|
Using electromagnetics to find oil Electromagnetic hydrocarbon detection shows promise, technique has potential to locate large oil and gas pools in rock strata offshore Alan Bailey Petroleum News
Despite major advances in techniques such as 3-D seismic surveying, the direct detection of underground oil and gas deposits has remained an elusive desire for exploration geologists and geophysicists: Exploration invariably involves drilling to test for the uncertain presence of oil in geologic structures appropriate to hydrocarbon entrapment.
But a technique called controlled-source electromagnetics, or CSEM, shows promise for detecting underground hydrocarbons in some situations. Working a bit like a giant metal detector, the technique involves sweeping the subsurface rocks with a low-frequency electromagnetic field and then detecting any resulting responses from the subsurface materials.
At the May meeting of the Geophysical Society of Alaska, ExxonMobil Research Co. Chief Research Geoscientist Leonard Srnka talked about the development of CSEM techniques and their modern application.
First tried in 1934 The first recorded use of CSEM occurred in 1934 when Schlumberger tried using a direct-current version of the technique to locate massive sulfide ore bodies off the Cornish coast of England, Srnka said. And in World War II the U.S. Navy used the technique to locate mines in nearshore waters.
Major breakthroughs in the technologies required for CSEM began around 1978, when scientists at the Scripps Institution of Oceanography at the University of California San Diego started building receivers to detect natural variations in electromagnetic energy in the oceans, Srnka said. The Scripps receiver designs still form the basis of the instruments in use today, he said.
And research at the University of Cambridge, England, into the processes occurring in the central ridges of the Earth’s oceans led to the design of equipment to source the electromagnetic energy that could be used in CSEM surveys.
Meantime, Exxon and Norwegian oil company Statoil independently initiated research into the potential use of CSEM techniques for detecting hydrocarbons.
Srnka joined the Exxon research team in 1979. In 1981 he reviewed research into the problem of electromagnetic disturbances and radio waves at the surface of the Earth obscuring the CSEM signals. Ideally, resolving this problem would entail burying both the transmitters and receivers 2,000 feet or more underground, but that arrangement was clearly impractical.
Rather than burying the equipment, might it be possible to eliminate surface noise by submerging the equipment under deep water?
“We had a conversation … around the coffee bar and said ‘what about seawater?’” Srnka said.
And computer simulations confirmed the concept.
“It became obvious that if you do this in a marine environment, in deep water — the deeper the water the better — it works out just fine,” Srnka said.
Got going in the 1990s Although Exxon obtained a patent on the technique that its research team had conceived, the oil price crash of the mid-1980s and the rise of interest in 3-D offshore seismic caused the company to stop further research at that time. But by 1996 improved electromagnetic receiver technology, improved computer technology, an interest in deepwater oil exploration and a desire by Exxon management to find “breakthrough opportunities,” led to a resurgence of interest in CSEM, Srnka said.
“The confluence was there so we launched a project called R3M, which stands for remote reservoir resistivity mapping … and off we went,” Srnka said. “… By 2001 we were in the water doing our own tests.”
Those tests have led to a successful technique that involves towing a piece of equipment called a horizontal electric dipole through deep seawater. Neutral buoyancy enables the dipole to “fly” 25 to 50 meters above the seafloor in water typically one-half to 3 kilometers deep (1,500 to 9,000 feet). A power source in the vessel towing the dipole pumps a low-frequency, low-voltage alternating electric current into the seawater, thus causing low-frequency electromagnetic waves to diffuse through the subsurface rocks.
An array of electromagnetic receivers in a pattern along the seafloor detects electromagnetic signals induced by rocks and fluids in the subsurface. The receivers may be offset 1 to 12 kilometers from the source dipole.
The receivers are dropped overboard from a surface vessel and each receiver sinks under the weight of a concrete base. With experience it is possible to position receivers about 25 meters apart, or to winch them down within a few meters of their desired positions, Srnka said.
