From seismic exploration to seismic monitoring
The seismic industry has been impressively dynamic and creative during its 60-year history. Although it is relatively a small sector within the oil and gas industry at large, it has made the most significant impact on increasing proven reserves and reserve-production ratios worldwide.
We shall now sketch a brief historiography of the seismic industry before we look ahead. The evolution of the seismic industry can be described briefly in decades of development and forward leaps from one theme to another as outlined in Table I-1.
In the 1960s, the digital revolution profoundly changed seismic acquisition. We were then able to record more data by increasing the number of channels and fold of coverage. The digital revolution brought about the need to use digital computers to analyze the recorded data. That came about in the 1970s when we switched from calculators to computers. Many of the data processing algorithms, including deconvolution, velocity analysis, refraction, and residual statics corrections, normal-moveout correction and stacking, and even migration, were implemented in those years. The computer before the seventies was a person using the calculator; now the computer is a machine and the person became the seismic analyst.
|From analog to digital
|From calculators to computers
|From 2-D to 3-D
|From time to depth
|From 3-D to 4-D
|From 4-D to 4-C
|From isotropy to anisotropy
In the 1980s, the seismic industry took another big step forward; it was now beginning to provide the oil and gas industry with 3-D images of the subsurface. We need only to examine the global reserve-production curves over the past decades to see that the 3-D revolution gave a big jump from 35 to 45 years for oil and from 50 to 65 years for gas. The seismic industry was already pushing the computer industry to the limit with its need for power to handle large-scale data volumes acquired by 3-D surveys.
Finally, in the 1990s, the seismic industry was capable of providing the oil and gas industry with images of the subsurface, not just in 3-D, but also in depth. It took years of exhaustive experimental research to test and field-prove numerous methods to accurately estimate an earth model in depth and use it to efficiently create an earth image in depth. Once again, the seismic industry has challenged the computer industry to provide cost-effective solutions for numerically intensive applications with large input-output operations, such as 3-D prestack depth migration.
As the seismic industry made one breakthrough after another during its history, it also created new challenges for itself. Now we record not just P-waves but also converted S-waves for a wide range of objectives. Using the multicomponent seismic method, commonly known as the 4-C seismic method, we are now able to see through gas plumes caused by the reservoir below. We are able to sometimes better image the subsalt and subbasalt targets with the 4-C seismic method. Using the converted S-waves, we are able to detect the oil-water contact, and the top or base of the reservoir unit that we sometimes could not delineate using only P-waves. We even go further now and attempt to identify fluid types in reservoir rocks, discriminate sand from shale, and map hydrocarbon saturation, again using the 4-C seismic method. Our ultimate objective is to use the seismic method, in addition to the production and geologic data, to characterize oil and gas reservoirs accurately.
Just as we may characterize oil and gas reservoirs seismically, we may also seismically monitor them. Given a set of time-lapse 3-D seismic survey data, which constitutes the basis of the 4-D seismic method, we can track flow paths and fluid distribution in the reservoirs throughout their lifetime. And finally, we have to acknowledge that the earth is anisotropic. By accounting for anisotropy, we can map fractures and increase the accuracy of velocity estimation and imaging techniques.
The topics on the 4-D and 4-C seismic methods, and anisotropy are for the road immediately ahead in the seismic industry with the aim of a rigorous, seismically driven reservoir characterization and monitoring.
- Introduction to Seismic Data Analysis
- Processing of seismic data
- Inversion of seismic data
- Interpretation of seismic data