Acoustic visibility of immiscible liquids in poorly consolidated sand

Abstract

Summary To investigate the acoustic visibility of non-aqueous phase liquids in poorly consolidated sands, laboratory experiments were conducted in a 0.6 m diameter confining cell with watersaturated sand. Crosshole data was collected before and after dodecane, a lighter than water non-aqueous phase liquid (LNAPL), was injected from the bottom of the cell. These experiments show a strong acoustic sensitivity of dod ecane for transmitted P-wave amplitudes (decreases of up to 65%) and a smaller, but measurable, acoustic visibility for velocity (decreases of up to 2%). Velocity difference tomograms were successful, but limited in resolution; they depict a low velocity region in the tank that corresponds to entrapped dodecane, as revealed by subsequent excavation of the sand cell. Effects of immiscible liquids on acoustic waves in unconsolidated sand Detection and characterization of non-aqueous phase liquids in poorly consolidated, water-saturated sands is of profound importance in petroleum and environmental engineering. Prohibitively expensive drilling procedures as well as the potential for accidental liquid mobilization during drilling procedures make non-invasive testing desirable for subsurface characterization. In poorly consolidated sand with weak intergranular cementation and low effective stresses, acoustic waves are sensitive to pore fluid properties. This is because the fluid phase within the pore space carries a large part of the wave energy as the frame stiffness decreases. Therefore, with sufficient contrast between pore liquids, their distributions can be visualized using tomographic inversion techniques (e.g., velocity contrast for travel time tomography, attenuation contrast for amplitude tomography, or impedance contrast for diffraction tomography). The success of these methods in practice is likely to depend on the fluid properties and the volumetrics and geometry of the target fluid. Ultrasonic measurements by Geller and Myer (1995) and Seifert et al. (1998) have demonstrated that the presence of both lighter-than-water and denser-than water non-aqueous phase liquids (LNAPL’s and DNAPL’s, respectively) in initially water-saturated sand packings produces changes in the P-wave velocities and amplitudes that can be explained and predicted. While the P-wave velocity decreases proportionately to the volume of LNAPL through which it passes, the impedance contrast between different phases does not operate on the velocity as it does on P-wave amplitude. In this manner, velocities are sensitive to liquid volume, whereas amplitudes are sensitive to liquid distribution, and could possibly detect the presence of relatively small volume NAPL fingers that form due to flow instabilities.