The rate of drilling varies with the hardness of the rock. Sometimes the bit may cut through as much as 200 feet an hour, but in a very hard layer progress may be as little as one foot an hour. Most oil wells are between 3,000 and 16,500 feet deep but wells as deep as four or five miles are sometimes drilled. Whenever possible, wells are drilled vertically, but sometimes, especially offshore, it is necessary to deviate from the vertical in order to reach a wide spread of targets from a single platform. Recent developments have made it possible to deviate as much as 90 degrees from the vertical.
Special care is needed during drilling as the bit nears a formation containing oil and gas.
The high pressure in an oil trap may force oil and gas up to the surface in a violent surge as the drill breaks through the impermeable rock. The supervisor in charge of drilling can anticipate the danger of a blow-out occurring when rock chippings from the well bottom show traces of oil, or when instruments on the derrick floor show rising pressures in the well. He can pump down heavier drilling mud to hold back the oil or close special valves, known as blow-out preventers, fitted to the top of the well casing. Drill cuttings which are returned to the surface are examined for traces of hydrocarbons and for their fossil content. Wireline logs examine the electrical, acoustic and radioactive properties of the rocks which give clues as to the rock type, its porosity and how much fluid it contains.