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Scientific Investigations Report 2012–5046

Isotropic, Anisotropic, and Borehole Washout Analyses in Gulf of Mexico Gas Hydrate Joint Industry Project Leg II, Alaminos Canyon Well 21–A

By Myung W. Lee

Thumbnail of report coverAbstract

Through the use of three-dimensional seismic amplitude mapping, several gas hydrate prospects were identified in the Alaminos Canyon area of the Gulf of Mexico. Two of the prospects were drilled as part of the Gulf of Mexico Gas Hydrate Joint Industry Program Leg II in May 2009, and a suite of logging-while-drilling logs was acquired at each well site. Logging-while-drilling logs at the Alaminos Canyon 21–A site indicate that resistivities of approximately 2 ohm-meter and P-wave velocities of approximately 1.9 kilometers per second were measured in a possible gas-hydrate-bearing target sand interval between 540 and 632 feet below the sea floor. These values are slightly elevated relative to those measured in the hydrate-free sediment surrounding the sands. The initial well log analysis is inconclusive in determining the presence of gas hydrate in the logged sand interval, mainly because large washouts in the target interval degraded well log measurements. To assess gas-hydrate saturations, a method of compensating for the effect of washouts on the resistivity and acoustic velocities is required. To meet this need, a method is presented that models the washed-out portion of the borehole as a vertical layer filled with seawater (drilling fluid). Owing to the anisotropic nature of this geometry, the apparent anisotropic resistivities and velocities caused by the vertical layer are used to correct measured log values. By incorporating the conventional marine seismic data into the well log analysis of the washout-corrected well logs, the gas-hydrate saturation at well site AC21–A was estimated to be in the range of 13 percent. Because gas hydrates in the vertical fractures were observed, anisotropic rock physics models were also applied to estimate gas-hydrate saturations.

First posted May 3, 2012

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Suggested citation:

Lee, M.W., 2012, Isotropic, anisotropic, and borehole washout analyses in Gulf of Mexico Gas Hydrate Joint Industry Project Leg II, Alaminos Canyon well 21–A: U.S. Geological Survey Scientific Investigations Report 2012–5046, 23 p.



Contents

Abstract

Introduction

Well Logs

Sand-Shale Porosity Model

Isotropic Rock Physics Model

Anisotropic Rock Physics Model

Washout Modeling

Well Log Analysis

Results and Discussion

Summary and Conclusions

Acknowledgments

References Cited

Appendix A. Porosity


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