Cable - tool drilling and rotary drilling techniques
Cable - Tool Drilling
The first steam - powered cable - tool rig was used by Drake and Smith for drilling Oil Creek site in Pennsylvania. The early drillers in California also used such techniques. The rocking motion of a seesaw best describes the principle of cable - tool drilling. Let us explore the concept in depth. You can tie a cable to the end of a seesaw and let the cable dangle straight down to the ground. A heavy chisel with a sharp point is attached to the dangling end of the cable. The seesaw is such adjusted that the dangling point of the chisel is just above the ground surface. Now release the seesaw, which makes the chisel to hit the ground to drill a hole. The process is quiet effective as the chisel slowly makes its way through the rock beds bit by bit.
A cable - tool rig operates much similar to a seesaw with a powered walking beam mounted on a platform. The walking beam is actually a wooden bar which rocks up and down on a central pivot. As the beam moves up, it raises the cable attached to the chisel, and when it moves down, heavy weights above the bit, called sinker bars helps the chisel to make a hole in the ground below. The repeated lifting and dropping helps the chisel to slowly make its way through the bed of rocks. The platform provides the space to raise the cable and pull the long drilling tools out of the hole dug, using one of several winches called the bullwheel.
Although the cable - tools drilling technique was famous in the olden day of oil well drilling, the system had some major drawbacks. The drillers had to stop periodically for removing the pulverized rock pieces from the bottom of the well. The bit is completely removed from the hole and a special basket called bailer had to put down in the hole. This bailer helped to remove the rock cuttings made by the bit. After bailing the cuttings, the bit was again inserted into the hole to resume the drilling of the well. The crew had to bail out the cuttings each time they dig into the hole. If the bailing is not done, the rock cuttings hinder the bit for further drilling into the hole. The cable - tool drilling worked best in the hard rock formations like in the eastern United States, the Midwest, and California. While working in soft foundations like sand or clay, the cable - tool had a major disadvantage. The loose sand caved in the hole hindering the rig from further drilling.
This limitation made the rotary rigs to come into the focus. The drilling fluid used in rotary rigs prevents the collapse in clay or sand formations, thus helping in further drilling. The use of cable - tool rigs diminished with the emergence of rotary rigs. They are only used for beginning or spudding the hole before the rotary rigs take over the process of drilling. This application of the cable - tool rig is commonly known as spudder.
A Rotary drilling is totally different from the cable - tool drilling. It uses a bit that is completely different. It contains rows of teeth or other types of cutting devices that penetrate the formation and then scrape out pieces of it. The rotary crew members attach the bit to the end of a long string of hollow pipe called a drill string which helps in the drilling of the hole.
With the bit on the bottom of the hole, the rig rotates the bit by using equipments like rotary table. A rotary table is a kind of heavy - duty turntable. Some may also use top drive equipment, which is a powerful built - in electric or hydraulic motor that runs the pipe and bit. Still others may use a narrow downhole motor powered by the pressure of the drilling fluid. A downhole motor can be used alone, or along with either a rotary table or a top drive.
Most of the rotary rigs use the top drive equipment today, to rotate the pipe and bit. But top drive equipments are a bit expensive to maintain in comparison to rotary tables. The downhole motors are used by rotary rigs when the owners want to rotate the bit without rotating the entire string of pipe.
The drilling job does not only involve the rotating of the bit at the end of the pipe. The cuttings made by the bit are to be removed for further drilling into the hole. In rotary rigs, the crew can bail cuttings without stopping the drilling process. The fluid circulated by the rig helps to carry the cuttings up to the surface. The pipes used with the rotary bit are hollow, which makes the liquid flow down the pipe to bring up the cuttings to the surface. A motor is used to pump the fluid down the pipes. Once the cuttings are brought up by the fluid, the pump helps to clean the fluid back again to drain down the pipe to repeat the process.
The fluid used for drilling is called drilling mud. In the early days, the drilling muds consist of plain water mixed with mud. But the modern drillings muds used are more complex blend materials. These drilling fluids, as it is commonly known as can be anything like liquid, gas or foam. The advantage of such rotary rigs lies in the fact that the drilling is not hindered even on soft surfaces like loose sand or clay. There are different kinds of rotary rigs today, which are operated for drilling wells on land and offshore.
Summary: Cable - tool rigs are the oldest way of drilling oil wells. This technique has been overtaken by rotary rigs with much more efficiency and advantages. Cable - tools rigs are now used only to spud or begin the hole for drilling the wells. The majority of the drilling work is completed by the rotary rigs.
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