Describe the methods of drilling wells

i) Percussion
Cable tool drilling, otherwise known as percussion drilling, is probably the oldest drilling method. Basically it involves lifting and dropping a string of solid steel drilling tools suspended from a wire rope to hit the bottom of the hole. This process drives the cutting bit, fracturing or pulverising the formation. The crushed material forms a slurry on mixing with water that has been added to, or is
naturally present in, the hole. The blow rate varies from 40–60 strokes per minute and, because of the characteristic left lay of the wire rope cable, the bit turns and strikes across a different section of the hole bottom at each blow. When the bit can no longer fall freely through the water–cuttings mix, the drill tools are withdrawn from the hole. A tubular bailer, which is run on a separate smaller wire
rope, is then used to pick up the slurry and cuttings and remove them from the hole
before drilling is resumed.
In cable tool or percussion drilling there are basically three major operations:
Drilling the hole by chiselling or crushing the rock, clay, or other material by the impact of the drill bit. Removing the cuttings with a bailer as cuttings accumulate in the hole. Driving or forcing the bore casing down into the hole as the drilling proceeds.
Because of the relatively low initial cost and simplicity of equipment used, the cost per unit drilled is relatively low. However, the technique is slow, and when the increased cost of labour is taken into account, there is usually little advantage over faster rotary drilling methods when drilling new bores.
Cable tool drill plants are used extensively for reconditioning because they:
- Are usually lighter than a rotary plant with an equivalent depth capacity
- Are easier to establish over a borehole
- Can also lower and retrieve tools to probe a bore more quickly than with a rotary plant
- Are able to work inside casings
- Are able to insert casing liners more quickly because of their better access around casing strings for screwing or welding a joint.

ii) Rotary
The principle of rotary drilling is based upon a rotating drill stem made of lengths of drill pipe about 15 feet long. A bit is attached to a heavy stabilizer or drill collar at the end of the column of drill pipe. The extra weight and larger outside diameter of the stabilizer just above the bit helps to maintain a straight drill hole. The drill stem is hollow and has a drilling fluid of either mud or air circulating down the drill stem out through the nozzles in the bit and up along the outside of the drill stem. The rotating action of the bit breaks up the material and the drilling fluid carries the
cuttings to the surface where they settle out in a mud tank.

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NatalieR answered the question on June 29, 2022 at 07:44