After completion of a survey, a latch releases each receiver from its base, thus allowing the receiver to float to the surface for retrieval, Srnka said. A biodegradable binder in many concrete bases causes the used bases to decompose over a period of a few months.
The use of low-voltage electricity appears to minimize any environmental impacts of the technique.
“There’s been a lot of work on the environmental aspects of this for marine life,” Srnka said. “All the studies so far say it’s benign. … We’re very encouraged by that.”
Detects electrical resistance Unlike a metal detector, which uses electromagnetic energy to find objects that conduct electricity, CSEM detects subsurface materials that resist the flow of electrical currents.
Essentially, a rock such as a shale that tends to conduct electricity absorbs the electromagnetic energy, thus hastening the energy degradation. However, a more resistive material such as crude oil causes much less signal loss.
“As soon as the energy gets to an (oil or gas) reservoir it propagates a very large distance very quickly,” Srnka said.
By placing electromagnetic receivers over a range of distances from the source dipole, geoscientists can see how the amplitudes and phases of the received signals vary with distance; unusually high amplitudes over a range of offsets from the dipole indicate the existence of resistive material such as an oil pool in the subsurface.
And the remarkable sensitivity of the receivers provides the key to enabling the technique to work.
“If you took the batteries out of your flashlight and put them together, connected one end to the ground and the other end to the sun, the instrument could measure that resulting electric field,” Srnka said.
Moreover, through computer processing of the CSEM data it is possible to produce maps of the subsurface electrical resistance. Those maps can be merged with seismic data, to evaluate the prospect of oil existing in geologic structures seen in the seismic.
Promising results So, how well does the technique work in practice?
Srnka described several CSEM surveys that Exxon has conducted, primarily in deep water off the coasts of Brazil and West Africa.
In one survey in a known West African oil field the oil pool showed up as a clearly defined electromagnetic amplitude anomaly that mirrored the actual shape of the reservoir. And in a Brazilian prospect, a CSEM survey pointed to an oil pool in a sand body identified from a seismic survey. Drilling then proved that oil pool’s existence.
However, the surveys can’t usually penetrate depths below about 3,000 meters (9,000 feet) below the mud line. And the spatial resolution degrades with depth — in general it’s only possible to see objects that are about as wide as they are deep. So, for example, at a subsurface depth of 3,000 meters it is only possible to see an object that is about 3,000 meters across.
“It’s like an elephant hunt at this stage,” Srnka said.
And although the technique can quite accurately depict the horizontal shape of an object, it provides little information about the vertical thickness of the object. Assessing the vertical thickness requires the use of other types of data, such as seismic data.
Because seawater conducts electricity, sea currents can disrupt the electromagnetic signals. And the technique does require relatively deep water to be effective.
However, the shallower the subsurface target, the shallower the water can be. Industry has worked in water depths as shallow as 75 meters (225 feet), Srnka said.
“The key is not to have super deep targets in very shallow water,” he said.
But the continuing development of improved data acquisition technologies should improve data quality in the future — Srnka drew parallels with the evolution of marine 3-D seismic technologies.
And there seems to be burgeoning interest in the use of CSEM. Srnka said that by the end of 2006 as many as 250 CSEM surveys had probably been carried out worldwide, with surveys happening at a rate of about three per month. Schlumberger; Offshore Hydrocarbon Mapping; and Electromagnetic Geoservices now offer CSEM services and two major seismic contractors are seriously considering adding the technique to their service lines, Srnka said.
One exciting future possibility is the use of the technique to monitor oil field depletion, rather like the use of 4-D seismic data.
“I think it’s going to spread into development, delineation and production, almost certainly,” Srnka said. “It’s highly repeatable data.”
Other possibilities include the simultaneous acquisition of CSEM data and marine seismic, and the development of cheap methods of CSEM data acquisition over large areas.
It all comes down to how the value of the CSEM data compares with the cost of the data from other methods such as seismic, or with the cost of drilling a well. But, results so far look good.
“The pre-drill success rate for us and for others has been very high … people are very encouraged,” Srnka said.
| 