86- www. world- petroleum. org 6.1- Understanding oil and gas and water and prevent a blow- out - high pressure downhole can cause oil and gas to spurt out of a well, often with dangerous results. The well casing is perforated at the right depths to make holes for the oil and gas to flow into the drilled shaft - or well-bore - and up to the surface. The first recovery phase is called pri-mary recovery. Underground reservoirs of oil, gas and water are under considerable pressure and their contents flow naturally once perforated. But eventually the reser-voir runs out of natural energy and the oil needs a helping hand to move to the sur-face. That's where enhanced- recovery techniques come into play ( see p67). Horizontal drilling In some places, in- fill drilling will work - sinking clusters of wells into the same area so the oil does not have to migrate as far through the rock to reach a wellbore. But in deep wa-ter, when price tags of up to $ 100 million start being waved around, it will not be economic to drill more than a few wells, so placement is the name of the game. In this situation, direc-tional wells, which can be steered downwards, sideways, horizontally and even upwards, are often used. In the right circumstances, they can prove a much more effective way of tap-ping an oil field than vertical wells. The North Sea's Clair field, in the Atlantic, off the Shetland Islands' west coast, was dis-covered in 1977. But although it was esti-mated to contain a staggering 5 billion bar-rels of oil - putting it on a par with the prolific Forties field, a giant of the North Sea - BP had to wait 27 years to start developing Clair. The problem? Clair's oil is contained in a very fractured reservoir and in the 1970s there was no way of producing commercially from any section of the field. Indeed, many experts predicted at the time that it would never be exploited. Improvements in seis-mic mapping and the arrival of horizontal drilling changed that. Horizontal wells cut through a greater length of the reservoir and can link up iso-lated sections. Well for well, horizontal drill-ing is far more expensive than vertical drill-ing, but in the right circumstances, productiv-ity gains make the extra investment worth it. The end- game Once the field's recoverable reserves are exhausted, infrastructure must be decom-missioned. After years of intense explora-tion, a wave of decommissioning is start-ing in mature provinces such as the US and the UK North Sea. It has become impera-tive for decommissioning to be handled with the utmost sensitivity to the local environ-ment. Yet, once E& P teams are long gone, oil fields have another use: they can serve as storehouses for the carbon that is pro-duced by fossil- fuel processes and removed through the evolving technology of carbon capture and storage. So they can be part of the future as well as part of the past. ?? Surface casing Perforations Oil enters through perforations Production casing Tubing Cement Oil produced to surface A typical oil well

87- www. energy- future. com 6.2- Understanding oil and gas Refining and petrochemicals: explained Crude oil straight out of the ground is pretty useless, but it becomes extremely useful after being refined into oil products such as gasoline, diesel and jet fuel. Indeed, it can be invaluable: if you ran out of fuel during a drive through the desert and were 200 kilometres from the nearest water source, would you rather have 50 litres of water or 50 litres of gasoline? Refineries come in many different sizes and configurations, depending on the size of the local market, the types of products needed and the types of feedstocks availa-ble for processing. But they all perform the same basic tasks: distilling crude oil into its various constituent fractions; chemically re-arranging low- value configurations of hydro-carbon molecules into high- value combina-tions to produce a variety of end- products, from gasoline to Tupperware; and treating those products to meet environmental and other specifications and standards by re-moving impurities such as sulphur. Carbon chains Crude oil can be split up into molecules of carbon and hydrogen in a variety of com-binations through the refining process. Depending on the length of the chains within them, they can be used in a variety of ways. For example, molecules used for cooking gas usually have up to four carbons, while gasoline for cars is a longer chain, of up to 12. Lubricants - motor oils, for example - are even longer, with perhaps 50 carbons. The different chain lengths in petroleum have different boiling points, so they can be separated by heating the crude and distilling the resulting vapour ( see p91). The first step is heating up the crude oil - once impurities such as water and salt have been removed from it. The heat is often gen-erated by burning fuel oil in a furnace. The vaporised petroleum, heated to about 350° C, is pumped into a fractionating tower - or atmospheric pipestill. As it rises up the tower, it cools down and its com-ponents condense back into several dis-tinct liquids, collecting in a series of trays. Lighter liquids, such as kerosene and naph-tha, a product used in chemicals process-ing, collect near the top of the tower, while heavier ones such as lubricants and waxes fall to the bottom. Vaporised petroleum, heated to about 350° C, is pumped into a fractionating tower