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1、THE NEW ROLE OF PETROPHYSICS IN GEOPHYSICAL INTERPRETATIONE. R. (Ross) Crain, P.Eng. Spectrum 2000 Mindware Ltd. 403 845 2527 Published in CSEG Recorder Sept 2003NEW ROLES NEW INTERACTIONS NEW INTEGRATION The role of petrophysics in seismic interpretation has taken a major leap forward in the past t
2、en years, resulting from important advances in seismic data processing techniques, particularly seismic inversion, attribute analysis, and amplitude versus offset methods that showed we could estimate reservoir properties from such data. Coupled with the recent advances in dipole shear sonic logging
3、, new vistas in seismic interpretation, dubbed seismic petrophysics, have opened.Geophysical well logs suffer from many borehole and environmental problems that need to be repaired before being used for calibrating seismic models or seismic interpretations. A primary aim of the geophysicist/petrophy
4、sicist is to create a synthetic seismic trace from EDITED log data that accurately represents the seismic response of the subsurface. This is accomplished by editing, repairing, or reconstructing the log data. Using unedited logs for seismic purposes is a waste of time and money and, in the worst ca
5、se, can lead to very expensive exploration and development mistakes. If the synthetic seismic trace is a good representation of the real seismic response, then the edited logs can be used effectively as aids to interpretation of the advanced seismic products.Consequently, the role of the petrophysic
6、ist has also evolved; she must now be competent in log reconstruction as well as conventional log analysis, and must understand the petrophysical needs and limitations of the inversion, attribute, or AVO results. Unfortunately, logs are not perfect measures of in-situ rock properties and seismic dat
7、a is severely band-limited compared to log data, so there are many compromises to be made. A significant change in mindset is also needed, as most of the log repairs (with the exception of fluid replacement) take place in the non- reservoir intervals - intervals that are not usually of interest to p
8、etrophysicists.Geophysicists engaged in seismic interpretation seldom use logs to their full advantage. This sad state is caused, of course, by the fact that most geophysicists are not experts in log analysis. They rely heavily on others to edit the logs and do the analysis for them. But, many petro
9、physicists and log analysts have no ides what geophysicists need from logs, or even how to obtain the desired results. Thats a particularly vicious “Catch-22“.Education, practical solutions, appropriate software, and practice are the keys to success. In order for geophysicists and petrophysicists to
10、 communicate well, each must know something of the others specialty. Chapters Twenty through Twenty-Five on my website provide theory, practical methods, and case histories to accomplish this goal. SEISMIC PETROPHYSICS AND SEISMIC MODELING Seismic petrophysics is a term used to describe the convers
11、ion of seismic data into meaningful petrophysical or reservoir description information, such as porosity, lithology, or fluid content of the reservoir. Until recently, this work was qualitative in nature, but as seismic acquisition and processing have advanced, the results are becoming more quantita
12、tive. Calibrating this work to well log “ground truth” can convert the seismic attributes into useful reservoir exploration and development tools. Since there are an infinity of possible inversions, it is pretty important to find the one that most closely matched the final edited logs or the compute
13、d results from those logs.A seismic petrophysics study aimed at quantifying porosity is shown below in Figure 1.FIGURE 1: Seismic petrophysics study for porosityThis example used a geostatistical package to distribute the dense “fuzzy“ seismic attribute data between the sparse, “accurate“ well log d
14、ata. The logs, or log analysis results, in turn are calibrated to core, well test, and production data before being used to control seismic interpretation. The use of geostatistics to map seismic attributes onto well logs is a relatively new phenomenon and is showing great promise.SEISMIC PETROPHYSI
15、CS AND WELL LOG MODELING Unfortunately, it takes a fair amount of effort to compare seismic results to log data. The logs will usually require some kind of editing or modeling or both. Comparison of seismic results to log data may indicate that further processing of the seismic is needed, and the ca
16、libration cycle is repeated often several iterations are needed. In other cases, it is the logs that need further editing.Log modeling or editing is required because logs dont see the same rock and fluid mixtures that the seismic signal sees. Drilling fluid invasion removes gas or oil near the wellbore, replacing it with water and altering the sonic and density log response from the reservoirs undisturbed
