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GCDifferentiating Fluid and Rock Boundaries with Seismic*

 

Bob Hardage1

 

Search and Discovery Article #40962 (2012)

Posted June 25, 2012

 

*Adapted from the Geophysical Corner column, prepared by the author, in AAPG Explorer, June, 2012, and entitled "Seismic and Boundaries: Is It Fluid or Rock". Editor of Geophysical Corner is Satinder Chopra ([email protected]). Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director. AAPG©2012

 

1 Bureau of Economic Geology, The University of Texas at Austin ([email protected])

 

General Statement

Identifying and mapping fluid-contact boundaries within a reservoir system with seismic technology are common objectives when doing a characterization of a hydrocarbon reservoir – and monitoring the movement of fluid boundaries during secondary and tertiary recovery processes of oil always has been essential for optimizing oil production. When attempting to analyze a fluid-contact boundary, a seismic interpreter must confront a challenging problem – how do you determine if a particular seismic reflection event is caused by a contact boundary between two different fluids, or by the contact between two different rock Previous HittypesNext Hit? Starting in the 1980s people began to see that an efficient way to answer this question was to acquire both P-Previous HitwaveNext Hit and S-Previous HitwaveNext Hit seismic data across a rock/fluid system that had to be interpreted.

General statement

Figures

Examples

Conclusion

Reference

 

 

 

 

 

 

 

 

 

General statement

Figures

Examples

Conclusion

Reference

 

 

 

 

 

 

 

 

 

General statement

Figures

Examples

Conclusion

Reference

Figure Captions

Figure 1. P and S reflection images recorded along the same surface track across a gas reservoir. P-Previous HitwaveNext Hit reflections (a) occur at gas-water fluid boundaries (labeled "Flat spot" and "High-amplitude reflection"), but S-Previous HitwaveNext Hit reflections (b) do not. Both images show a high-amplitude reflection at a known lithological boundary (labeled "lithology"). In this example, an interpreter could use well control to identify which P-Previous HitwaveNext Hit reflection marks a fluid-contact boundary. In areas with little or no well control an interpreter will benefit by having both P and S images and using a comparative method such as demonstrated here to identify reflections associated with fluid-contact boundaries. Data examples taken from Ensley (1984).

Figure 2. P and S reflection images recorded along the same surface track across a gas reservoir. P-Previous HitwaveNext Hit reflections (a) occur at gas-water fluid boundaries (labeled "Flat spot" and "High-amplitude reflection"), but S-Previous HitwaveNext Hit reflections (b) do not. Both images show a high-amplitude reflection at a known lithological boundary (labeled "lithology"). In this example, an interpreter could use well control to identify which P-Previous HitwaveNext Hit reflection marks a fluid-contact boundary. In areas with little or no well control an interpreter will benefit by having both P and S images and using a comparative method such as demonstrated here to identify reflections associated with fluid-contact boundaries. Data examples taken from Ensley (1984).

Examples

An example of a petrophysical interpretation that can be made from a combined analysis of P-Previous HitwaveNext Hit and S-Previous HitwaveNext Hit data is illustrated in Figure 1. These seismic profiles, published in 1985, follow the same track across a known gas field. Three reflection events are labeled on the P-Previous HitwaveNext Hit profile; two of these profiles are absent on the S-Previous HitwaveNext Hit profile. The common reflection that appears on both the P-Previous HitwaveNext Hit and S-Previous HitwaveNext Hit data is caused by a contact between two different rock Previous HittypesNext Hit and is labeled as “lithology.” The two events that appear on the P-Previous HitwaveNext Hit data but not on the S-Previous HitwaveNext Hit data are contact boundaries between brine and gas. A second example that also appeared in the 1980s is presented as Figure 2. In this prospect area, the challenge was to determine if bold reflection events seen on P-Previous HitwaveNext Hit data were caused by gas or by coal. If the cause was gas, the reflecting interface was a contact boundary between two fluids – gas and brine – embedded in the targeted sand interval. If the cause was coal, the reflecting interface was a contact boundary between two different lithologies – coal and its host sand. Wells were drilled that confirmed the following important findings:

  • When a reflection event appeared on both P-Previous HitwaveNext Hit and S-Previous HitwaveNext Hit data, the event was caused by the contact between two different lithologies.
  • When a reflection event appeared on P-Previous HitwaveNext Hit data but not on S-Previous HitwaveNext Hit data, the event was caused by a fluid-contact boundary (brine and gas in this instance).

Conclusion

The P-Previous HitwaveNext Hit and S-Previous HitwaveNext Hit seismic data displayed on Figure 1 and Figure 2 illustrate some important principles.

  • First, P-Previous HitwaveNext Hit seismic wavefields reflect from boundaries created by the contact between two different lithologies and also from the contact between two different pore fluids embedded in a constant-matrix host rock.

In contrast, S-Previous HitwaveNext Hit seismic wavefields reflect from boundaries between contrasting lithologies but do not reflect from fluid contact boundaries unless there is a significant change in bulk density across the fluid boundary. Even when there is an appreciable change in bulk density between two contacting fluids, an S-Previous HitwaveNext Hit reflection tends to be weak compared to the bold nature of its companion P-Previous HitwaveNext Hit reflection from that same fluid-contact boundary.

  • Second, when it is critical to identify and monitor fluid-contact boundaries, both P-Previous HitwaveNext Hit and S-Previous HitwaveNext Hit seismic data should be utilized.

S-Previous HitwaveNext Hit data are needed to identify which P-Previous HitwaveNext Hit reflections are associated with fluid boundaries; P-Previous HitwaveNext Hit data are needed to map and quantify calendar-time changes in any reflection event that has been identified as a fluid-contact boundary. These well-established seismic principles are becoming more important now that there is increasing emphasis to sequester CO2 in brine-filled reservoirs.

Reference

Ensley, R.A., 1984, Comparison of P- and S-Previous HitwaveTop seismic data: A new method for detecting gas reservoirs: Geophysics, v. 49, p. 1420-1431.

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