4. Read and translate the text. Text a drilling

Because of the subterranean origin of petroleum it must be extracted by means of drilling wells. Until an exploratory well, or wildcat, has been dug, nobody knows whether or not petroleum lies under a particular site. In order to reduce the number of exploratory wells drilled, scientific methods are used to discover the most promising sites. Sensitive instruments, such as the gravimeter, the magnetometer, and the seismograph, may be used to find subsurface rock formations that can hold crude oil. Drilling is a complex and often risky process. Some wells must be dug several miles deep before petroleum deposits are reached. Many are now drilled offshore from platforms standing in the ocean bed.

Usually the petroleum from a new well will come to the surface under its own pressure. Later the crude oil must be pumped out or forced to the surface by injecting water, air, natural gas, steam, carbon dioxide, or another substance into the deposits. Improved recovery techniques have increased the percentage of oil that can be extracted from a field.

The well bore length exceeds many times its diameter. The opening of the well bore at the surface is called the well head, and its lower part the bottom hole. The depth of an oil-well is controlled by the depth at which oil is found. Deep holes and high pressures are characteristics of petroleum drilling. In general, it becomes more difficult to control the direction of the drilled hole as the depth increases, and additionally, the cost per foot of hole drilled increases rapidly with the depth of the hole.

The hole direction must be controlled in order to reach a desired target at depths as great as 25,000 ft (7600 m). Inclined layers of rocks with different hardness make the direction of drilling deviate; consequently, deep holes are rarely truly straight and vertical. The drilling rate generally increases as additional drill collar weight is applied to the bit. A so-called packed-hole technique has been used to reduce the tendency to hole deviation. One version of this technique makes use of square drill collars that nearly fill the hole on the diagonals but permit fluid and cuttings to circulate around the sides. This procedure reduces the rate at which the hole direction can change.

Advances in technology and the need to accomplish special objectives have led to drilling horizontal wells in deep oil or gas reservoirs. New drilling methods include studies of rock failure by mechanical, thermal, hydraulic, fusion and vaporisation, and chemical means. Jet piercing is widely used for drilling very hard rocks, such as taconite. Other methods include the use of electric arc, laser, plasma, and ultrasonic drills.

After a well is drilled, it should be completed. Completion means the operations that prepare a well bore for producing oil or gas from the reservoir. The goal of these operations is to optimise the flow of the reservoir fluids into the well bore, up through the producing string, and into the surface collection system. The well bore is cased with steel pipe, and the annulus between well bore and casing is filled with cement. Properly designed and cemented casing prevents collapse of the well bore and protects fresh water beds above the oil and gas reservoirs from becoming contaminated with oil and gas.