Ultrasonic seismic wave attenuation (Q) for better subsurface imaging, energy exploration, and tracking of sequestrated carbon dioxide

Abstract

Parameters related to seismic and ultrasonic elastic waves traveling through a porous rock material with compliant pores, cracks and isometric pores are subject to variations which are dependent on the physical properties of the rock such as density, porosity, permeability, frame work moduli, fluid moduli, micro structural variation, and effective pressure. Our goal is to understand these variations through experiments completed using rhyolites, coal, and carbonate samples. Understanding these lithologies are relevant to enhanced oil recovery, enhanced geothermal, and CO2 storage activities. Working in the COREFLOW laboratory at the National Energy Technology Laboratory (NETL) of the United States Department of Energy (DOE) we performed several experiments on these rock types with various different pore filling fluids, effective pressures, and temperatures. We measured P, S1 and S2 ultrasonic velocities using a New England Research (NER) Autolab 1500 device and calculated the lame parameters (Bulk modulus (K), Young’s modulus (E), Lamè’s first parameter (Λ), Shear modulus (G), Poisson’s ratio (ν), P-wave modulus (M)). Using an aluminum reference core and the ultrasonic waveform data collected, we employed the spectral ratio method to estimate Q. This method uses the ratio of the amplitude-frequency spectrum (obtained via fast Fourier Transform and processed using Matlab) of the rock core compared with the amplitude-frequency spectrum of the aluminum reference core to calculate the quality factor (Q). The primary focus of these calculations was P waveform attenuation. The quality factor is a dimensionless value that represents the attenuation of a seismic wave as it travels through a rock. Seismic attenuation is dependent on wave velocity, the path length or time the wave is in the rock, and of course the physical properties of the rock through which the wave travels. Effective pressures used in our experiments varied between 0.01 MPa and 50 MPa and temperatures varied between 21° C to 80° C which allowed us to more accurately represent subsurface conditions. Pore-filling fluids consisted of deionized water, oil/water mix, gas, and supercritical CO2. Carbonate samples were tested dry (atmospheric gas as pore fluid) and with deionized water, oil, and CO2. By understanding how seismic waves attenuate we can better understand what collected seismic signals traveled through. This knowledge and understanding of seismic wave attenuation could prove to be a powerful tool for better subsurface imaging, tracking of sequestrated CO2, and energy exploration.