The Leading Edge; 2005; v. 24; no. Supplement;
p. S86-S91; DOI: 10.1190/1.2112396
© 2005 Society of Exploration Geophysicists
The rapid rise of reservoir geophysics
Wayne D. Pennington
Michigan Technological University, Houghton, USA
In 1980 and again in 1985, on the occasions of the 50th anniversary of the Society of Exploration Geophysicists and the 50th anniversary of publication of GEOPHYSICS, special issues of that journal were published. In both those times, as now, the science was flourishing. The science described in those issues was directed toward exploration, but many of the methods were to form the basis for a new application, here called reservoir geophysics. In 1980, oil prices were at record highs, and in 1985 they were about to plummet; at the time of this writing, prices are again at local highs, accompanied by a renewed enthusiasm for the sound application of the science.
The acceptance of 3D seismology as a cost-effective tool for reservoir management was the single most important aspect in the growth of reservoir geophysics. As such, most of the history of reservoir geophysics parallels the history of 3D seismology. On the other hand, a wide variety of different techniques within specialty areas of geophysics was developed simultaneously; although these are not as widespread or well-known as 3D seismic, they are extremely valuable tools in the arsenal of reservoir management.
This brief history first describes the evolution of the acceptance of 3D seismic techniques for reservoir management, and then summarizes a number of other geophysical techniques used for reservoir engineering purposes. Of course, any retrospective is strongly colored by the personal experiences and biases (whether or not they are recognized as such) of the author, who assumes full responsibility for any errors, particularly errors of omission.
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Defining reservoir geophysics
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Reservoir geophysics can be defined as the application of geophysical techniques within a known hydrocarbon reservoir. This implies that at least one well has been drilled into that reservoir, and may (or may not) be available for geophysical applications. It is this access to wells and/or to internal information about the reservoir that distinguishes reservoir geophysics from exploration geophysics, as well as the overall scale of the surveys. We can further subdivide "reservoir geophysics" into "development" and "production" geophysics, depending on the immediate application: Development geophysics is applied to the initial efficient development of a field, whereas production geophysics is applied to the understanding of the field as it evolves during production. (In some instances, authors may use the term reservoir geophysics as a synonym for "time-lapse seismic." This usage should be discouraged; time-lapse seismic is simply one aspect of production geophysics.)
In 1980, the typical sequence of reservoir development followed a "classical" flow of information from one specialty to another, as shown in Figure 1.
In 1980, the flow of information was linear, from one person (and specialty) to another. There was very little feedback between, say, the engineers involved in development and the geophysicists who may have been able to assist them.
This has changed, of course. In fact, the new edition of the Petroleum Engineering Handbook, to be published by the Society of Petroleum Engineers, will include a chapter on reservoir geophysics, specifically to inform engineers of the assistance that geophysicists can provide. While the transition from exclusively exploration-oriented geophysics to reservoir geophysics may seem fast and furious, concentrated in the 1990s, a more-detailed reflection indicates that the movement had already begun by the early 1980s, when several developments took place in academia, industry, and the economy.
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The academic role in developing reservoir geophysics
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Three key participants were:
- By 1977, Amos Nur had founded the Rock Physics group at Stanford, and was later rejoined by his former student, Gary Mavko. An expansion into borehole geophysics in 1986 created SRB, the Stanford Rock Physics and Borehole Geophysics Project. This group has done (and continues to do) much to allow the interpretation of geophysical data in terms of rock and fluid properties, and of stresses around boreholes, both key applications of reservoir geophysics.
- In 1982, M. Nafi Toksöz at the Massachusetts Institute of Technology founded the Earth Resources Laboratory, which by 1984 included the Full-Waveform Acoustic Logging Consortium under Arthur Cheng, and by 1985 the Reservoir Delineation Consortium under Roger Turpening. Both these consortia actively developed and tested new geophysical methods for the evaluation of reservoir and nonreservoir rocks through borehole geophysical techniques.
- In 1985, Tom Davis formed the Reservoir Characterization Project at the Colorado School of Mines using multicomponent (and, later, time-lapse) seismic studies in reservoirs to define internal attributes such as fracture density and fluid content. This group is now on its tenth "phase," having studied at least seven different fields.
