Facts, Figures and Explanation Drilling for Oil Once the environmental and seismic surveys are completed, and if the results look good drilling an exploratory well is considered. Even at this stage, it remains an uncertain business with no guarantees. There is still a high risk that nothing at all will be found, or that the oil will be in such small quantities that it would not be worthwhile extracting it. In the North Sea only about 1 in 8 exploration wells find quantities of oil and gas that are economic to develop. Drilling Click on the thumbnail to view a larger image Drilling exploratory wells Three different ways of drilling exploratory wells: Jack-up unit This is a barge with legs that can be raised and lowered. It is usually towed into position, its legs lowered and then once they are sitting on the seabed, the barge is raised out of the water creating a stable drilling platform. Jack-up units are usually used in up to 100 metres of water. Semi-submersible drilling rig These usually have an engine so they don't need to be towed. The working platform is supported on vertical columns that are attached to submerged pontoons. Once in position the pontoons can be flooded with water to lower the whole unit further into the water. The lower the pontoons are beneath the water, the less likely they are to be affected by wave action. This makes them stable in rough seas and in depths of 300 metres or more. Drillships These look more like a conventional ship and can be easily moved between locations. The latest drill ships can drill in water depths of 1500 metres or more, but they can be unstable in rough seas. On Board Click on the thumbnail to view a larger image Spudding the well The first step is to drill a 90cm hole and then put a wide pipe into the seabed to guide the drill and the drilling fluid. As each section of the well is completed it is lined with a heavy steel pipe casing that is cemented in place to prevent it caving in. This process is called "spudding the well". The derrick The derrick is the tall structure that supports all the drilling operations. It is designed to haul the drillstring - lengths of drillpipe with the drill bit on the end - in and out of the hole. Sections of drillpipe or drillstring are added as the hole gets deeper. Each piece of drillstring is typically 10 metres long, about the length of a bus which explains why the derrick, which needs to accommodate 3 lengths of drillpipe, has to be so tall. The drillstring can weigh several hundred tonnes, so a powerful motor is needed to winch it up and down. The drill bit The drill bit is attached to heavy drill collars that put weight onto the bit. The drill bit is rotated either by turning the whole drillstring or by a turbine down the hole that is driven by drill fluid. Depending on how hard the rock is, the drilling rate can be less than 30 centimetres an hour or as much as 60 metres an hour in relatively soft rock. The drill bit needs changing every few days, or maybe every few hours, depending on the hardness of the rock. When the drill bit has to be changed, the whole drillstring is pulled back up, uncoupled in sections, stacked up, the drill bit changed and the whole process starts again. This is known as a 'round trip' and can take 10 hours or more. Use of 'mud' Drilling fluid, which is commonly known as 'mud', is continuously pumped at high pressure down to the drill bit to lubricate it and keep it cool. The mud also flushes out the rock cuttings and brings them back to the surface. Geoscientists are able to inspect and analyze these tea leaf sized samples and gain more information about the rock structures and the presence of hydrocarbons. The person who does this is known as a mudlogger. Another important consideration is that the force and weight of the mud that is pumped down the drillstring into the well, balances the pressure of the crude oil and gas in the surrounding rocks and so significantly reduces the risk of a blow out. Controlling the well Because the oil and gas, deep below the Earth's surface are at high pressure great care has to be taken to control the pressure. In cinema films they often show what is called a 'blow out'. This can occur when a drill enters a reservoir and the pressure causes the oil and gas come spurting out of the well. The result is potentially very dangerous. Although blow outs are very unusual, all wells are fitted with an emergency valve designed to prevent this from happening. Core samples If the geologists find something particularly interesting they can ask for a core sample. A hollow drill, called a core barrel, is attached to the drillstring and as it goes down a core of rock forms inside - a bit like using a giant apple corer! This core sample gives a continuous record of the different layers of rock and therefore more detailed information than the rock cuttings. Collecting a core sample is expensive and time consuming because it involves a complete 'round trip'. Logging Before a well is finally capped, vital information is gathered by lowering measuring devices contained in what as known as a sonde down the hole on a wireline. As the line is pulled back up the hole, the sonde transmits information to a computer on the surface about the porosity and other qualities of the rock it is cutting through. This information provides a survey of the well and gives more information about the presence of hydrocarbons in the pores. This technique is called logging. Field Appraisal If an exploratory well shows that hydrocarbons are present, more seismic data is gathered and then a number of appraisal wells are drilled and more data is collected. From this data it is possible to estimate how much oil and gas the field contains, how difficult it will be to extract and what percentage of the oil and gas can be extracted. Unfortunately, it's not possible to get every last drop of oil out of a reservoir - in the North Sea, for example, operators expect to recover about 40 - 50% of the total reservoir. This figure may seem quite low, but remember there isn't a lake of oil and gas. Instead it is trapped in the pores of the reservoir rock and you can appreciate that it isn't possible to get every last drop out. Think back to the idea that oil is held in pores in much the same way that water is held in a sponge. We know that it isn't possible to get every last little drop of water out of a sponge - it's always a bit damp. Oil exploration involves lots of people with different skills, for example geologists, geophysicists, surveyors, mudloggers, computer scientists, marine biologists, drilling engineers, the drilling crew. Alongside these are all the people employed back onshore for example economists, planners, lawyers, IT specialists, environmental advisers and safety advisors to name a few. Facts, Figures and Explanations Index Next Page

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