The goals of this project are to develop an understanding of seismic loss mechanisms, measure attenuation in the laboratory, extract time-lapse attenuation estimates from field data, and interpret the results in terms of pore fluid and reservoir changes.

The threefold goal is to:

• Perform innovative state-of-the-art laboratory measurements of seismic attenuation at seismic frequencies on well-characterized rock samples under reservoir conditions.
• Perform state-of-the-art theoretical modeling in order to understand and quantify the detailed mechanisms responsible for the attenuation.
• Invert crosswell and vertical seismic profile data for attenuation as a function of frequency and analyze targeted reflectors overlying hydrocarbon reservoirs to quantify the nature of the frequency dependence of the reflection coefficients.

Using the models developed and confirmed by steps above, the project seeks to quantify the material properties required to explain the field measures of seismic attenuation.

Colorado School of Mines (CSM), Golden, CO
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA
Chevron Energy Technology Company, San Ramon, CA
University of California at Berkeley (UCB), Berkeley, CA

Seismic attenuation continues to gain interest in the exploration and reservoir-monitoring community. Partly this is because seismic data quality has improved substantially, to the point where quantitative measures of attenuation can be extracted. This interest is also demonstrated by the number of reports, meetings, papers, and service company activities dedicated to extracting and analyzing attenuation through producing reservoirs. However, it is still apparent that little understanding of the underlying attenuation mechanisms has developed. Of great concern is the potential for misuse of attenuation attributes. As is typical with developing technologies, many excessive claims are being made which, when unfulfilled, will impede valid application of this attribute.

This project plans to provide a scientifically thorough and systematic test case for the use of attenuation. It is a unique combination of theoretical and numerical analysis, laboratory analysis, and application to real field data that will help test and calibrate the process.

Results 
Seismic attenuation is directly linked to fluids and fluid motion. Both the researchers’ theoretical analysis and laboratory measurements have demonstrated that large amounts of attenuation and velocity dispersion are possible when mesoscopic fluid motion occurs. Such mesoscopic motion requires heterogeneous saturations or matrix compliance. This implies that the greatest attenuation signature in the reservoir will be with distributed gas pockets and cementation variation. The source of data for this project, the Genesis Field, is now below bubble point, resulting in patches of elevated gas saturation. This patchy saturation should maximize the time-lapse attenuation signal.

Benefits
Seismic attenuation has great potential for aiding in hydrocarbon exploration and reservoir monitoring. Current seismic technology has difficulty distinguishing between economic gas reservoirs and uneconomic reservoirs with partial gas saturation.

Attenuation is a seismic attribute that could greatly aid in both assessment of potential reservoirs and monitoring the distribution of fluid phases after production has begun. What will be provided by this project is a thorough test case where the mechanics of seismic losses are understood and the ways that systematic methods of extracting attenuation values from field data can be demonstrated.

Summary
Time-lapse seismic data have been acquired over Genesis field, Green Canyon 205, in the deepwater Gulf of Mexico (Figure 1). The reservoir is a series of stacked turbidite sands of Nebraskan (N1, N2, N3U, and N3L) and Pliocene (P14200 and P14800 sands) age. Genesis field is providing a rich set of real data to which test concepts can be applied. The good data quality over the field should enable development not only of an example of applied attenuation analysis but also a toolbox of calibration data and modeling techniques that can be applied to other, similar reservoirs.

Geomechanical alteration is one issue that arose influencing our time-lapse data. The Genesis field has been producing for several years, and the pore pressure has been drawn down significantly due to the weak water drive locally. As a result, the reservoir has experienced significant compaction. This produces velocity changes not only in the reservoir intervals but in the overlying shales as well. These geomechanical effects are being examined both in the laboratory and in the field data before attenuation information can be extracted.

At this point, several tasks are underway. Chevron’s low frequency measurement equipment is up and running. Researchers have procured most of the needed data and reservoir information about Genesis field. Analog samples to field core have been selected and are being prepared at Chevron for shipment to CSM. Both numerical and theoretical models have been developed (Figure 2). These modeling results have been submitted for publication. Forward seismic models have been built and run. These calculations will be compared to field data.

(July 2007)
Much of the needed field data have now been procured. Geomechanical analysis and lab measurements have been conducted to help prepare the data for attenuation analysis. The low-frequency system at Chevron is again functional. Standard sandstone samples have been exchanged between Chevron and CSM and low-frequency attenuation measurements are in progress. Theoretical models have been developed at LBNL, and laboratory tests are conducted. Forward modeling of the seismic data is completed. The project is not funded in FY07.

Accomplishment:
The Final Report has been completed. Study shows that attenuation attribute of seismic data can provide information of fluids in reservoirs and assist in the process of hydrocarbon exploration and production. Results are presented at professional societies. Publications have been submitted.

Funding
This project was selected in response to solicitation DE-PS26-04NT15450-2A, Subsurface Imaging, with research specifically aimed at measuring and understanding the nature of seismic attenuation in sedimentary rocks relevant for hydrocarbon production.

$385,827

$135,000

NETL - Purna Halder (Purna.Halder@netl.doe.gov or 907-452 2084
CSM - Michael Lee Batzle (mbatzle@mines.edu or 303-384-2067

Publications
Ruiz, Begoña, “Comparison of Measured and Modeled Frequency Dependent Moduli in Sandstones,” Proceedings, Rainbow in the Earth—2nd International Workshop, Lawrence Berkeley National Laboratory, Berkeley, CA, August 17-18, 2005.

Duranti, L., Ewy, R., and Hofmann, R., “Dispersive and Attenuative Nature of Shales: Multiscale and Multifrequency Observations,” Society of Exploration Geophysicists Annual Technical Meeting, 2005.

Batzle, M., Hofmann, R., Prasad, M., Gautam, K., Duranti, L., and Han, D-h., “Seismic Attenuation: Observations and Mechanisms,” Society of Exploration Geophysicists Annual Technical Meeting, 2005.

Masson, Y.J., Pride, S.R. and Nihei, K.T., “Finite-difference modeling of Biot's poroelastic equations at seismic frequencies,” J. Geophys. Res., 2006 (accepted for publication).

Masson, Y.J. and Pride, S.R., “Computation of seismic attenuation and dispersion due to mesoscopic heterogeneity in porous materials,” J. Geophys. Res., 2006. (under review).