These and other groups in many different countries laid the foundation for much of the seismic work now included in reservoir geophysics. Their timing was good for the science, although funding was always (and presumably continues to be) a challenge. But the combination of new methods of seismic acquisition and processing with new and evolving interpretational aspects of rock physics was key to the ease of industry acceptance of the seismic aspects of reservoir geophysics. It was fortunate for the industry at large that a few far-sighted professionals within various companies championed these and other consortia at a time when funding was scarce and the applications of the science were not always entirely apparent.
The education of geophysicists in universities continued during the low-hiring period of the mid-to-late 1980s, providing a workforce for those companies that did hire them. In part due to the poor job market, most schools were training their students broadly, without early specialization into certain niches, allowing them breadth of choice in employment upon graduation. This ran counter to the demands of some recruiters, who tended (and still tend) to request students who were trained in one specific software package or one highly specialized niche area, in order to fill a certain immediate need. The generalized backgrounds of many of these students served them and their companies well when the discipline rapidly evolved, and these new employees migrated into interdisciplinary positions bridging geology, geophysics, and engineering.
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The oil and gas industry
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In the 1980s, the big money was being put into exploration, not reservoir, geophysics. There was a widespread conviction that the price of oil would never drop, and that finding new oil was the best way to make money. But then, suddenly, the Ekofisk platform in the North Sea was observed to be "sinking" due to subsidence associated with reservoir compaction. Understanding the interior of the reservoir was suddenly a multibillion dollar question, at least for one company, and all large oil companies became aware of their limited knowledge of reservoir-rock dynamics. Many geophysicists had been schooled in earthquake seismology, and the transition to reservoir mechanics was natural. As it turned out, petroleum engineers also needed input for well-completion designs, and this was becoming available through full-waveform acoustic logging. Once again, the classical earthquake training of many industry geophysicists made them well-suited for understanding the normal-mode propagation of waves in the borehole (compared with the ray-theoretical approximations suitable for most surface reflection studies) and the strength of rock and stresses in the formations—these values were needed by engineers working on hydraulic-fracture design, predictions of wellbore stability, and simulation studies incorporating the compressibility of the reservoir rock.
Many engineers and geophysicists developed good working relationships within their companies as a result of these mutual interests and capabilities, and each learned the advantages the other could bring to their work. These relationships proved to be extremely useful in the next stage—the acceptance of 3D seismic studies by the engineering community. (While 3D seismic methods were gaining the most popularity and attention, there were significant advances in several other aspects of reservoir geophysics, including borehole-based seismic, microgravity, electrical, electromagnetic, and passive seismic; these will be discussed in a later section of this paper.)
The first 3D seismic surveys were performed as subjects of research and have been discussed in various reminiscences published in TLE's "From the Other Side" column. Most geophysicists knew by the late 1970s or early 1980s that 3D seismic was technically feasible, and some dramatic examples were shown at various meetings, mostly directed toward enhancing exploration, rather than production (see, for example, the abstract describing 3D seismic exploration in the Austin Chalk by Calcote and others in SEG's 1982 Expanded Abstracts). By 1983, the SEG Annual Meeting and Expanded Abstracts included a session "Seismic 16" (available for online browsing through the SEG Digital Library) with seven papers presented on 3D seismic methods and case histories. Of these, about half could be considered applicable to reservoir geophysics, and about half to exploration. One of the papers described the first time-lapse seismic study reported in the literature: "A study of fireflood efficiency" (Greaves and others, paper S16.1; also later published in GEOPHYSICS). A number of presentations at the 1983 Annual Meeting (sessions Seismic 20 and Seismic 21) also described computer techniques that allowed interpreters to manage and view 3D seismic data, a necessary feature for wide application, of course.
Although the value of 3D seismic for field development was recognized publicly as early as 1984 ("The value of 3D seismic in field development" by Gaarentstroom, SPE 13049), an important milestone occurred with the publication of "Modern technology in an old area: Bay Marchand revisited" by Abriel and others (first, as an abstract, RES 2.7 in 1990, then as a paper in TLE in 1991). In this study, the Chevron team demonstrated that 3D seismic studies and interpretation applied to a field—one that had been under production since 1949 and in decline since the early 1970s—resulted in nearly doubling the daily production and clearly demonstrating that reservoir geophysics was a cost-effective tool for the management of producing assets.
