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Tuesday
Jan152013

Rocks, pores and fluids

At an SEG seismic rock physics conference in China several years ago, I clearly remember a catch phrase used by one of the presenters, "It's all about rocks, pores, and fluids." He used it several times throughout his talk as an invocation for geophysicists to translate their seismic measurements of the earth into terms that are more appealing to others. Nobody cares about the VP/VS ratio in a reservoir. Even though I found the repetition slightly off-putting, he succeeded—the phrase stuck. It's all about rock, pores, and fluids.

Fast forward to the SEG IQ Earth Forum a few months ago. The message reared its head again, but in a different form. After dinner one evening, I was speaking with Ran Bachrach about advances in seismic rock physics technology: the glamour and the promise of the state-of-the-art. It was a topic right up his alley, but suprisingly, he seemed ambivalent and under-enthused. Which was unusual for him. "More often than not," he said, "we can get all the information we need from the triple combo." 

What is the triple combo? 

I felt embarrased that I had never heard of the term. Like I had been missing something this whole time. The triple combo is the standard set of measurements used in formation evaluation and wireline logging: gamma-ray, porosity, and resistivity. Simply put, the triple combo tells us about rocks, pores, and fluids. 

I find it curious that the very things we are interested in are impossible to measure directly. For example:

  • A gamma-ray log measures naturally occuring radioactive minerals. We use this to make inferences about lithology.
  • A neutron log measures Compton scattering in proportion to the number of hydrogen atoms. This is a proxy for pores.
  • A resistivity log measures the conductivity of electrical current. We use this to tell us about fluid type and saturation.

Subsurface geotechnology isn't only about recording the earth's constituents in isolation. Some measurements, the sonic log for instance, are useful because of the fact that they are an aggregate of all three.

The well log is a section of the Thebaud_E-74 well available from the offshore Nova Scotia Play Fairway Analysis.

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Reader Comments (6)

I'm surprised about the "we can get all the information we need from the triple combo".

The whole point of seismic rock physics technology is to interpolate *away* from wells. You cannot get all you need from the triple combo because you simply do not have anything there. Wells are our calibration, Seismic data is what enables us to interpolate with some degree of confidence.

Brings me to the data set you link to. I can see all sorts of data, including horizons picked on seismic data. But not actual seismic data itself. Not interesting? Trust us, it's all you need and the horizons are good. Just examine the logs and do some kriging ...

January 16, 2013 | Unregistered CommenterBert Bril

Great point Bert. I agree with you. The Canadian government seems to publish well data readily, though the QC and conditioning is left up to others. Seismic data, on the other hand, is simply hard to come by. I suppose this is due to the business behind selling and re-selling seismic data after it has been acquired. Perhaps the scarcity and inaccessibility of seismic data too, adds to the value and the demand for it. But my impression of open data sets is that there is an intangible value in releasing it to the public. A different kind of economy emerges around attention and knowledge that doesn't fit into to the standard business models. Not yet anyway.

January 16, 2013 | Unregistered Commenterevan

I understand what you're saying, IMO it's weird that there is so much acceptance of the situation, but OK.

Anyway, leaves me puzzled about the "More often than not, we can get all the information we need from the triple combo.". Does that mean they don't use seismic data to interpolate between the wells? Or extrapolate? And why not? Lack of tools?

January 16, 2013 | Unregistered CommenterBert Bril

@Bert,
I don't think he was meaning he didn't support the notion of using seismic to interpolate between the wells, or extrapolate. But his statement did surprise me too. In fact, it may have been his commentary on the accepted belief, for better or worse. Also, I think his world view probably stems from mostly offshore wells, where there is almost always a VSP to provide a time-to-depth relationship, to calibrate the well so-to-speak, enabling Kriging and so on. Maybe it's more palatable to interpret his statement as, "more often than not, we get 'enough' information from the triple combo (plus a VSP or checkshot)". Though his heart and his work show a dedicated push to getting more out of seismic technology, both inside and outside the borehole.

January 16, 2013 | Registered CommenterEvan Bianco

@Evan,

Are you suggesting it's common to have VSPs spanning the distance between wells?

I don't really understand your 'calibration' here. The issue is not to calibrate the well, it's to calibrate your seismic rock physics relationships. You don't even need a 100% correct Time2Depth model for that.

I can see the triple combo being enough in areas densely sampled by wells, like mature production areas. For the rest, not using seismic data to predict rock properties between wells is, well, ignoring what you've got.

January 16, 2013 | Unregistered CommenterBert Bril

Evan and Bill,
Interesting. The comment was made in China by Ikon's Henry Morris. It's the company mantra, which has now evolved into "its all about the dynamic behaviour of rocks,pores and fluids..." Hearteningly,the message is really getting over and despite the rather odd comment by Ran the fact is that almost everyone making big decisions on expensive wells has to make a prediction about pore pressure and calibrate their seismic ahead of drilling - so rock physics based rock,pore and fluid based prediction is the only option rather than the triple combo.

January 17, 2013 | Unregistered CommenterMartyn Millwood Hargrave

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