MEASUREMENTS OF SEISMIC WAVE ATTENUATION IN BUBBLY LIQUIDS: WIGED MECHANISM

Abstract:

Wave-Induced Gas Exsolution Dissolution (WIGED) has been observed in some experiments that were designed to measure attenuation in saturated sandstones at low-frequencies (0.1Hz-100Hz). Nevertheless, no experiments directly measured the attenuation of seismic waves in bubbly-water to prove the WIGED attenuation mechanism. If verified, WIGED would be key to investigate the presence of multiphase fluids in the subsurface, such as in magma or hydrocarbon reservoirs. We designed an apparatus that can measure the deformation of a bubble due to WIGED. The bubbly water is injected into a glass-made tight pressure syringe, and a linear actuator generates the pressure disturbance. A MATLAB code synchronizes the linear actuator, the recording of the pressure from the sensor connected to the syringe and the image acquisition from a camera set above the syringe. We detect bubble from the recorded images to estimate the variation of the bubble size and location. Here, we show the results of one experiment of a mixture of water and air. First, the bubbly water in the syringe is pressurized at an initial pressure (1.88 bar). We then wait a few seconds to achieve thermodynamic equilibrium (i.e., to ensure that bubbles are stable in size and position). Then, the actuator moves the syringe piston to reach the new target pressure (2.32 bar), and the bubble shrinks quickly as a response: its diameter decreases by 6%. The Arduino measures the pressure and controls the syringe piston position to maintain the target pressure for over 1 minute, and the bubble keeps shrinking by ~0.5% in this period. We suggest that such a creep or anelastic behavior is due to the dissolution of gas into the water and the diffusion away from the bubble. Such a process lasts for a time much longer than the pressure stabilization until a new thermodynamic equilibrium is achieved. Such a process introduces a time delay between the pressure change and the bubble deformation resulting in the attenuation of seismic waves. We also model the variation of bubble size according to the WIGED theory and the model fits the data accurately. We can conclude from our work that in rocks saturated with bubbly liquids, WIGED can cause attenuation of seismic waves in reservoirs and could be used as a diagnostic tool to monitor the formation, migration, and growth of gas bubbles at depth.