Converted-wave data are inherently more difficult to process than P-wave data due primarily to the asymmetry of the travel path as indicated in Figure 6. Standard processing techniques for P-wave data are not directly applicable, and algorithms and processing flows require modification. Significant improvements in converted-wave processing algorithms and methods are now producing improved subsurface images and facilitating more effective integration of P- and S-wave data. Further work remains in developing and implementing true vector processing methods for the full elastic wave field to fully exploit the value of converted-wave data. Such methods must necessarily treat data processing and interpretation in a parallel rather than serial manner.
Figure 6: Ray Diagram Showing Incident P-wave and Reflected S-wave Conversion Points at Reflection Boundaries
The success and sustainability of multicomponent methods is ultimately tied to value demonstrations and to the development of interpretation tools and work processes, which allow shear-wave information to be effectively utilised. Specialised elastic wave interpretation packages, such as Hampson-Russell’s ProMC, are helping to promote the extraction of information from multicomponent data. The reality is that current typical interpretation workstations are not designed to effectively and efficiently manage and evaluate multimode data. Continuing demonstrations of value from multicomponent data will undoubtedly catalyse further workstation development.
Multicomponent Applications
The literature provides a broad range of convertedwave applications. (9–10) Additionally, Veritas DGC has actively pursued the development of applications over the past two years. Some of these are discussed in the examples which follow.
North Emerald 3-C3-D Test – Anadarko Basin Gas Play
The Anadarko Basin is a major gas production province with both mature fields and on-going exploration efforts. Production is from Ordovician through Permian-age clastics and carbonates. In the North Emerald 3-C3-D study area, thin sands (less than 8m each) produce natural gas from the Mississippian Springer Formation at depths nearing 3.2km. Production is from offshore sandbars trending NW–SE that were bisected by NNE–SSW trending erosional systems during periods of lowstand. The Springer interval is approximately 70m thick containing three reservoir units – the Cunningham, Britt and Boatwright sands. Production is primarily related to porosity development. Wells with greater than 2m of 8% to 12% porosity tend to be commercial while porosities less than 8% are nonproductive. Natural gas production is through pressure depletion.
The authors observed changes in both P-wave (PP) and converted-wave (PS) reflectivity associated with gas production. Their hypothesis was that porosity development in these lithified Paleozoic sediments primarily changes the rigidity with little effect on compressibility. Both PP and PS reflectivity ‘see’ the change in rigidity, but the PP reflectivity is damped by the large compressibility term.
The authors used a neural network waveform classifier to quantitatively determine whether using the PP and PS data together could provide a better empirical estimate of gas production than using PP data only. For 17 wells in the 3-C3-D image area with Springer penetrations and tests, a simple log trace was created representing cumulative gas produced. Using cumulative gas as the target, a neural network was trained and the most significant seismic attributes were identified. For validation, each well was removed from the process and tested based on the relationships determined by the remaining population.
Figure 7 shows the empirical results for predicting cumulative gas production. Results using both PP and PS data correctly identified the three wells with higher cumulative gas produced. Crossplots of estimated gas compared with actual gas production show that using PP and PS data together produced a more accurate linear fit than when using the conventional P-wave data alone.
Figure 7: Predicted Gas Production Based on Seismic Data Attributes
The authors conclude that changes in both PP and PS reflectivity associated with gas production fit their hypothesis that porosity development in these lithified, sands primarily changes the rigidity with little effect on compressibility. The development of porosity allows for commercial quantities of gas to be produced. (11)
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