85- www. energy- future. com 6.1- Understanding oil and gas the stratigraphic sequence outlined by seis-mic, through a process called well logging. Derived from the word log in the sense of a record, logging involves lowering a tool down the well on an electrical wire to measure the properties of the rock around the borehole. The core measurement is resistivity - essentially the same as the breakthrough innovation made by Conrad and Marcel Schlumberger in 1927. The brothers Schlumberger measured the electricity resistivity of rocks in oil wells to determine the nature of that rock and whether it could, theoretically, hold oil. The measurement of the speed of sound along the borehole wall and radioactivity logs also yield data on the thickness and depth of res-ervoirs and their probable content. After a discovery has been made, ap-praisal wells are drilled to determine the size and composition of the reservoir, which will consist of water and either oil or gas - of-ten both. The all- important question that the exploration company needs answered is whether the reservoir can be produced prof-itably. There are lots of reasons why a dis-covery might not be economic even if oil and gas are present. The field might consist of multiple reservoirs and faults, which is tech-nically more difficult - and expensive - to produce. If it's offshore, it may not be prac-tical to drill the necessary number of wells from one platform. Perhaps the oil is too thick and viscous to pump to the surface without special - and expensive - equipment. Heavy duty After drilling, steel pipe called casing is set in the hole and is cemented into place ( see diagram). A heavy- duty system of valves called a Christmas tree is positioned at the wellhead to control the flow of the oil, gas TheDiscoverer Deep Seas drill ship. Drilling is the only way of getting hard proof that hydrocarbons are present

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