrust applied or the tension maintained in the drill string at the collar of the hole
• rotary torque at the collar of the hole
• diameter of the drill string in relation to the hole diameter
• rotary speed
• rate of movement of the drill string into or out of the hole
• shape of the drill string and the weights of its component parts.
The problem here is that the forces applied at the collar can change in size and direction by the time they reach the bit
Conduct of the luid
Many types of drilling use luid low to assist in making a hole. In each of these, the drill string directs the low. In some cases it causes the low.
In cable systems the motion of the drill string causes circulation of luid in the botom of the hole and over the bit. The luid velocity is controlled by the:
• drilling action
• viscosity of the luid in the hole
• shape and size of the watercourse.
The lift and drop action of mud scow bits and bailers causes luid low, which helps to move cutings up into the body of the tool.
In pipe systems, three systems of low are used in drill strings:
1. Standard circulation
2. Reverse circulation
3. Dual pipe, usually with low down between the pipes and up the centre.
Cable system circulation when drilling (left) and bailing (right)
These three pipe system lows are illustrated in Figure 1-2.
Pumps are used to provide the desired low in most cases. The drill string conducts and directs the low. In the case of hollow rod drilling, the movement of the string is used to generate luid low.
Figure: 1-2: Pipe system lows
Conduct of the cuttings
The luid types of drilling use luid low to carry the cutings. The drill string itself conducts the luid, and controls its direction of low, and thus the direction of movement of any cutings carried by the luid.
Bailing is a mechanical method of clearing cutings, but it depends on luid low to assist in illing the bailer.
Types of drilling that use the drill string itself to convey cutings are auger drilling and shell sampling.
Auger drill strings may carry cutings upwards along the helix as it is rotated (see Section 3.9), or the auger may be pulled when the light is judged to be carrying an adequate load.
Shell samplers, casing samplers, and earth sockets rely on compression of the cutings or solid sample to retain them in the sample tube.
In these situations the drill string must be carefully handled to avoid bumps or vibration that could dislodge the sample.
Drill string lexibility
When the drill string is being used to lift a load of cutings, many drillers ind that the string is suiciently rigid to transmit unwanted jerks, bumps, and vibration. But when the long slender drill string is extended down into the ground, its lexibility, relative to its length, is more like the lexibility of a one metre piece of string or cooked spagheti.
The direction of the pull or push provided by the rig at the surface of the ground has no efect on the direction taken by the bit 1000 metres (3300 ft) below. However, the driller can use an amount of thrust or pull to help control the direction of the hole.
Tasks performed down the hole
The drill string is the ‘extended arm’ of the driller. It reaches down the hole to:
• ind out what is there
• pick up lost or broken objects
• change the direction of the hole
• install liners to protect the hole
• sample formations or luids
• place packers
• install screens
• develop aquifers
• carry out any other task the driller requires.
Most of these tasks will require a special tool or iting to be atached to the drill string. Such tools must be suitable for the type of drill string in use, and they must have the correct connection. Figure 1–3 illustrates six drill string itings and tools. They include:
- An impression block to obtain an imprint of an object in the hole
- A rope spear to recover broken rope or cable
- A whipstock or wedge to delect the hole
- A casing spear to place a liner in the hole
- A screen suitable for seting, using a latch on thebotom bail
- A jeting tool used to develop through the screen
Figure 1-3: Drill string ittings and tools
The drill string provides the clues
Every driller rapidly learns about the ‘feel’ he has through the drill string. Feel is the most important way of understanding what the drill string is doing and what is happening down the hole.
On cable-tool rigs and on light rotary rigs the driller must be able to feel the drill string, and to do this he may place his hand directly on the string. For example:
• The cable-tool driller feels the nature of the blow on the botom and the turn of the tools by holding the drilling cable.
• When the diamond driller holds the rods he can feel the core break or the inner tube latch-in.
A driller can feel what it is like at the botom of the hole or the sides of the hole by carefully turning the pipe, either by hand or by using a handheld wrench.
When the equipment is too heavy for the driller to receive clues by holding the drill string with his hand, he can get the feel of what the bit is doing by watching the instruments to see changes.
Sometimes these changes are mere licks on the weight or feed indicator. Other times the changes or licks are in the torque or hydraulic pressure gauge. The driller must always look and listen.
For example, when the drill string is moved:
• there may be hesitations or bumps
• the luid pressures may change
• the hoisting or rotating equipment may creak or falter.
A suitable drill string
Drill strings must be selected to suit the:
• machine driving them
• type of drilling to be used
• size and depth of the hole
• formations to be penetrated
• information to be obtained
• tasks to be performed downhole.
Cable-tool strings: When drilling using the spudding method, the tool string weight should be well within the weight capacity of the machine.
Select the largest joint size (tool thread diameter) consistent with hole size. The length of the tool string will depend on the weight required. Usually this should not exceed half the height of the mast.
Strings requiring a strong stirring action should include a bit with a long watercourse.
Pipe strings: Pipe strings must have a diameter that is suited to the type of drilling and the hole size. They must have the strength to:
• provide the required stability
• transmit the necessary torque
• support the weight of the string plus an allowance for friction and other downhole resistance.
The total weight of the string must be within the handling capacity of the machine.
Drill string inspections: Drill strings wear, and they are damaged by contact with the hole and by careless handling. A driller must therefore understand which points in the string are critical. He must inspect the string regularly to ensure that it remains capable of doing its job. Later in this chapter we examine how this can be done.