On land in Oman:
"Land-based seismic surveying can be carried out using a variety of acoustic sources, such as dynamite, thumpers [dropping heavy weights on the ground] and Vibroseis®, which is the method used exclusively at Petroleum Development Oman [PDO]," says Andy McGinn, Chief Geophysicist at PDO, a company in which Shell has a 34 per cent interest.
"Vibroseis® works by using large trucks, from the center of which a large steel plate is lowered to make contact with the ground. A jacking system then raises the wheels of the truck partially off the ground, placing a great deal of pressure on the plate and providing a good coupling between the plate and the ground.
An acoustic signal is transmitted through the plate into the ground. The signal is received by thousands of geophones, which are laid out on the ground.
"We use nine Vibroseis® trucks at the same time, with two spares in case of breakdowns. These trucks work in groups of three and drive up and down between lines of receivers, known as the 'receiver spread'.
"The 'receiver spread' consists of four parallel lines of cables laid on the ground 300 meters apart and 15 kilometers in length. Attached to each cable, at 50-meter intervals, are arrays of geophones which themselves cover a surface area of 50 by 50 meters. Each array contains 18 geophones and their combined signal is digitized locally by an analogue to digital converter.
"With the cables laid out in their 15 kilometer pattern, the Vibroseis® trucks then drive between the centre cables in a zig zag pattern, with each source group generating a signal at intervals of 100 meters on average. This operating cycle takes a truck 46 seconds to complete, from lowering the plate, firing the acoustic signal, raising the plate and then moving to the next location 100 meters away.
"It is like a ballet of trucks moving and raising their back wheels off the ground."We use two seismic contractors, namely Western Geophysical and Veritas/DGC, to conduct our surveys for us. They both use Slip-sweep, a system of seismic surveying, which was developed by PDO geophysicists in partnership with our contractors and equipment manufacturers.
This system allows us to shoot and receive data 50 per cent faster than normal practices and translates into an annual saving in excess of US $10 million.
"The acoustic signal from the trucks propagates into the earth, is reflected by different geological formations and received by the geophones on the surface. The signal is then digitized and transmitted along the cables to a recording truck, which stores the data on tape for later examination and analysis.
"Land seismic surveys are quite different from marine surveys. They are operationally more difficult because the terrain is not flat like the sea and cables have to be physically picked up from one location to another rather than towed behind a boat.
"In some areas where we work, such as the Sabkha [salt flats] of Marmul, you can lose a truck by its sinking into the soft ground. There are also problems with mountainous areas and sand dunes, but we usually manage to get our equipment where we want it - even if we have to use mountaineers and helicopters.
"At PDO, we mostly shoot 3D seismic but we still use 2D for some areas which may not have been surveyed for a long time."The data from the seismic recording trucks is brought back to PDO headquarters, where it is analyzed by CGG, a French firm, who have some 50-plus staff based in our offices to process the data. We also have 15 PDO staff working on high technology processing.
"Oman has a diverse and complicated geology, and this makes the extraction of its oil and gas resources relatively difficult and expensive. Unlike Saudi Arabia, where the oil tends to be in very large concentrations, here in Oman we have some reasonably large fields but some of these are nearing the end of their lives.
It is, therefore, our job at PDO not only to new reserves of oil and gas but to develop existing finds and to optimize their recovery.
"Oman is a fantastic place, and for a geophysicist it has to be the most challenging posting within the whole Shell Group," he concludes.
Offshore in Iran:
"We hope to drill our first well in the Soroosh field by the end of October," says Calum Macdonald, Geophysicist with Shell Iran. "We will eventually be looking to drill 17 deviated production wells in the Nowrooz field and 10 horizontal wells in the Soroosh field.
"We have analyzed the 2D surveys carried out by the National Iranian Oil Company [NIOC] in the early 1990s, and have concluded a geophysical survey of the sea floor of the two fields. We are now conducting a new 3D seismic survey, using the ocean bottom cabling [OBC] method.
"There are two reasons why the geophysical survey came before the 3D survey. Firstly, it was to ensure that when the contractor laid the OBCs on the ocean floor they would not be laid on top of debris which could damage the cables.
There used to be 10 drilling platforms in the Nowrooz field and 10 in the
Soroosh field, but both fields and platforms were the subjects of aerial bombing during the Iran-Iraq War. So there are structures which have been destroyed in the area. The geophysical survey also looked for unexplored ordnance.
"Secondly, it provided us with information to assist in planning the final locations of the 27 development wells and identified sub-sea obstructions which would interfere with the construction of the drilling platforms and the pipelines that will connect them all together."
Moving on to describe the 3D survey, Calum says, "It is being carried out by CGG. Ocean bottom cabling is being used, as opposed to the towed array method, because the presence of so many structures can damage the equipment used in a towed array."
The Soroosh and Nowrooz 3D seismic survey is only the second ever undertaken in the Gulf. It covers 600 square kilometers, and will take three months to complete. Nine vessels are being used: three cable laying vessels, an acoustic source vessel, a survey vessel, an accommodation/processing vessel, a recording vessel, and two crew/chase vessels.
At any one time, there can be 190 people involved in operations offshore, and local resources are being used, with the port of Bushehr serving as a supply base and some 12 Iranians working in the crew.
Calum goes on to explain, "With the OBC acquisition method, OBCs are laid on to the sea floor from two cable laying vessels, which lay parallel lines of cable 400 meters apart, each cable being six kilometers long. Three lengths of cable are laid end to end, with a total of 18 kilometers of cable from each boat on to the seabed.
"These cables have two types of sensors built into them at 50 meter intervals. The hydrophones measure pressure, and the geophones measure vertical ground motion.
"In normal streamer operations, only pressure is measured. As the OBCs are laid on the solid floor of the seabed, both pressure and ground motion can be measured. This has advantages during the data processing stage and leads to better resolution in the sub-surface images.
"Once the two cable laying vessels have each laid their cable, a third source vessel sails perpendicularly back and forth across the two cables, making passes at intervals of 300 meters each time.
Every 25 meters, it fires compressed air shots under the water to create downward moving sound waves to penetrate the sea floor. These continue down through the sub-sea floor structures to a depth of some 4,000 meters.
"The sound waves produce signals which return to the OBCs and are read by the hydrophones and geophones. They are then transferred along the cables to the recording vessel, where the data is recorded on to magnetic tapes. These tapes then go to a processing center where the final 3D sub-surface image is generated.
"The Nowrooz and Soroosh fields were discovered in the mid-1960s and between them have produced in excess of 1.6 million barrels of oil. Following the Iran-Iraq War, production has continued on Nowrooz until today, but from only two wells and at a modest level.
"Shell, in an agreement with the NIOC, plan to jointly develop the two fields so that, by Autumn 2003, the Nowrooz field will be producing 90,000 barrels per day and the Soroosh field 100,000 barrels per day," he concludes.
Source:Shell.com/me
© 2000 Mena Report (www.menareport.com)
