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How do well tractors work? |
 Over the past several decades, the oil and gas industry has adapted the technique of drilling
horizontally through the reservoir in order to increase the surface area in contact with the reservoir as compared to drilling vertically through it. One of the resulting side effects from this technique is that it became very difficult and costly to obtain measurements in this horizontal environment, as gravity will no longer put the electric line tools to the bottom of the well.
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Logging-while-drilling (LWD) |
 Formation evaluation and drilling mechanics information in real time are critical for effective geosteering (the act of directing a drill bit along a defined path while drilling a well). The path is chosen to maximise both the geological information acquired in a well and the surface area of any oil or gas-bearing reservoirs that are in direct contact with the wellbore, so increasing the ultimate production rates.
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 Drilling fluid (mud) is a mixture of clay, usually bentonite, and water or oil used to carryrock cuttings to the surface and to lubricate and cool the drill bit. Special chemicals are added to the mud to compensate for the varying composition of the water and the formation being drilled and to increase the weight of the mud column. The drilling mud also helps prevent the collapse of unstable formations into the hole and the intrusion of formation fluids. Fluid is circulated with mud pumps and a tank, called a suction pit, which makes up any losses.
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 After perforating, a drillstem test (DST) is performed to test the producing characteristics of a formation. It involves lowering a special tool downhole attached to drill pipe or tubing without removing the drilling mud from the hole. Offshore the DST is run safely in the casing string and the well may be produced for several days. The test information ultimately allows an assessment of the rate at which a well will flow hydrocarbons and for how long.
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 Designing the best drill bit programme for a well is also a specialised job. In exploration wells where the formations to be encountered and their thicknesses are liable to be uncertain a full selection of drill bits, which are highly engineered tools, must be available at the rig site to ensure the most effective drilling speeds through the entire section.
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Hazardous climatic conditions |
 Deep water currents are a major hazard in the Gulf of Mexico. Loop current events come through the straits of Yucatan, loop up into the eastern Gulf, then reverse and head down to Florida and the Keys. Here they enter the Gulf Stream travelling across the Atlantic to northern Europe.
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 After an area has been licensed, a combination of pre-existing and newly acquired seismic data, nearby and regional well data and regional geological analyses are used to define where to drill exploration and subsequently appraisal and development wells with the lowest risk of failure. When a well location has been selected a geological programme is written defining both the direct objectives and the information that must be collected,
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 Full-hole cores are cylinders of rock cut by a coring bit and retained in a core barrel behind the bit. Standard core barrels recover 20m or 30m cores although longer barrels have also been developed. There are many types of core barrel that can be used depending on the formation being cut.
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The depth of a well is measured by two different methods as drilling proceeds. The driller measures the length of all the components of the drill string (called the drillers tally) and adds them up to calculate the measured depth whilst the wireline logging engineer records the length of cable plus logging tools used to reach the bottom of the well.
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Valves at the surface, in the drill string and on the wellhead are used during drilling for well control, specifically to prevent the potentially disastrous consequences of a kick, which is the sudden influx of large amounts of gas into the well bore. The valves are designed to close the well exactly when required and under any circumstances although how it is done depends on what is in the well at the time.
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 If the reservoir appears impermeable there are remedial measures, called stimulation, which may be employed to induce flow. In the 19th and early years of the 20th century simple dynamite charges were used onshore to accomplish this task. It was later found that the walls of a borehole tend to rupture when the pressure of fluids in the hole exceeds a certain critical value.
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 At stages as the well is deepened casing will be run and cemented into the hole. Also at total depth after drilling has been finished a final casing string will be used if the well is to be tested and/or completed. Casing is metal pipe of progressively reduced diameter, cemented into a borehole at intervals. The casing point is the depth at which the base of each casing string is set.
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