But the real confirmation that the industry was going to adopt the new technology and apply it to reservoir development and production arrived in 1991 when Shell described its experiences with 3D seismic. Figure 2 is from Nestvold's "3D Seismic: is the promise fulfilled?" SEG Expanded Abstract which stated that "... it is recognized that 3D is a powerful tool for appraising a field and for providing valuable input into the development plan itself." It was inferred that Shell would conduct 3D seismic surveys over every major asset, as well as being used earlier in the exploration process. This caught the attention of the managements of most oil companies, and geophysicists were finally brought into the discussion of reservoir engineering and production on a larger scale.
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Figure 2. The growth of 3D seismic surveys in Shell (outside of North America), from the expanded SEG abstract published by Nestvold in 1991.
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Most producing companies had, by this time (1991), developed some experience with 3D seismic methods, and contractors were able to deliver the service worldwide. The additional pieces required to make reservoir geophysics a mainstream aspect of reservoir management were (1) confidence of management in the geophysicists' capability to understand and appreciate reservoir engineering needs, and (2) direct lines of communication between the geophysicists and engineers. Fortunately, in many companies, these were already in place as a result of their earlier experiences in geomechanical and well-completion studies. Most companies were already familiar with the appropriate technologies through participation in academic consortia, if not through their own efforts. The rest, as they say, is all in the details. Of course, the details varied among companies and even among different management groups. Some companies and managers made opening of the lines of communication easy; others, no doubt, made it difficult.
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The gorilla in the room—economic issues
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The development of many of the techniques that ultimately found applications in reservoir geophysics had begun when the price of oil was very high, in the late 1970s and early 1980s. But then the price of oil collapsed in 1986, and the attention of most oil companies and oil-service companies was directed to cutting costs ... to the bone. Exploration was a primary target of cost-cutting, because of its long payback time. Reservoir geophysics was seen by those geophysicists remaining in the business as a possible avenue to continued relevance and employment. Companies had to be convinced that there was actually an economic benefit to be realized in applying reservoir geophysics. The correlation between the drop in oil prices and the rise in use of 3D seismic surveys (Figure 3) is only partially spurious, but the dramatic rise in seismic surveys applied for reservoir studies was no doubt accelerated by the need to develop existing assets as budgets tightened.
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Figure 3. Comparison of the growth in 3D seismic surveys (approximated from Figure 2) and the price of oil (first-purchaser's cost, in constant year-2000 dollars; from the Energy Information Agency, U.S. Department of Energy).
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How did this affect the geophysicists who were needed to apply their science to the improved development of reservoirs? A few scientists actually found positions as geophysicists attached to engineering departments, but this was the exception, rather than the rule. Strong economic pressures helped drive geophysicists into making use of their talents in ways which they had not previously envisioned, in areas such as full-waveform acoustic logging, borehole stability, reservoir geomechanics, and rock-physics integration with reservoir simulation. These applications all became directly engaged in what we now call reservoir geophysics.
As companies began to depend more on increasing productivity from their existing assets and less from finding new fields, the pressure also increased on reservoir engineers to ensure that they made use of all the relevant data that could be obtained. Their relationships with some geophysicists allowed them to have confidence (although perhaps limited) in the field in general, and most were open to considering the use of geophysics in their reservoir evaluations.
Following the oil-price collapse of the 1980s, oil prices remained more-or-less steady through 2003 (Figure 4), although volatile in the short term. The groundwork for reservoir geophysics was laid during the price collapse of the 1980s. The science matured during the postcollapse period of the 1990s, and this continues today. (Speculation about the relationship of reservoir geophysics with the oil-price run-up under way in 2004–2005 is premature at this time, and will not be attempted by this author!)
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Figure 4: Price of oil (first-purchaser's cost, in constant year-2000 dollars; from the Energy Information Agency), showing short-term volatility and long-term stability from 1986 through 2003.
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As reservoir geophysics matured, it became increasingly "standard operating procedure" at most companies. With increased scrutiny of asset statements, it is likely to become more integrated with traditional reservoir management schemes over time. Although hard figures are impossible to come by, it may be that more financial and human resources are being invested in reservoir geophysics than in exploration geophysics at this time ... less than 20 years after the phrase came to popular attention.
