Drilling for Oil and Gas

Drilling for oil is a costly process, especially offshore. The object of a well is to prove the presence of, or produce, oil or gas. Sometimes wells are also drilled to inject water, chemicals or steam into the reservoir during production. Even a well which fails to find hydrocarbons (a dry well) is still of great value, because of the information it provides about the rocks in the area. This information forms part of the basis for the geological maps and profiles which are used in further exploration for oil and can be sold or exchanged for data from other companies. This is the reason why oil companies wish to keep the geological results of oil drilling confidential for some years after a well has been completed.

Oil drilling used to be carried out largely on land, but now offshore drilling takes place not only on our continental shelves, but also in deep water (1–3 km). This type of drilling is many times more costly than drilling in shallow water or on land. This has led to increased efforts to gain maximum information from wells. The cost of analysing samples and logs is small in relation to the cost of drilling the well.

We shall not go very deeply into the technical aspects of drilling for oil here, but merely look briefly at some of the most important principles. When drilling commences at the surface, the diameter of the well may be 20"–30" (50–75 cm), but decreases downwards to 3"–6" (7–15 cm) at great depths. Normally a roller bit is used, which crushes the rock into small pieces (about 2–5 mm) called cuttings. Core samples are only taken when drilling through especially important rock strata (usually reservoir rocks) where large intact samples are needed for detailed examination.

A circular diamond core drill bit must then be used. This takes time and costs a lot more per running metre, as the entire drill string has to be recovered to get each core section to the surface. Only the most critical sections are therefore cored. Drilling mud is pumped down through the drill string into the well during drilling. This mud has several functions. When one drills several hundred or a 1,000 m down into rock, one encounters water, gas or oil which may be under high pressure.

The drilling mud acts as a counterweight to prevent the uncontrolled gush of water or petroleum into the well and up to the surface in a blow-out. The pressure exerted by the drilling mud must exceed the pressure of oil and water in the surrounding formation. Heavy minerals such as barytes are frequently added to increase density; the main components of drilling mud are montmorillonite (smectite) containing clays, with a large number of different additives. The drilling mud also serves to cool the drill bit, and cuttings are brought back to the surface suspended in the circulating mud. The cuttings are then washed out from the drilling mud onto a sieve (shale shaker) and the mud can be used again.

Not all the cuttings which come up with the drilling mud have necessarily come from precisely the strata being drilled through at that time. There may also be contamination due to the caving in of overlying strata into the rising drilling mud. This means we can find material from younger rocks with a different composition mixed in with the formation being drilled, together with younger fossils. This demands considerable care when making stratigraphic interpretations based on microfossils identified in cuttings. The safest way is to register the first occurrences of a species when proceeding downwards from the top in the well. The last occurrence of a fossil may be the result of cave-ins from younger strata.

Since the pressure of the drilling mud is being monitored and adjusted to prevent oil and gas from penetrating into the well, significant oil and gas occurrences may be drilled through without being registered. This should be detected on the logs but it may be advisable to carry out special tests in the most promising strata to find out if there is petroleum present, and in what quantities. As drilling proceeds, the well is lined with steel casing to prevent rock and loose sediment falling into it, but prior to casing, each section of the well has to be logged with different logging tools which require physical contact with the wall of the well. Radioactive logs, however, can also be run after the casing has been installed. It is useful to note when the different casings are installed, because that limits the strata which could have “caved in” and contaminated the cuttings. If the well is going to produce oil, a production pipe is used and installed running through the petroleumbearing strata. It is then perforated by shooting holes in the steel casing (in the oil column) so that petroleum can flow into the well.