Air Percussion Casing HammerThis method is also called drill through casing driver. It combines the borehole stability of the cable-tool rig and the speed of an air rotary rig. The use of a casing driver permits the casing to be advanced during drilling, but both drilling and driving can be adjusted independently depending on the nature of the formation. Drivers can be used to drive upward to remove the casing or expose the screen for well installation. The method is particularly suitable for drilling in stratified deposits that have large differences in particle sizes. Cuttings are blown upward inside the casing and discharged through a pipe at the surface. When air-drilling techniques are used, it is easy to see how much water is being blown out with the cuttings at a given depth. From this observation, it is possible to assess when the borehole is deep enough to produce the desired yield. The air-operated casing hammer requires lubrication (hydrocarbon) by means of in-line oilers. Unlike the down-the-hole hammer, the casing hammer does not input oil into the formation directly as oiling proceeds at surface. The method is thus applicable for groundwater monitoring work. It is possible, however, that oil can be added indirectly to the formation if proper cleaning of the casing is not performed. Drilling limitations are essentially the same as for the down-the-hole hammer method. The first method oil drillers use is called Cable-Tool Drilling. It is the simplest way to drill and is used to create shallow wells in soft beds of rock. A bit held to a long steel cable by an iron rod called a stem is raised and dropped again and again. The force of the drop drives the bit deeper and deeper into the ground. The jagged bit crushes soil and rock and the drillers must occasionally pull it out to pour water in the opening to flush it out. The bailer, a long pipe, takes out the water and loose rocks and soil. The second and most widely used method is Rotary Drilling. The equipment used is more complicated, but a rotary drill works more like a screwdriver, turning rapidly to force its way down. The rotary table rests on the floor of the oil derrick and is turned by an engine to turn the bit. The rotary table holds the kelly, the uppermost part of the hollow drill stem. The kelly attaches to the drill pipe, which connects to the drill collar that holds the bit. The draw works is what lowers and raises the drill stem out of the hole. A cable runs from the hoisting drum at the bottom of the derrick over two sets of pulleys farther up. A hook connects the lower pulley to the kelly to allow it to be raised and lowered. As the bit drives into the ground, the kelly will begin to sink down level with the rotary table. The drilling crew must stop, pull up the kelly and disconnect it to add another section of drill pipe. This is done about every nine feet the bit drills. Also, mud is forced down the drill stem to carry pieces of rock out of the hole. The mud plasters the sides of the hole, helping to prevent cave-ins. Another way of preventing a cave in is to line it with steel pipe called casing. This also keeps the oil well from contaminating ground water. The bit may become dull or encounter a different rock formation, requiring the drill crew to pull the pipe and change it. If the bit or drill pipe breaks off in the hole, the crew will try to pull it out with "fishing" tools. If that fails, they will pour cement into the hole over the obstruction and lower a tool called a whipstock down. They will try drilling again, passing the bit through the whipstock and around the obstruction. Once they have the drill down to where the oil reserve is, the drill crew can pull the drill pipe and focus on bring the oil to the surface.
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