I hope that all this is not another case of me being somewhat slow on the uptake. Although people have been banging on about 4D seismics for years, as yet the technology has lacked a public airing of its’ proof. No longer. Our ‘take home’ from the SEG was what we judge to be the ‘coming of age’ of the 4D process. A seminal paper by BP’s David Whitcombe outlined the technical justification for a step-change in the seismic work flow.
4D (the extra dimension is time) seismics involves repeating seismic acquisition over an oilfield at intervals during its production. The premise is that the seismic method is now so sensitive, that the changes in reflection strength, as water encroaches on the oil, can actually be observed.
Well, as they say, seeing is believing, and I would just like first to digress a little on when this is not the case. Recently NASA has shown pictures generated by the Cobe satellite that purport to show what the universe was like a few billion years ago. You probably have seen the oval image of wormy colors - signifying, well, nothing! Because we haven’t a clue what the image should look like, we have no idea whether it has any validity. Such images could just be background noise for all we know.
The opposite is true of BP’s 4D imagery as shown in a talk by BP’s Ian Jack. We all have a more or less intuitive grasp of how water moves in on an oilfield, and the time-lapse imagery produced from the BP’s seismics is instantly believable. Jack wound-up his talk with a Power Point tour de force – animated 4D movies from half a dozen of BP’s 4D surveys all flashing their way through their production histories*.
The migrating waterfronts made a compelling argument indeed. So compelling in fact that BP is no longer content with this kind of long interval ‘snapshot’ approach to seismic reservoir monitoring. The next step is for permanent seismic arrays over producing fields, with more regular seismic acquisition performed on demand by a shooting boat.
We summarize the BP analysis of oil/brine reflection coefficients below. Most N. European oilfields operated by BP show values in the range of 0.02 to 0.06. BP reckons that until recently, these values were barely above the seismic signal to noise threshold of around 0.02. But BP has found that ‘seismic technology gets better the more it is pushed’ and estimates a current value for the S/N threshold as 0.006.
Order of magnitude
This puts reflection coefficients for all the fields studied a comfortable order of magnitude above the seismic threshold, and gives numerical justification for Jack’s Power Point proof.
More next month
One insider told us that the impact of 4D would be even more apparent at next month’s SPE New Orleans meet. The belief is that petroleum engineers will be abandoning their cellular numerical models in favor of seismically-derived representations of what is really happening in the reservoir. PDM will be there!
*One wonders incidentally if this is not some sort of a record in data synthesis, has so much data ever been so tellingly condensed into a single Power Point slide?
Regarding the tragic events of September 11th, we received the following from Bjorn Wygrala of IES - we thought he summed up our feelings too, and Bjorn has kindly let us reproduce his message.
“It is difficult to return to ‘business as usual’ without referring to the recent tragic events in the USA and the current risk of increasing national and cultural tensions. As members of the petroleum E&P industry, an industry that is possibly more international than most, we believe that we are in a unique and privileged position to prove that nationalities and cultures are not incompatible. For example, we were recently involved in a project in the Middle East in which 16 geoscientists from 14 nations worked together. There is not really anything special about this in our industry ... and that’s the way it should be.”
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