Carbonate evolution

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Qatar’s vast North Field and Iran’s South Pars hold 30% of the world’s known natural gas reserves.

On top of all these factors impacting the carbonate structures is the issue of tectonic activity. "When the plates beneath us move they tend to buckle, bend or crack under the huge pressure.

This forms fractures and these structures can have a huge impact on the recovery factors. When seawater is injected to push the oil towards the production well, under natural pressure it may go into these cracks and get directed away from the matrix or body of oil you are targeting."

Heterogeneity is the number one challenge when dealing with carbonate reservoirs. The secret to maximising production in these fields lies with accurately mastering the reservoir description.

"Being able to locate where those big fractures are, knowing how the different rock types are distributing the reservoir, and from that being able to create a complete description of the permeability of the reservoir is making a step in the right direction."

Ultimately, if this can be achieved, then placing the wells and pinpointing where to inject water or gas will have a potentially massive impact on the production yield.

Field evaluation

A reservoir may be 30 km East - West and 50 km North - South, and maybe 1 kilometre deep. When you are looking at a field on this scale its vital to understand how the fluids are going to move within it.

"A good analogy is to imagine the reservoir as a roadmap of a country, and understanding the vehicular traffic through it," says Montaron. "For example, throughout a country there will be lots of small roads running all over, but that's not an efficient means of getting around. If any significant distance is going to be covered, then highways are the quicker routes.

Just like a road map, within the matrix of the carbonate reservoir are fractures, which act as highways. Most of the movement happens along these fracture corridors formed by tectonic activity. Some of these major arteries could be 10 metres wide, three kilometres long, and if fracture corridors can be found and tapped, then you hit these highway for fluids."

When drilling, if by sheer luck, the vertical well hits the fracture corridor then you have an area where permeability and the flow of oil from all the surrounding cracks will migrate - essentially taking advantage of a giant natural feature of the reservoir to drain all of the oil towards the well.

"However, if you drill just a hundred metres away you could be in a very tight matrix of the reservoir, and the productivity of the well could be 50 times less that the one that struck a fracture corridor. Of course, it could still produce 2000 bpd, but if you found the natural highway for the oil, it would be more like 20 000 bpd."

In carbonate reservoirs it can be quite common that out of 25 wells drilled, just one of those may produce 60% of the total yield, and this is an illustration of the difference striking a corridor could make.

"Of course, on the other hand, it could also happen that the fracture corridor is connected to water, in which case your well will produce and recycle a lot of water, which is far from ideal, so understanding which fluids these structures will bring to your wells is essential."

Knowing that the fracture corridors act as conduits for oil or water is just the first step. The major problem is that in the scheme of an oilfield seismic report, these fractures are very difficult, almost invisible to see on seismic images.

Understanding the big structures and taking advantage of high-resolution seismic imaging, and new workflows to characterise the reservoir is a fundamental step towards improving recovery yields.

"It's only very recently with seismic we've been able to see these fracture corridors. The seismic signature is very small, and they are easily confused with noise. To get a picture of these structures you can't filter anything out of the pictures; you want all the noise and a very smart way of pinpointing these hard to see structures.

Fracture corridors are impossible to see with the naked eye and even the best seismologists will only be able to see the folds above, but the corridors remain near invisible.

"Smart processing of high resolution seismic - a new workflow called FCM for fracture cluster mapping - will show you these structures, but you need to understand each rock layer within the reservoir. Comprehensive, detailed reservoir characterisation is the answer. Having a complete picture of what you're dealing with is absolutely crucial."

This is achieved through full data integration. Seismic will give you an image of the big and interesting structural features, but then you have several other tools such as wireline data logging.

Precision monitoring of the downhole conditions is the best way of understanding the rock types and the drilling environment.

An example of this data integration is available through WesternGeco's Q-Technology surveys. "Each survey is a unique combination of acquisition, processing, and inversion technologies required to produce solutions to reservoir problems, whether defining reservoir geometry, characterising reservoir properties, or monitoring fluid movements. Acquiring the right survey the first time increases its value as a reservoir-management tool at every subsequent stage in the field's life."

These technologies are rather more advanced than standard seismic surveillance methods because they record the data from individual seismic detectors without summation.

The seismic wavelet is controlled through source signature monitoring. Together, this results in the highest quality seismic fidelity and the maximum suppression of noise. Hence, subsequent Well-Driven Seismic processes such as well calibration, inversion, and classification techniques all produce more accurate results.

"Using this sort of technology enables incredibly powerful data analysis which in turn generates resolution that could not be generated by more traditional bundled seismic systems," says Montaron.

A new workflow has been developed by Schlumberger to improve fracture characterization and to effectively model carbonate reservoirs. The FCM Fracture Cluster Mapping workflow integrates Q-Technology services, borehole measurements and Petrel seismic-to-simulation software with expert interpretation and flexible work processes, resulting in improved production performance.

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