Photographs from aircraft and satellites are used to begin the onshore search for oil and gas which is underground. This cuts down the time spent searching on the surface. The photographs are studied very carefully for the structures where oil might be found. If an area shows promise, then teams are sent to find out more about the rocks.
Geologists and geophysicists work closely together using a variety of methods. All the information is carefully considered, with the help of computer analysis, before any decisions to drill are made. A geologist collects small samples of rock. Sometimes the samples of rock are dug out by hand or cylindrical coresare drilled to give samples which can be cut and studied under a microscope. These help them to find out where the rocks have come from, what they are made of and how the rocks are arranged in strata.
Geologists also find out about the physical and chemical propertiesof the rocks and the fossil record from ancient times. All these clues give information to build up a picture of the area being surveyed. A geophysicist adds to the information of a geologist by studying the physics of the Earth. Surveys are made of the magnetic field, the gravity and how waves travel through the layers.
Magnetometers measure very small changes in the strength of the Earth's magnetic field. Sedimentary rocks are nearly non-magnetic and igneous rocks have a stronger magnetic effect. Because of these different effects on the magnetic field, measurements can be made to work out the thickness of the sedimentary layers which may contain oil.
Gravitometers measure the strength of the Earth's gravitational pull. This is not the same all over the Earth because of the different densities of the rocks. Igneous rocks like granite are denser than sedimentary rocks. Granite near the surface will have a stronger pull than the same lump deeper down, so measurements help to build up more information about the layers of rock.
Shock waves or seismic waves are used to help give a picture of deep rock structures. The idea is to make artificial shock waves and record how they travel through the Earth. The shock wave travels through the water and strikes the sea bed. Some of the energy of the wave is reflected back to the hydrophones. The rest of the wave carries on until it reaches another rock layer.
The time taken for the waves to travel from the source to the hydrophones is used to calculate the distance traveled - hence the thickness of the rock layers. The amplitude of the wave gives information about the density of the reflecting rock. A survey using artificial shock waves is called a seismic survey. The data from a survey is recorded and displayed by computer as a pattern of lines, called a seismograph.
Sometimes, surveys show that a structure is present which may contain oil and gas. If so, an exploratory well or wildcat well is drilled. Very few exploration wells find oil. Even in areas like the North Sea, where we know a great deal about the geology, only one in every eight wells which are drilled will find oil or gas in quantities worth developing.
Drilling is a very expensive activity, with each well on average costing several million dollars. Even with today's technology, there is still a low probability that oil or gas will be found. Most oil wells are between 900 and 5,000 metres deep, but it is now possible to drill 8 km below the surface, an achievement made possible by skilled operators using powerful equipment and advanced technology. However, the costs of drilling can double or treble when in very deep water, hostile environmentsand when high pressure or temperature isencountered.
The rock is drilled with a rotating drill bit, similar to those that are used to drill a hole in wood. The drill bit is attached to a string of steel pipes, each approximately 9 metres long. The derrick, the structure that stands above the hole, must be strong, as the drill pipe and bit are suspended from it. Only a small proportion of the total weight of the drilling string is allowed to bear on the drill bit. This proportion will vary depending on the rock formation being drilled. The derrick must also be tall enough to enable the individual lengths of drill pipe to be added to or removed from the string.
The drilling process is lubricated and cooled by a carefully constituted mud. This passes down inside the pipes to the drill bit and then returns to the top of the hole between the pipe and the sides of the hole, bearing rock debris with it. This provides the geologists with rock samples to indicate the kind of rock the drill is passing through.
The weight of the mud also prevents the escape of oil or gas if it is found. Usually the gas or oil is under pressure in the ground. To stop wasteful and dangerous gushers, a set of valves called a Christmas tree is fitted to the well head to control the flow of fluids from the well.
(Material is supplied by the Institute of Petroleum)
Дата добавления: 2015-09-10; просмотров: 4 | Нарушение авторских правAcquisition | Processing | Answer the questions. | Read and give the Russian equivalent of the given word-combinations. | Latest developments | Terms and Vocabulary | Acquisition of Seismic Data | SHOT HOLE | WORDLIST | APPENDIX. Use the table to tell about the four major geophysical methods used in oil exploration. |