No stone unturned: Advanced exploration methods

24 October 2011 (Last Updated October 24th, 2011 18:30)

Gaining a clear picture of the contents and traits of a deposit is no mean feat. But, with some of today's most advanced technology, we can develop our understanding of reservoirs and subsequently improve overall production rates, as Sarah Blackman reports.

No stone unturned: Advanced exploration methods

In an age where companies are racing to find economical quantities of oil as demand outstrips supply, there is no room for error. It is vital that oil firms improve their understanding of reservoirs in order to determine accurate estimates of production rates and prevent the risk of dry well drilling.

Of course, companies are unable to directly observe the physical properties and traits of a basin with their eyes, but with the help of advancing technologies, the picture is becoming clearer.

Avoiding risk

"There are several ways in which an oil company can detect fluid movements in a reservoir."

There will always be an element of uncertainty in decisions over reserve estimations and new field development plans, but good quality reservoir management can help mitigate risk when it comes to exploration.

Industry Technology Facilitator (ITF) director of strategic technology David Liddle said: "You can get to the point where you know that if you've put a well into the ground you are going to find something, and that is what it's all about. Getting a better understanding of reservoir characteristics leads to improved development and production methods."

In some cases, studying land formations can help geologists gain knowledge of rock formations before they move offshore. Currently, the University College Dublin is examining fault zones in rock outcrops that could compartmentalize a reservoir and cut off certain nations of the rock formation.

The study, Quantitative Parameterisation of Fault zones in Outcrop for Inclusion in reservoir Flow models (Quaff), aims to consider the potential impact on fault zone structure and related complexities on fluid flow.

Defining gravity

There are several ways in which an oil company can detect fluid movements in a reservoir. One solution is to use geophysical techniques to measure gravity.

Liddle explained: "Gravity can tell you a lot about fluid movements in a reservoir. It is measured in gals, but we can now look at developing technologies which can get down to micro gal measurements. It's essentially like trying to measure a fly landing on a whale's back."

UK-based geophysical service company ARKeX offers an airborne gravity gradiometry imaging solution called BlueQube, which is able to pinpoint oil, gas and mineral deposits by examining geological structures buried deep underground from the air.

This solution is also designed to work in marine environments to explore small density variations in underlying rocks quickly, accurately and cost effectively, with no adverse environmental impact.

In a recent report published in OilEdge, geophysicist David Bamford said: "Today, some of the world's leading international and national operators are specifying that GGI [gravity gradiometry imaging] should be part of their exploration plans. They are using GGI as an effective complimentary technology to seismic in some of their most complex and challenging geological situations."

Bamford noted that 2D and 3D seismic continues to be the principal geophysical technique of choice for explorationists, but conventional seismic also has its limitations, ranging from the high cost, long turnaround times and access issues.

He added: "From the sub-salt discoveries deepwater offshore Brazil and the Gulf of Mexico to the deserts and mountains of Africa, all too often the seismic wavefield is distorted and illumination irregular."

Liddle argues, however, that the industry is moving on from 2D and 3D seismic imaging and is entering 4D territory. "Seismic techniques are getting more reliable, more accurate and more resolute. 4D adds the dimension of time, so it's not just about understanding what's there; it's about understanding how they change over time," Liddle said.

"There are some interesting projects now that look at techniques, such as acoustic zooming technology, which basically allows you to pinpoint a particular area in a reservoir."

Other 4D seismic methods allow you to 'listen with light'. In February 2011, Stingray Geophysical launched its fibre-optic FosarFocas system which monitors waterfloods and fractures in a targeted well zone.

The sensing array is permanently installed on the seabed and acquires time-lapse data to help oil companies improve their levels of oil recovery.

Electromagnetics

While seismic methods measure acoustic velocities and provide essential geological information, electromagnetic exploration methods measure electrical conductivity, which can help determine the nature of the fluid inside the rock.

There are two types of electromagnetic methods, according to Steven Constable, professor at Scripps Institution of Oceanography in California. "Magnetotellurics use the natural variations in the earth's magnetic field and the controlled-source electromagnetic (CSEM) method uses a man-made source of electromagnetic energy that is towed near the sea floor.

"Then what you do is install electromagnetic recorders on the sea floor, which measure how quickly the electromagnetic fields decay as a function of distance and frequency," said Constable.

In order to better understand and characterise offshore reservoirs, scientists have developed their ability to integrate seismic and non-seismic methods, over the last decade.

Ten years ago, it wasn't always possible to combine the two techniques into one project, as CSEM methods proved to be unreliable in measuring electrical conductivity in water.

"In an age where companies are racing to find economical quantities of oil as demand outstrips supply, there is no room for error."

Constable explained: "The controlled-source method is a little unusual that in water - because of the fact that you've got electrically conductive water above your instruments instead of electrically resistive air - it does in fact behave very differently. That's just the nature of the environment, but recognising how that worked was a bit of a breakthrough. Chip Cox, my predecessor, worked that out."

As a result of this breakthrough, the CSEM method was developed as a deep water (greater than 1km) application. But, because of the poor interaction of signals in shallow water, the method could not be integrated in water depths of 300m or less, where many of the world's oil fields, particularly in the UK Continental Shelf, are located.

To resolve this problem, ITF launched a joint industry project dubbed SWOP in 2003, to develop and test methods for applying the CSEM method in shallow water.

Marine EM specialist OHM worked in collaboration with ITF and came up with a solution, which transmits a low frequency signal from a dipole source towed close to the seafloor, and, as a result, can operate in water depths of 100m or less.

Nanotechnology and beyond

In the hope of moving exploration technologies forward, in order to get a more accurate picture of the inner properties of a reservoir, some scientists are looking at how nanotechnology can be used as a sensing device.

Oil giant Saudi Aramco has developed Nanobots which have been able to penetrate reservoir pore spaces that have so far gone untouched by other exploration technologies.

Liddle said: "The next stage in development is whether we can turn these nanoparticles into some form of sensing device to give us some information about the reservoir.

"The company has a 20-year horizon of where they want to get to, which is really what the rest industry should have in many respects."