Low frequency elastic wave experiments with gas kick and its implications for downhole detection

Early and reliable detection of gas kick is critical for drilling safety, particularly in deep and ultra-deep wells where surface indicators are delayed and downhole conditions are highly complex. This study investigates the propagation characteristics of low-frequency elastic-wave in gas–liquid mixtures and evaluates their potential for downhole gas kick monitoring. The theoretical framework was revisited, including the Wood model and the Commander–Prosperetti model, to describe acoustic velocity and attenuation in bubbly fluids. A high-visibility laboratory apparatus was developed to reliably control bubble size, gas volume fraction, and excitation frequency. Low-frequency elastic waves were generated using an electromagnetic piston exciter. Multi-sensor acquisition and signal preprocessing, including mean removal, moving-average filtering, and wavelet denoising, enabled reliable time-delay estimation via cross-correlation. Experimental results show good agreement with theoretical predictions. Acoustic velocity decreases nonlinearly with increasing gas volume fraction, exhibiting a sharp reduction at very low gas fractions (approximately below 1%) and a more gradual decrease thereafter. This indicates high sensitivity to early-stage gas entrainment. In contrast, within the 20–200 Hz range, frequency has only a weak influence on velocity, suggesting limited dispersion and improved robustness to frequency variations under these low-frequency conditions. Based on these findings, we propose a downhole gas kick monitoring concept utilizing low-frequency elastic wave responses. The results support frequency selection, feature interpretation, and tool configuration for downhole applications.