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Specific aspects of reservoir geophysics
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Reservoir geophysics differs from exploration geophysics in three main areas: well control, rock-physics control, and survey scope and design. The targets of reservoir geophysical surveys are more clearly identified, and the existence of at least one well means that the surveys can be focused, calibrated to depth, and calibrated for rock physics correlations. The availability of one or more wells also opens up new geometrical options for the surveys. As a result of these factors, reservoir geophysics has expanded the application of 3D seismic and opened new opportunities for borehole seismic and nonseismic techniques.
Many of these techniques are due largely to the tenacity of a few dedicated visionaries of geophysics. Each specialty has repeatedly been declared "dead" by practitioners and management, only to resurface again with improved technology and resolution. The dedication of these people cannot be overstated, and the field of reservoir geophysics owes them their appreciation. Their funding sources varied, but included their own personal credit, corporate support, venture capital, and government funding. Government funding for the development or improvement of many of these techniques was often through the U.S. Department of Energy and its national laboratories, in an effort to decrease the decline of US-based petroleum resources (for author's disclaimer, see acknowledgments). Currently, research support in reservoir geophysics is also provided through the European Union, reflecting the importance of North Sea assets.
3D surface seismic has five main benefits:
- Attributes: While seismic attributes have become increasingly important for exploration geophysics, they are de rigueur for reservoir geophysics. Spatial variations in lithology and fluid content are among the primary goals of reservoir geophysics, and these are typically established through calibrated seismic attributes, including inversion results.
- Geostatistics: With well calibration comes the opportunity to provide estimates of confidence in the results of correlation of rock properties through (calibrated) rock-physics relationships.
- Time-lapse seismic: The repeated surveying of a reservoir has allowed changes in attributes to be related to changes in reservoir properties due to production. Some changes are the result of fluid substitutions, while others are due to pressure changes, and still others may, in some unusual circumstances, be due to chemical and physical changes in the reservoir matrix material.
- Ultrathin beds: As the targets become more focused, the ability to use the natural bandwidth within the seismic wavelet increases. Commonly grouped under the label of "spectral decomposition," these methods exploit the highest-frequency components of the wavelet and their tuning effects in thin beds, rather than just the dominant frequency component.
- Multicomponent seismology: The use of three-component receivers, whether to record shear waves generated by a specialized source or shear waves generated by conversion upon reflection, has been demonstrated to enable imaging beneath gas clouds that overlie some reservoirs and to map fracture patterns and densities.
Borehole seismic has three primary functions:
- 3D VSPs: Getting either the receiver or the source closer to the imaging target (and below the weathered layer) results in a much higher-resolution image. Placement of a string of seismic receivers in the borehole (vertical seismic profiling or VSP) or a source in the borehole (reverse VSP) accomplishes this, and allows for 3D imaging if the surface components (sources for VSP and receivers for reverse VSP) occupy appropriate large swaths of the surface. Development of extremely high-quality multichannel receiver strings has made the service affordable by minimizing acquisition time, which often requires loss of production.
- Crosswell seismic imaging: The deployment of a string of receivers in one well, and a source in another well, allows the imaging of the plane between the two wells. The timing of the first arrivals allows a 2D image of interval velocities to be obtained as a velocity tomogram, and the reflected events can then be migrated into proper positions for a crosswell reflection image. The primary advantage comes from a tremendous increase in resolution, often exceeding a full order of magnitude improvement over the surface data in the same area.
- Passive seismic monitoring: Some reservoir management activities result in microseismic (and occasionally macroseismic) activity—small earthquakes—usually not detectable at the surface of the earth. Deployment of sensors in boreholes has allowed detection of these events. When the seismic events that accompany stimulation for hydraulic fracturing are located, the result is a temporally changing map of the fracture during its creation. The mapping of events from other reservoir practices (usually, although not always, injection) can also be accomplished, although the relationship of these events to information that is deemed useful to the improvement of reservoir performance is not always apparent. While hydraulic-fracture monitoring services can be considered "routine" by reservoir-geophysics standards, the application of other microseismic services is still developing.
