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5 Seismic Response of Fractured Sandstone During Geological Sequestration of CO2 : Laboratory Measurements at Mid (Sonic) Frequencies and X‐Ray CT Fluid Phase Visualization
Seiji Nakagawa and Timothy Kneafsey
Earth and Environmental Sciences, Lawrence Berkeley National Laboratory, Berkeley, California, USA
ABSTRACT
A series of laboratory supercritical carbon dioxide (scCO2) injection experiments (pore water drainage experiments) on small sandstone cores investigating the effects of a single discrete fracture is presented. The orientation and aperture of the fracture are varied across the series of tests. The dynamic Young's modulus and shear modulus along the axis of cylindrical core samples and their related attenuation are determined within a sonic frequency band of ~1 kHz to ~2 kHz, using a resonant bar technique. Concurrently, the distribution of scCO2 injected into the cores is examined using X‐ray CT. The orientation of the fracture with respect to the scCO2 migration and the wave propagation direction is shown to have a large impact on how the seismic wave velocities (or moduli) and attenuations change as a function of scCO2 saturation of porous, fractured rock.
5.1. INTRODUCTION
During geological sequestration of CO2, velocity and attenuation of seismic waves can be monitored to detect the invasion of supercritical CO2 (scCO2) and to determine its saturation in the reservoir rock. The scCO2 introduced in fluid‐saturated (typically by fresh or saline water) porous rock reduces the bulk modulus of the rock, causing reductions in the compressional (or P‐wave) wave velocity. These reductions are usually related to the amount of CO2 in the pore space through simple quasi‐static rock physics relationships such as the Gassmann's fluid substitution model (Gassmann, 1951). However, these models do not always provide satisfactory results, particularly when the seismic waves used by the measurements have relatively high frequency (e.g., Cadoret et al., 1995; Azuma et al, 2013).
Many laboratory experiments for the dynamic properties of CO2‐injected rock have been conducted at ultrasonic frequencies, for correlating a variety of reservoir conditions to seismic signatures as a function of scCO2 saturation. Wang and Nur (1989) conducted laboratory ultrasonic measurements during CO2