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PGS – working with broadband seismic

Friday, June 17, 2016

Seismic service company PGS is encouraging oil and gas companies to record seismic over a wide range of frequencies, or ‘broadband’, because it enables them to get a much better understanding of the subsurface

Recording more high frequency seismic makes it possible to understand the subsurface with more accuracy. Recording low frequency seismic can be used to calculate actual reservoir properties about the reservoir, said Cyrille Reiser, reservoir characterisation director with PGS.

He was speaking at the Finding Petroleum forum in London on April 18, “Transforming Subsurface Science”.

If you look at a real outcrop, you will see it has a very complex structure, with rock layers centimetres thick and with various rock properties. But seismic data can get a maximum of 10s of metres resolution, and usually about 25-30m depending of the reservoir depth.

The higher the seismic frequency, the more detail you can understand the subsurface, because the distance between peaks of the wavelet becomes shorter.

With interpreted high frequency seismic data, you can start to pick out the exact top and base of the reservoir, he said. You can also get a better understanding of rock properties, for example the sandiness or the porosity of a reservoir.

Broadband frequency seismic data (rich on the low and high frequency) help significantly to make the absolute measurement of subsurface properties, such as elastic properties easier and with less ambiguities and more predictability away from a hard control point,

This enables you to reduce the amount of “a priori” or previous information required to do an interpretation, quantitative seismic interpretation. Usually, seismic interpreters use data recorded from well logging tools to help with seismic interpretation.

Also, the more you can directly calculate the reservoir properties from the seismic data, the less subjective interpretation is required. Subjective interpretation takes a great deal of geoscience understanding, which is becoming less and less available these days, Mr Reiser said.

Ideally, with broadband data (if acquire and process adequately) you could get all the information you need to make a decision mainly based on the seismic survey, he said.

PGS’ believes that broadband seismic (and more specifically the multi-component dual-sensor streamer) can provide more insight to the subsurface without making too many ‘a priori’ information a company might have and increase their probability of success.

The data workflow is usually to develop a simple ‘low frequency’ model of the subsurface (a simple model based on low frequency seismic), and use this as a basis for the pre-stack seismic inversion and the understanding the lithology (rock structure) and fluids which might be contained in it.


More data

High bandwidth seismic means recording much more data, he said. You now have two sensors (hydrophone and vertical particle velocity sensor) recording seismic, recording the wave field up-going and down-going, where previously there was just one (the combination of the two).

The data is recorded with many more sensors, located on longer streamers, and you have more streamers (more than 12 streamers). The recording sampling rate can be 2ms instead of 4ms.

Altogether, you can be recording twenty times more data than 10 years ago, he said.

The seismic processing is much more complex, with different products generated at different stages of the processing.


Notches

Another problem PGS has solved is the “notches” which are found in seismic recordings at specific frequencies, perhaps every 50 Hz depending of the streamer/seismic and source depth. This means you get zero amplitude response at these frequencies, due to the seismic wave bouncing up to the sea surface and down to the streamer.

PGS has come up with one solution to this, a dual-sensor or multi-component streamer (hydrophone and a vertical particle velocity sensor) which allow the separation of the the up-coming and down-going wavefield. This is combined together with the usual hydrophone for seismic recording.

With a little processing, you can use this to separate the seismic waves which are reflected from the subsurface from all the other recording, he said.


North Sea example

Mr Reiser presented an example of how broadband seismic was used to try to de-risk a possible reservoir prospect off the West of Shetland Islands, UK.

The information most important to geoscientists is the size of the reservoir, its porosity, and the ‘net to gross’ (how much of reservoir actually contains oil).

In the seismic interpretation, it was possible to compare the P wave velocity with the S wave velocity (Vp/Vs) for different areas of the prospect. The Vp/Vs ratio gives a good indication of the lithology and fluid. From your understanding of lithology you can assess porosity.

The low frequency model (based on the 0-2 Hz data) can be used to get acoustic/shear impedance low frequency model. The absolute properties can be used to make some “simple cross plots”, which will show for example which of the acoustic impedance and Vp/Vs calculations are for shale and which are sand. The lowest acoustic impedance and Vp/Vs is probably hydrocarbons. “It’s a very efficient way to do it,” he said.

Using methods like this, geoscientists they can get useful data as quickly as possible from 3D seismic, without introducing too much interpretation and more importantly some a-priori or pre-conceive ideas in their decision making process.

Mr Reiser also showed the workflow being used on a tertiary reservoir in the Forties Sand (North Sea). One well had already been drilled in the area, and did not find the hydrocarbons which were expected, predicted by this acquired broadband pre-stack seismic inversion.

PGS’ seismic processing “predicted very nicely the presence of hydrocarbons,” in other locations he said, and it also did not show any hydrocarbons in the reservoir which the company had previously drilled.

“Based on this type of analysis you probably would think twice about drilling this one, you get a flag, there might be a problem.”

Once you have all the results, you can use the well data for final refinements, he said.



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