Working out the grain of the rock
Feature Articles, May  29  2009 (Digital Energy Journal)

- Geotrace has developed a technology for working out the direction of the grain of subsurface rock – useful for estimating the direction of natural fractures, and which direction the rock could be fractured in

Rock, like wood, has a grain to it – a direction in which it fractures most easily. Seismic data processing company Geotrace has developed a processing method to work out which direction the grain is running.

Knowing the direction of the grain is very helpful - both to know how it is likely to be fractured already, and to know in which direction it will be easiest to fracture or drill.

Fractures which already exist are very important for carrying oil and gas around the subsurface – collecting it into reservoirs and draining them into wells.

Dr Jaime Stein, chief geoscientist with Geotrace.

“The earth goes through many stresses and strains through its years of evolution – and the rocks just break –there’s fractures everywhere,” says Dr Jaime Stein, chief geoscientist with Geotrace.

Then, if we’re planning a horizontal drilling or fracturing operation, “you need to know if you’re cutting with the grain or against the grain,” Dr Stein says.

You want a man-made fracture to extend into the reservoir – not out of it, perhaps helping more water to flow into the well, this could be disastrous.

“If rock fractures in the wrong direction you might lose your production,” he says. “People might say – “I wish I knew this before I fractured the rock”.

“This information is going to be very valuable to the petroleum engineers – it’s an area we are very interested in exploring,” he says.

“If we can connect our seismic expertise with the drilling expertise – there might be very synergistic relationship. Engineers don’t normally see much value in what we do.”

“The engineers we’ve spoken to are extremely excited about this information – because it affects directly the drilling decisions that they make. Clients have made drilling decisions based on these kinds of displays.”

The service has been available for around a year so far. There are already companies using the system in various plays of the US lower 48 states (including Texas, Oklahoma and Louisiana) and clients in the Middle East, Dr Stein says. For all of these, Geotrace is providing the data processing services itself.

Geophysicists have always been aware that it should be possible to calculate the grain of the rock, but they haven’t done it due to lack of available data and processing power, but this is all starting to change, he says.

“It was one of those things ahead of their time,” he said. “We did not have sufficient (wide azimuth) seismic data to support their use in any way shape or form, so it was just an oddity.”

Data acquisition and processing

You normally need wide azimuth data for it to work – which has generally only been available recently.

“We are exploring ways of maybe relaxing conditions on the data acquired,” he says. “Wide azimuth surveys are expensive to acquire.”

“With wide azimuth acquisitions – there’s a new richness in the data that we didn’t have before,” he says. “And these new dimensions that the data has, has allowed us to develop new data attributes that help us detect anisotropy and fractures t.”

Traditional seismic processing methods, such as with common mid points and offsets, ignore the azimuthal information, – but with offset vector tiles and gathers, the seismic data retains all its azimuthal information allowing us to construct these new vector quantities. “These objects are a lot more complicated than they were before,” he says.

It is helpful if Geotrace can get involved in the planning of a new seismic survey, because it can make sure that it is made in an optimal way for the data processing.

You have to record which direction the rays came from. “The big difference between the data we have today and the data we had before – is we can preserve the orientation and geometry of the acquisition,” he says.

“In the past, if you look at marine data – the boats were sailing, pulling an air gun and long cables and the data was treated as if it had no azimuthal information.”

“Now people say it does make a different – it matters how we plan the trajectory of an acquisition – where do you position the sources and receivers makes a big difference to the final outcome.”

The processing is made by sorting the data up into discrete tiles or ‘bins’, that have offsets and azimuths in a range of values, and processing the data, including time and depth migration in such a way that all the information is preserved in what we call Offset Vector Gathers.

Fracture maps

Geotrace puts together fracture maps, which indicate the direction of fractures in a cross section of the reservoir.

See the rock grain in different reservoirs

In the images above (ref images A and B), you can see a reservoir (purple shading) where the rock grain direction has been calculated (shown as pick marks). On figure A, the rock grain is mainly in line with the direction of the reservoir.

This means that you could drill through the reservoir quite easily going top left to bottom right. Once you had done that though, if you wanted to fracture the rock in a direction perpendicular to the well (ie going towards the top right or bottom left), you would be going against the grain. It would be possible that the fracture would bend to follow the grain and quickly leave the reservoir.

In image B, the grain is in the opposite direction relative to the reservoir. So drilling through the reservoir (eg top right to bottom left) would be harder work. But the rock should fracture much more easily in a direction perpendicular to the well.

Geotrace calculates the direction of the grain by dividing the subsurface up into small squares (called tiles) and modelling offset vector gathers.

Geotrace

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