David Bamford
Nearly seventy years ago, one of the more notable events in the history of the global oil & gas industry occurred when the first offshore seismic was shot in the shallow waters of the Gulf of Mexico. Of course, seismic (typically refraction or 'radial' shooting around salt domes) had been acquired onshore in the USA for the previous twenty years but Shell Oil took the adventurous step of going offshore. I was told this story nearly twenty years ago by a gentleman called Sid Kaufman who was Shell Oil's chief geophysicist at the time. As I recall, the decision was made to emulate the onshore approach, with a shooting party and a recording party, and so two shrimp boats were hired in Galveston for the princely sum of $50 or so. After a successful first project, expenses were submitted and came back with the comment from the 'boss' (an accountant for sure!) "too expensive, next time use one boat! And so towed streamer seismic had to be, and was, invented, allowing just a single boat to be used"!
Well, it's a story and my memory is not what it used to be but it set me thinking about the phases of progress in the seismic technology business and whether we could describe these phases as the drillers do their offshore rigs I guess they are up to '5th Generation' by now. My thinking goes somewhat like this:
1st Generation Seismic = 2D
2nd Generation Seismic = 3D
3rd Generation Seismic = 4D.
Now I know I have skipped across several technology 'leaps' here for example, from onshore to offshore, from explosives to vibroseis and airguns, and of course from analogue to digital recording but I'm trying to define phases that when they were introduced produced a quantum shift in the exploration and exploitation of oil and gas and for which oil & gas companies were prepared to pay a premium for access to the new approach. Of course, in time and in turn, each of these approaches become a commodity, with many competing offers, and bid mainly on price.
So what might be described as 4th Generation Seismic? Perhaps we could start by asking what it is that oil & gas companies seem willing to pay a premium for nowadays (as opposed to accepting the currently high prices for, say, conventional 3D driven by an out-of-kilter supply/demand balance). I would like to focus on just four examples Continuous Seismic Surveillance; Wide-Angle; Multi-Azimuth; Multi-Component (2C and 4C).
What do each of these offer?
Continuous Seismic Surveillance extends 4D methodology. Offshore, permanent sensors (with up to 4 components: X, Y, Z and pressure) are installed, usually in a cable, in the sea-bed above any oil or gas field in a one-time installation before production starts. Then a relatively inexpensive seismic source vessel can sail above these sensors whenever required and generate a "repeat 3D". In theory, this could happen as often as once a week at a repeat cost of as little as $100k, giving real-time data for analysis of reservoir dynamics throughout a field's life. Onshore, a similar approach can be envisaged although permanently installed down-hole sensors are perhaps more likely. Such surveillance should be thought of as a component of the Digital Oil Field which I discussed at some length in a ROGTEC Issue 11and so will not pursue further here.
Wide-Angle and Multi-Azimuth Seismic were originally conceived of as two different ways of addressing a similar problem, namely how to image beneath lithologies such as salt or basalt that limit the ability of conventional 2D or 3D seismic to 'see through them' to image structures below. An example might be where a structural feature such as a rift system lies below a layer of salt, for example in the Santos basin, offshore Brasil or the Gulf of Suez, offshore Egypt, or beneath basalt, for example in the Rockall basin, offshore Ireland here Wide Angle Seismic seems to deliver. Elsewhere, more rugose surfaces (such as the Messinian unconformity in the Eastern Mediterranean) or salt bodies (Gulf of Mexico; Angola) might prevent illumination of deeper geology when data is acquired along a particular azimuth here Multi-Azimuth Seismic seems to deliver.
However, as the apparent leader in this area CGGVeritas makes clear, these two approaches have somewhat merged and have benefits beyond what was originally conceived. There seem to be two reasons for this. First of all, survey design is key so as to provide the optimum combination of target illumination, sampling, data quality and cost of acquisition. Second, the processing of Wide- and Multi-Azimuth has challenged conventional processing techniques, leading to the adoption of true 3D algorithms and workflows. The combination of this true 3D approach to processing, combined with improved illumination and data quality during acquisition, leads to great improvements in image quality in all sorts of geological settings, especially complex ones.
Multi-Component Seismic is seeing a boom offshore where the sector-leading companies such as RXT have figured out the logistics of operating Ocean Bottom Cables (OBC) efficiently and effectively, finally offering the opportunity to realise the potential of multi-component data to delineate 'hidden' reservoirs by imaging beneath gas clouds or imaging reservoirs that are 'transparent' to conventional (wave) seismic and to locate and identify reservoir fluids by yielding rock physics parameters.
OBC deployment offers significant data quality advantages from the use of geophones/accelerometers in addition to hydrophones and from avoiding the noise arising from towing a streamer and from the sea surface itself. Also, RXT uses new sensors that have excellent characteristics down to very low frequencies, crucially important for imaging deeper images. OBCs can be deployed in areas where towed streamers are impractical, for example in very shallow water or where there are obstructions such as producing platforms, moored rigs and so on: also OBC enables a full Multi- and Wide-Angle capability. Obtaining high quality multi-component data and then processing and analysing it has always been seen as something as a challenge in the industry and indeed the latter has always occupied somewhat of an 'academic' backwater in both the Majors' R&D departments and the bigger geophysical service companies. However, it has now moved into the mainstream.
There is significant overlap between the topics discussed above and we could say that 4th Generation Seismic equals Total Seismic, the ability to acquire data at any azimuth, any offset, for any depth, for any geological target and to use massive computing power to process and analyse it. Seismic remains "King" and I recall the advice of my first boss in BP who said "If you have a $100 to spend on geophysics, spend $99 on seismic and the rest on a good cup of coffee at least that is how I remember it!"
Finally, it's worth noting that Total Seismic seems to involve the use of more than one boat; perhaps it wouldn't have taken us nearly seventy years to get there if Shell Oil's 'bean counter' hadn't stood in the way some things never change!
posted by The Rogtec Team @ 12:21
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