(MEBG)- Drilling for oil and gas is a highly skilled operation, often carried out in remote or difficult terrain. If the find is promising and commercial conditions are right, a field will be developed and brought into production.
Wells are drilled with rotary drilling tools, which work on the same principle as the carpenter's brace and bit. The cutting tool is the drilling bit, which has tough metal or sometimes diamond teeth that can bore through the hardest rock. The bit is suspended on a drilling string consisting of lengths of pipe, which are added as the bit goes deeper. The bit is turned either by a rotary table on the drill floor or, increasingly, by a downhole motor.
In time, the bit gets worn and has to be replaced. The whole drilling string, sometimes weighing over 100 tons, must then be hauled to the surface and dismantled section by section as it emerges. The new bit is fitted and slowly lowered as the drill pipe sections are re-assembled. In a deep well, this operation—known as a "round trip"—can take up most of a 12-hour shift. Until recently, the drilling crew carried it out manually. But automated drilling rigs, with mechanized pipe handling and computerised controls, are now being introduced to reduce drilling costs.
One of the essential supplies for the drilling crew is "mud", or drilling fluid. This is a special mixture of clay, various chemicals and water, which is constantly pumped down through the drill pipe and comes out through nozzles in the drilling bit. The stream of mud returns upwards through the space between the drilling string and the borehole, carrying with it rock fragments cut away by the bit. At the top, the returned mud is sieved and then re-circulated through a pump.
The cuttings left on the sieve indicate the kind of rock the drill is passing through, and they may show traces of oil as the bit nears an oil-bearing formation. The drilling mud keeps the bit cool and prevents the escape of gas or oil when the bit enters an oil trap. The drilling rig is a substantial piece of equipment, Therfor access roads may have to be built before the start of drilling in remote areas.
Nowadays, much smaller installations called slimhole rigs might be built to reduce transport costs. The rate of drilling varies with the hardness of the rock. Sometimes, the bit may cut through as much as 60 metres an hour, but in a very hard layer progress may be as little as 0.3 metres an hour. Most oil wells are between 900 and 5000 metres deep, but wells as deep as seven or eight km are sometimes drilled.
Wherever possible, wells are drilled vertically. But sometimes, especially offshore, wells have to be drilled which deviate from the vertical in order to reach a wide spread of targets from a single platform. This is known as deviated drilling.
Recent developments have made it possible to deviate from the vertical by as much as 90 degrees. Known as horizontal drilling, this technique can, in some instances, increase the productivity of a well. 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. Such blow-outs or gushers—common in the early days of the oil industry—pollute the environment, carry a high fire risk and waste hydrocarbons, so drilling technicians are now trained to prevent them.
The drilling 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. To avoid the blow-out, the supervisor 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.
During drilling operations, valuable information about the field at various depths is collected by a procedure known as logging. Drill cuttings returned to the surface are examined for traces of hydrocarbons, as well as for their fossil content. Wireline logs examine the electrical, acoustic and radioactive properties of the rocks, giving clues as to the rock type, its porosity and how much fluid it contains.
Sometimes, rock specimens—known as core samples—are extracted for laboratory examination. The first well to be drilled in an area is known as an exploration well. If oil is discovered, appraisal wells are then drilled to establish the limits of the field. If the field is developed, some of these appraisal wells may be used as production wells.
However, there are many factors to consider before a field is taken into production. How much oil does a reservoir contain and how much will it cost to extract (costs depend, among other things, on depth and how easily the oil flows to the surface)? How close is the field to potential markets? How many wells will be needed and where should they be located? What treatment facilities will be required?
© 2000 Mena Report (www.menareport.com)
