Oil & Gas UK Education Information Leaflets Fact Sheet 5 - North Sea Oil - Drilling For Oil Introduction In some ways, drilling for oil is like digging a hole in the sand at the beach. At first it is easy, but as the hole gets deeper the sides cave in or it fills with water. Lifting the sand out from the bottom is hard work as the hole gets deeper. And if you hit gravel or stones is gets harder to dig. Now imagine drilling a hole in the North Sea over 3000 metres deep. You cannot even see the top of the hole, because it is under a hundred feet of ocean. And how are you going to make sure that you drill in the right direction? History Wells were first drilled using a heavy chisel-shaped tool which was raised by a rope over a hoist and dropped into a hole in the ground. The constant pounding chipped away at rock to deepen the well. However, even when steam engines were introduced to haul back the chisel for the next drop, it was a slow and tedious process, which limited the depths to which drillers could operate. Tools and Techniques Luckily a century of practice has helped us solve some of these problems. Nowadays we stop the sides of the well caving in by lining the hole with steel pipes, which we fix in place with cement. The drillbit (which is covered with diamonds or tungsten - among the hardest materials on earth) spins at the bottom of the hole, and it is attached to the surface by long lengths of tubing called drillpipe. If the motor which makes the drillbit spin is on the surface, the whole drillpipe has to spin round to make the drillbit at the bottom rotate. If the motor is near the bottom of the hole, then it only has to turn the drillbit, and this takes less energy. The drillbit reduces the rock to tea-leaf sized particles called cuttings. To bring them to the top of the hole we pump drilling fluid down the hole under high pressure. When it gets to the bottom it picks up the cuttings - in the same way as a river picks up mud as it flows along - and brings them back to the surface. The drilling fluid also helps to cool the drillbit, which gets very hot as it spins. The drillbit wears out quite often, and to change it you have to pull the whole drillstring out of the hole, section by section, take off the old drillbit, put on a new one and put the drillstring back together to send the new bit back down to the bottom. Sometimes this whole job takes more than ten hours. If you want to see what sort of rock you have reached, you can either have a look at the drill cuttings, or you can pull out a 'core' of solid rock and have a geologist analyse it. Where to Drill? It is very important to end up in the right place, so we use special computers and equipment to tell us how deep to drill and in what direction. And in the last few years we have learned how to change direction while we are drilling. For example, we can drill downwards, and then gradually turn the direction of the hole so that it ends up being horizontal. This can be very useful for reaching new areas of rock which are far away from where the top of the hole is. If you are drilling a well under the sea you have to build a drilling rig to put your people and equipment on. Sometimes the rig stands on the seabed on very long legs, and sometimes it floats on the surface. When you have finished drilling the well, the rig can be moved to drill a well somewhere else. Leaflets Index Next Section

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