While most reported 4D effort focuses on 4D processing and interpretation of 4D seismic results, perhaps it is time to look at the problem the other way around. What 4D results are likely to arise from common reservoir depletion and displacement conditions? By better understanding the engineering cause of the geophysical effect, it may be possible for engineers to better contribute to the processing and interpretation of 4D data. This paper looks at this issue through reservoir simulation studies. The reservoir simulator is a powerful tool to predict how fluid saturations and pressures are distributed throughout the volume of the reservoir over time. This information can be used in conjunction with a properly scaled rock physics model to generate seismic attribute maps that reveal the patterns of 4D results that would be expected under various depletion, displacement, and heterogeneity conditions. The preliminary findings of this work are that pressure effects do obscure fluid saturation changes and exacerbate the effects of porosity variations in the reservoir. In addition, it is shown that care must be taken to accurately represent the poor vertical resolution of surface 3D seismic to avoid overly optimistic predictions of likely 4D seismic results. Introduction Time-lapse 3D, or 4D, seismic is an evolving geophysical technique involving the acquisition, processing, and interpretation of repeated 3D seismic data. The technique has developed considerably in the geophysical community since the early 1980's when it was first introduced, to the point where several successful case studies have been published recently1,2, including reporting the economic benefit to the asset. But even with the advances in seismic processing that have produced these 4D successes, there continue to be two aspects of the problem that deserve special attention - will 4D work in a given field, and interpretation of the 4D result. Interestingly, these are also the two aspects of 4D that require input from non-geophysical disciplines, primarily reservoir engineers and petrophysics. The first of these, ‘Will 4D work in a given field?’, has been the focus of several papers2,3,4,5,6,7 and excellent rock physics research and development8,9. But most of this work focuses on developing a rock physics model for a field, and then predicting the change in acoustic response for an assumed ‘average’ change in reservoir properties. For example, a saturation change from 0. to 0.8 will give an 8. 5% change in acoustic impedance, Z, a pressure change from 5586 psi to 4116 psi will give a 4% change in Z, and the combination of a saturation and pressure change will result in a 12% change in Z2. This ‘average reservoir’ approach is useful as an initial screening tool, but it is possible to go much further2,10,11. Specifically, when a reservoir simulation of the reservoir is available, it is possible to synthesize the acoustic response in much more detail than the ‘reservoir average’ approach. In this way, it is possible to predict both the magnitude and location of the acoustic change to give a better estimate of whether ‘4D will work in a given field’. This approach will be described in detail in the sections that follow. It turns out that this same approach is useful in addressing the 2nd aspect needing attention, the interpretation of the 4D result. By synthesizing the acoustic response of common reservoir depletion processes, it may be possible to describe the ‘4D signature’ of the different processes. This may allow a more informed interpretation of 4D results. However, the scope of this paper is limited to describing the approach and demonstrating two simple cases. Studying the Cause/Effect Relationship Knowing the relationship between the fluid changes in the reservoir (Cause), and the acoustic response on the 4D seismic technique (Effect) is critical in both feasibility and interpretation stages of a 4D project. But quantifying this relationship is difficult using field measured results because it is impossible to know what the Cause actually is, and the Effect is obscured by noise.
This paper was prepared for presentation at the 1999 SPE Annual Technical Conference and Exhibition held in Houston, Texas, 3–6 October 1999.
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