An interview with Jerry Harris - Stanford University

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INTERVIEW Cont’d TECHNOLOGY IS IMPORTANT Continued from Page 15 The problem with migration of crosswell data is the limited aperture of the crosswell survey. Unlike medical imaging or surface seismic we are trying to image a complicated structure from data collected along two lines. We don’t have the vertical aperture you would ideally like to have for migration, so we you use is a limited aperture migration. Well, if I keep limiting the migration aperture more and more, the migration turns into a CDP reflection mapping process. So instead we tend to start with a CDP reflection mapping and then open the aperture until we start seeing unacceptable migration artifacts. One last comment is that we have incorporated Fresnel zones into the tomography algorithms, though not routinely, to capture finite bandwidth effects on the inversion. S: So it is a sort of trade off? JH: It is a trade off between wanting to collapse the Fresnel zone effects with a migration technique, but being forced to control the aperture to reduce the artifacts with a limited migration aperture. Again, the conceptual advantages and disadvantages of migration, say regarding Fresnel zone effects, are the same as for surface seismic but the practice is different. S: You referred to anisotropy determination in your talk. How much effort is being put into including anisotropy in processing crosswell data? JH: Not nearly enough in my opinion. We can extract anisotropy in the plane of the survey but not done routinely. Azimuthal anisotropy is a different beast. Tomoseis is recording crosswell using multiple wells, so they can in principle detect azimuthal anistropy. One challenge though is to separate anisotropy from heterogeneity. More sophisticated modeling is required. Nevertheless, this is one area where the detailed understanding that you get from the crosswell survey could be used to enhance the value of surface seismic by unraveling how scale affects anisotropy. As you know, heterogeneity below the scale of resolution, say due to aligned fractures, may appear as seismic anisotropy. In the simple cases of laminated shales that I have shown, high frequency crosswell data can resolve some scales of heterogeneity while other smaller scales still appear as anisotropy. Of course, the scales we resolve are an order of magnitude smaller than surface seismic so we may be able to resolve heterogeneity that appears as anisotropy in lower frequency seismic data. S: Are we in a position to do 3D imaging with crosswell technology and if yes, does that justify the cost that may be incurred? JH: We do have the technology to do 3D imaging, at least the velocity and attenuation tomograms in 3D. The issues with migration are more complicated. Now that we can survey several wells simultaneously, the aperture issues improve but sampling is still a problem. The basic technology is there but data are scarce and the devil is in the details. Now, I’ll move to the other part of your question. The cost is acceptable if the result you produce has value or answers the questions being asked. For example, someone might ask, “Do I have to 16 CSEG Recorder December, 2002 shut the wells in to do this, because that will cost me money?” The answer is, yes you must make the wells available to do this imaging. But shutting the wells is not a problem if the results add value. If the engineer asks the operator to shut in for a well test, there wouldn’t be any question because they know the value. So, if we can establish the value crosswell brings to reservoir analysis and monitoring, the cost for 2D or even 3D will not be an issue. S: With this 3D coverage, is it possible to get an idea about azimuthal anisotropy? JH: The limitation of crosswell in this respect is constrained by where the boreholes are located; you will not be able to get uniformly sampled azimuthal data and this lack of uniform coverage will be a problem for estimating azimuthal anisotropy. From the point of view of tomography, the rate of convergence of the different complements of the anisotropy model may be horribly different. S: It will depend on the location of the borehole. JH: Yes, and you’ll want to keep the geometry as uniform as possible. You do not want to shoot between wells a few 100 m apart and interpret anisotropy with data from other wells that are 1000 m away. So what it means is you are forced to work with the geometric pattern you have for the wells. Remember too, in surface seismic this anisotropy you are seeing may be due to heterogeneity. It appears as anisotropy because you are not resolving the heterogeneity, say fractures. It may not be intrinsic anisotropy at all. If I have the higher resolution imaging that crosswell brings you may actually image the isotropic zones between the fractured zones or see other wave phenomena associated with the fractures like guided waves, etc. So it is no longer an anisotropy problem, it is a heterogeneity problem. S: Apart from Tomoseis, what other companies offer crosswell imaging service? Continued on Page 18
INTERVIEW Cont’d TECHNOLOGY IS IMPORTANT Continued from Page 16 JH: There is Schlumberger and Paulsson Geophysical and companies that do 3D VSP, though others have not been doing surveys for as long a time or as routinely as Tomoseis. The national laboratories have active programs. In addition to their work in oil and gas, the labs also work on near surface shallow environmental applications. S: Your present assignment as SEG/AAPG Distinguished Lecturer takes you to different places in North America and UK. How does this help you professionally? JH: It is very interesting. In some ways, it is an opportunity for me to see what other people are doing, what their research interests are. I am enjoying it, so far, though I am only on my 7th or 8th stop. I have 26 in all. S: This will take you to North America and UK or beyond? JH: This program is limited to North America and Western Europe, even though I had requests from South America and the Far East. I think the SEG should be responding to these distant requests, but there may be a combination of financial and political reasons for not doing them this year. S: I was wondering if you would like to share with us what research you are doing now? JH: I am giving this lecture on crosswell imaging now, but in fact much of this work was done 5 or more years ago at Stanford and elsewhere. Most of the work we do in crosswell at Stanford is related to environmental and near surface applications. Moreover, my personal interest is mostly on attenuation of seismic waves. This interest is driven by the observations of attenuation that we see in crosswell data. The problem is that even when we produce an attenuation tomogram, we have no good idea about how to interpret it. We cannot go to the laboratory and measure attenuation on a piece of rock, because the way attenuation is typically measured on a core is with ultrasound waves in the megahertz range. Attenuation scaling is just not reliable as velocity scaling. I have developed this technique called Acoustical Resonance 18 CSEG Recorder December, 2002 Bruce Marion (Tomoseis) and Satinder in conversation with Jerry Spectroscopy, for measuring attenuation or Q of a small sample of rock in the lab at frequencies of 1000 Hz or so. This will provide some ground truthing for later being able to interpret the in situ attenuation data. We can also measure Q and velocity on irregularly shaped pieces of rock, like cuttings from a borehole. It is difficult to measure those in the lab even at high frequencies because of their irregular shape. We are looking for a technology that we can potentially locate at the well site, so that as the cuttings come out from the well, we can determine their acoustic properties. So, we spend most of our time working and thinking about numerical and physical models for attenuation in porous media, and how to process data to produce attenuation images. The other area where we have funding for is related to problems related to carbon sequestration, that is, using geophysical methods to monitor the injection of carbon dioxide in depleted oil and gas fields and aquifers. So, you might say that crosswell data, because the quality is so good, opens the door for a lot of basic research in terms of how waves propagate in porous media. I am spending more of my time looking at those basic issues than say specific applications. S: Jerry, what message do you have for young geophysicists entering our industry? JH: It is a question that I hear often. My response is that technology is important. They need to understand to some extent and not fear to use it when a problem calls for leading-edge technology. Moreover, they must support its development and think ahead longer term than just a few months. The majors need to be competitive and leaders in developing and applying technology, and should support research and education. Young geophysicists should follow their hearts in choosing a career path. They should build and basics and strive to be the best at what they do. S: Jerry, we thank you for sharing your experience and spending this time with us. We wish you good luck with your work. JH: I enjoyed it. Thank you R
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An interview with Jerry Harris - Stanford University

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