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Business case for permanent reservoir seismic by OCTIO

Thursday, March 26, 2015

It may be easier to justify spending on permanent reservoir seismic if it is used for overburden monitoring, or monitoring drill cuttings injection, rather than to monitor the reservoir, says Helge Brandsaeter of Octio

Using digital cables on the seabed, to record seismic data and get a picture of how the oilfield is changing as it is produced, is generally thought to enable an increase in oil and gas recovery of 5 per cent over the lifetime of the field, said Helge Brandsaeter, president of OCTIO, speaking at the Digital Energy Journal Stavanger conference on December 10, 'People and Subsea Data'.

But from an investment point of view, systems are not being installed because too often it is seen as 'nice to have' rather than a necessity, he said.

Most oil and gas staff are busy meeting their short term objectives, and don't have time for longer term ones.

But the short term business case can be much stronger if you see it as a way to monitor the overburden (rock between the reservoir and seabed) and avoid costly environmental fines, he said.

Octio is majority owned by Statoil Technology Invest.

Overburden problems

The seismic recording can help spot problems leading to possible overburden leakage.

There have been a number of well publicised cases in Norway where a water injection well put more pressure into the rock than it was able to handle, leading to craters opening up in the subsea, he said.

Now fields in the Barents Sea has shallow reservoirs, and so there is a higher risk of oil leaking through the rock to the seabed

'You see faulting going from the reservoir to surface directly,' he said. In reservoirs like this, 'The overburden is as important as the reservoir."

In Brazil, the overburden is 'young in geological terms' which means that if water is injected at too high pressure you can open pre-existing faults to the seabed, and water and oil seeping to the seabed.

By monitoring the seabed with digital cables, you can see how the rock is fracturing and if there is a chance the fracture will reach the surface. 'It's not difficult to see that seismic can map any such seepage,' he said.

You can see how the cracks develop subsurface. 'This will give the operational teams a notice that something has to be done by the injection,' he said.

'It is reasonably easy to set up a business proposition which is reasonably sound,' he said.

Drill cuttings

The system is being used on the Oseberg field (140km Northwest of Bergen), to monitor the injection of drill cuttings and waste water.

Drill cuttings are milled very finely at the platform and injected into the reservoir.

This provides a much less expensive option for managing drill cuttings than transport back to land.

'The average cost for a North Sea field for transportation of cuttings and water waste is close to $20m a year,' he said. 'The alternative is to ensure safe injection, for $2m to 3m.'

But Norway has 'zero tolerance' for any waste pollution. 'If you can't ensure safe injection you have to transfer all the fluids onshore,' he said.

Seabed infrastructure

Octio typically installs a mesh of seismic sensors on the seabed, 50m apart, which communicate with a hub and send the data to surface.

One system has 172 sensors in a W shape around the well on the seabed.

The sensors have 4 components, 3 component accelerometers and hydrophones.

The sensors have an 'active' mode for recording seismic data in a survey, and a 'passive' mode for continually listening to seismic data.

Everything is managed remotely, with data transmitted back to shore.

The installation of the system basically comes down to the cost of leasing vessels, and installation cost is a third of the lifetime operating costs.

The data communications infrastructure can also be used for any other subsea equipment.

'If you build such an area wide infrastructure we can use it for all types of communications and types of sensors,' he says. 'You can drop down a sensor and communicate to surface.

'We have made basically an ethernet on the seafloor, you can interface any system to us,' he said.

Octio is working with a number of standardisation committees including SWIG (Subsea Wireless Group) and SIIS (Subsea Instrumentation Interface Standardisation).

Watch Helge's talk on video at www.d-e-j.com/video/1502.aspx



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