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Electrical and electromagnetic surveys
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The single most significant physical property that distinguishes hydrocarbons from brine is resistivity—hydrocarbons are virtually insulators while brine is an excellent conductor. The differences can be orders of magnitude (compare this with the fractional differences of seismic properties), and mapping of reservoir fluids from electrical and electromagnetic should be easy, it seems. The problems with using these techniques are associated with their inherent poor resolution (they should be considered dispersive, with essentially very large wavelengths) and the prevalence of steel-cased wells in oil fields. Still, amazing progress has been made, and, while not quite routine, time-lapse electromagnetic surveys of reservoirs are now possible, and case histories have been published. This area can be expected to continue to improve in capability and availability in the future, as improvements continue to be made and case studies conducted.
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The role of SEG
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SEG promotes the advancement of the science of geophysics and the ethical practice of applied geophysics. It is strongly driven by the desires and needs of its membership. But the word "exploration" is in its name. In the 1980s, a small group of geophysicists decided that the overwhelming attention paid to exploration geophysics was resulting in the neglect of geophysics applied to reservoir development and production, and they formed a new committee, called the Development and Production Committee, to address their needs. (This is how things work in SEG. If there is a need for something, a group can be formed to address it. It is a highly democratic institution.) This committee rapidly grew in size to more than 200 members, almost all of whom were active in one form or another. It initiated the "Development and Production Forum" (D&P Forum) in 1991, where attendees were united by common goals, rather than common technologies. In this sense it was remarkably unique and beneficial—the participants in these week-long meetings (held at resort locations) included geophysicists, geologists, engineers, and occasionally management. They were virtually forced to sit through presentations and discussions involving technologies with which they were not necessarily very familiar, because there were no alternative sessions (other than truancy, which was frowned upon). The effect was a tremendous cross-fertilization of ideas and expertise. For example, the electromagnetic researchers learned how to present results in ways that reservoir engineers could see a benefit. Seismologists learned about the problems facing the engineering community, and found out that these were not always the same as the "problems" that the geophysicists had been working on. And so on. Most meetings were highly successful, although some did not break even financially, causing a strain on the concept of dedicated small meetings sponsored by the larger society.
A brief timeline drawn from session titles of SEG Annual Meetings and special sections of TLE shows a number of accomplishments (see box, right).
There were times that the D&P Committee recommended that the SEG Executive Committee change the name of SEG to something more encompassing (my personal favorite is SEG: the Society of Extraordinary Geophysicists—where we are all above average). This proposal was usually met with disdain, but occasionally good-natured laughter. The D&P Committee no longer feels the need to exert its influence in these matters. Instead, the question presently facing the group is this: Now that development and production geophysics has become a major force—perhaps the major force—in petroleum geophysics, is there still a need for such a committee? Should the committee "declare victory and go home"? We are nearly all reservoir geophysicists now.
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Examples of typical advertisements published in GEOPHYSICS during the 1940s.
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Suggested reading
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The best contemporary accounts of the development of reservoir geophysics can be found in the annual special sections of TLE from 1992 through 2004. The journal is available for browsing through the SEG Digital Library (http://segdl.org/). Some readers may be interested in comparing the reflections made in this paper with the predictions made by Gordon Greve in "Geoscience in reservoir development—a sleeping giant" (TLE, 1992).
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Acknowledgments:
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The author gratefully acknowledges all the people who worked on the D&P Committee through the years, and who actively promoted the discipline of reservoir geophysics. Each person's recounting of the historic record will vary, and this article presents just one view. Preparation of this manuscript was supported by project DE-FC26-04NT15508 from the U.S. Department of Energy, Fossil Energy Program, through the Tulsa office of the National Energy Technology Laboratory with Purna Halder as program manager. The views and opinions of the author expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
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Footnotes
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Wayne Pennington has degrees in geology and geophysics from Princeton, Cornell, and the University of Wisconsin-Madison. His career has been divided between academic and industry employers and he is currently a professor of geophysical engineering and department chair at Michigan Technological University. He was an early advocate of reservoir geophysics and chaired the 1992 D&P Forum on Monitoring Reservoir Changes Over Time. Pennington was guest editor for several TLE special sections on development and production geophysics and wrote the reservoir geophysics chapter in the new Petroleum Engineering Handbook (soon to be published by SPE).
Copyright © 2008 by Society of Exploration Geophysicists
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Defining reservoir geophysics
The academic role in...
