An interview with Jerry Harris - Stanford University

INTERVIEW Cont’d
TECHNOLOGY IS IMPORTANT
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to develop reservoir models for flow simulation and of course monitoring.
When you need to estimate small-scale features, crosswell
may be the only technology capable of producing the kind of reservoir
models that are needed for flow simulation. Crosswell images
are actually pushing the limits in terms of resolution of grid blocks
in the flow simulator itself. Where else do you get this small-scale
information between wells? So crosswell profiling is the only direct
measurement technology available today for this problem.
S: What type of sources are used in crosswell (imaging) data
acquisition?
JH: I have used a number of different sources: airguns were probably
the first modern sources to be used, but piezoelectric sources
are probably the best and easiest to use and also are relatively
powerful and easy to operate. The piezoelectric sources run sweep
waveforms or pulse codes that are easy signals to distinguish from
background noise so high peak pressures are not required.
There are other sources of course. One approach was to take low
frequency surface seismic sources like hydraulic or pneumatic vibrators
and repackage them for downhole use. Others have tried this
with limited success. Instead, I took the approach of repackaging
high frequency logging technology, making it more powerful and
lower frequency. With logging-based technologies, I could build on
all the expertise and experience of logging operations, that is high
temperature, high pressure, and wireline operation. Of course the
traditional sonic logging tool is not capable of transmitting the long
distances required in crosswell, say up to a 1000m. So, we had to reengineer
the piezoelectric devices to produce the right frequencies,
100 - 2000 Hz, a feature that repackaged surface seismic technology
was never able to accomplish.
BM: You spent a lot of time as you were developing this technology.
You would have many stories to tell.
JH: Not only about sources, but downhole detectors and operations
were issues as well. Some of the first downhole receivers we
used were actually Teledyne marine streamers and hydrophones.
14 CSEG Recorder December, 2002
There were some good stories and bad stories about them. They
worked very well as detectors but sometimes we would get things
stuck in the borehole and some of the stuff we put in first actually
came out first.
Apart from the bad stories, we had some good times as well,
for example the first time we detected a 1000 Hz signal over 500 m
or so. Others could not believe that we could see those high
frequencies over those distances. It was pleasantly surprising for
all of us. In fact the piezoelectric source that we use now was
primarily built as a reference source in many of those early tests.
We were comparing a number of different sources and needed one
as a reference. I built the piezoelectric because it was very repeatable
and reliable. And it turned out to be the best of all the sources.
So, what started off as a reference tool ended up as a tool of choice.
And we compared that source to hydraulic vibrators, airguns,
explosives, all kinds of fancy exotic sources literally from around
the world. From Norway to Japan and other places, the piezoelectric
always emerged as the best source.
S: What type of source spacing are we talking about?
JH: Typically, 1m; it depends on the imaging objective, that is
whether only tomography or reflection processing is needed. The
sampling interval is dictated by the frequency content and the desire
to avoid spatial aliasing of the low velocity events. Another problem
is the depth control needed for for small source intervals. Imagine
now that you are trying to position the source at 1 m intervals at
5000m depth. How do you do it? If you just start at 5000m and call
for the source to move 1m at these depths, sometimes it moves,
sometimes it sticks and doesn’t move the entire 1m. So having a
source that operates like a logging tool, keeping it moving continuously
in the borehole, takes care of the problem and keeps the source
on depth with repeatability of a few inches. As the source moves, the
computer is telling it to fire on depth. Again we borrowed that operation
from well logging but we, at Stanford, were the first to use it
with a modern crosswell source while others were operating by
stopping, starting and sometimes clamping the source at depth. This
shooting on the fly became very important to data quality because
we have to go back and occupy those shot points several times. By
keeping the downhole source continuously moving, we found we
could repeat locations and the event moveouts in common-shotgathers
became much smoother. We introduced that approach,
shooting on the fly, at Stanford with the piezoelectric source. Now
it’s used with other downhole sources, such as the airguns.
S: Is there a limit to the distance between the wells to get good results?
JH: Certainly there is a limit. But again, it depends on what you
are trying to image and how you hope to accomplish that.
Eventually high frequencies are going to be attenuated to a level
below delectability. So, if you are willing to use the same frequency
as surface seismic, say 100 hz, then you will be able to see the signal
over distances comparable to the distances you see in surface
seismic, say 1000s of meters. But in my opinion, the advantage of
crosswell comes when you use the higher frequencies. The higher
frequencies will still have good signal-to-noise ratio over the shorter
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