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Sat 27 Mar 2010 12:00 AM

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Fractures focus

Iain Bush says Schlumberger's Q-Technology services, combined with new processing methods, allow 3D detection of the position of fracture corridors within the reservoir.

Fractures focus
Iain Bush is an expert in developing workflows for naturally fractured reservoirs.
Fractures focus
Schlumberger’s Carbonate Research Centre is in Dharan, Saudi Arabia.

Iain Bush says Schlumberger's Q-Technology services, combined with new processing methods, allow 3D detection of the position of fracture corridors within the reservoir.

Most carbonate reservoirs are naturally fractured. The fractures exist at all scales, from microscopic fissures to kilometre sized structures called fracture swarms or corridors, creating complex flow networks in the reservoir.

As a consequence, the movement of hydrocarbons and other fluids is often not as expected or predicted. Just a few very large fracture corridors can act as highways for fluids in the middle of a carbonate reservoir; therefore, knowing their exact position is critical for planning new wells and for simulating and forecasting reservoir production.

"In fractured reservoirs our ultimate goal is to provide a complete geometric and dynamic characterisation of the fracture network. Recent advances in seismic technologies for enhanced reservoir delineation, characterisation and monitoring is doing much to bring us closer to that objective," says Iain Bush, business development manager, Naturally Fractured Reservoirs at Schlumberger.

Until recently, understanding of fractures in carbonate reservoirs has been hindered by low data density, which prevents effective fracture attribute mapping. Image logs and new core-recovery techniques, fracture data are frequently incomplete and sometimes misleading.

"We have been focused on integrating workflows for fractured reservoir characterisation and developing business around that," explains Bush. The core challenge of fractured reservoirs is that they are extremely complex, in so far as the fractures are distributed in networks. You can have fractures which are very small, almost micro-fractures, up to very large fracture faults which you can see at the macro scale on the surface.

"The real challenge is to characterise those fractures within the network which are contributing to the fluid flow. With well measurements and boreholes it is possible to get a good understanding of the fractures around the wells - the difficulty comes in the space between them."

These are visualised as swarms or clusters of fractures tightly packed in. They have a very high permeability but could easily be missed by a well if it doesn't strike through it. "It's important to be able to detect these in the interwell space, and being able to do that is playing a major role in understanding reservoir dynamics," says Bush.

"There are improvements in all areas. On the surface the measurements themselves allow us to see what we're interested in, which is the seismic reflections from the fractures," he says. This is being done through deploying a wider bandwidth - incorprating both higher and lower frequencies - and also a cleaner signal, which means less noise in the data.

"This provides a better quality and sampling of data enabling better discrimination at the reservoir level. These improvements allow better capture and analysis which allows us to extract the subtle signatures of the fracture corridor and clarify the data which then allows hugely improved overall characterisation."
The analysis and detection from the measurements is mainly from new algorithmic development.

"Of course the increase in processing power allows much greater data volumes to be processed, but that isn't the key factor here. Numerical processes - essentially algorithms which highlight the signature in 3D seismic is what is delivering the real breakthroughs. Detecting these small changes through noisy data is the key factor."

Middle East Hub

Schlumberger has been at the forefront of tackling the specific challenges of carbonate reservoirs and has a network of research laboratories and technology centers which is actively involved in carbonate projects. In the Middle East, the Dhahran Carbonate Research Center is entirely dedicated to carbonate research. The firm also has a regional technology centre to support this commitment. The centre in Abu Dhabi is primarily focused on carbonates and enables customers to work alongside the Schlumberger's experts to solve specific challenges for this region.

"The carbonate research centre in Saudi Arabia and regional technology centre in Abu Dhabi have very much been the hotbeds for our focus on carbonates and the various technologies. CarbonatesQ (Carbonates to the power of Q) is anexample of the sort of technology we can derive from what we learn through those technology centres," Bush concludes.

Some of the technology like fracture cluster mapping, which is critical for detecting carbonate fractures in reservoirs was developed out of the research centre in Dharan.

"These centres really are right at the core of our strategy, and the work that's coming out of Saudi Arabia and the UAE is both extremely important, and generating a huge amount of interest in the region," says Bush.

Despite the ample reserves, the recovery factor remains very low in carbonate fractured fields. "You may have a reservoir where some wells have watered out and are just producing water. This is a major concern because having drillied what may be an expensive well, if you are not producing oil then you have to start over. We can help avoid that by understanding where fracture corridors exist."

So far clients have been very receptive. Looking at fractures, we've had some good case studies from Kuwait where we able to resolve the fractures within the Jurassic reservoirs. In Saudi Arabia there is a field where the full story of the carbonate fractures is not completely understood they are trying out our technology there to see what they can achieve - so there is tremendous interest."

Much of the technology is applicable and extremely useful when you are looking at tight gas fields where the rock type has a low permeability.

"This is also important for CO2 storage. When you store CO2 under the ground you don't want the CO2 to leak through into other fractures, so there is huge potential for this kind of work," Bush concludes.

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