Kurt Nihei¹, Larry Myer¹, Zhuping Liu², Liviu Tomutsa¹

(1) Lawrence Berkeley National Laboratory, Berkeley, CA
(2) University of California, Berkeley, CA

ABSTRACT: Seismic Properties of Partially-Saturated Sands with CO2 Gas in the 1-9 kHz Range

Re-injection of CO2 into high permeability sand aquifers has emerged as a candidate strategy for CO2 sequestration. Critical to the success of this approach are the abilities to evaluate the sealing capabilities of the sand aquifer prior to injection, and to monitor the transport of the injected CO2 in the sand. High-frequency singlewell and crosswell seismic imaging are two of several possible technologies that may prove useful in these efforts. The focus of this research is to provide a physical basis for analyzing the observed changes in seismic velocities and attenuation of 1-9 kHz waves for poorly consolidated sands with varying saturations of CO2 gas and for a range of consolidation conditions.

Extensional wave attenuation and velocity data on a high permeability (3.4 D) Monterey sand was obtained for a range of gas saturations for imbibition and degassing conditions. This data consists of extensional wave pulse propagation and resonance tests performed over the 1 - 9 kHz frequency range for a hydrostatic confining pressure of 8.3 MPa and corresponding X-ray CT images of CO2 gas distributions. Analysis of the extensional wave data and the CT images showed that large attenuation resulted from partial gas saturation, with larger attenuation at a given saturation resulting from heterogeneous fluid distributions, and extensional wave velocities that are in basic agreement with Biot-Gassmann theory for homogeneous gas saturations and patchy saturation models for heterogeneous gas saturations. Modeling efforts to explain the observed attenuation, as well as ongoing efforts to measure torsional wave properties in the 1-9 kHz range, will be reported.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado