Influence of gas accumulation on rock elasticity and control factor analysis for gas chimney formation

ABSTRACT Gas chimneys can often be observed in marine seismic profiles, which are usually related to gas accumulation in shallow layers and their impacts on rock elasticity. To clarify the impact of various geologic factors on elastic wave propagation, an experimental study is conducted to elucidate the variation in rock elastic parameters in gas-accumulated rocks. Gas accumulation simulations and real-time ultrasonic monitoring experiments are conducted in laboratory experiments. The control factors for gas chimney formation, such as pressure, porosity, fractures, and fluids, are analyzed. It is found that high pore pressure provides favorable conditions for the formation of gas chimneys. Within the lower range of pore pressures, the P-wave velocity is more sensitive to gas changes, whereas the S-wave velocity is relatively insensitive. When the pore pressure is within a moderate range, the S-wave velocity is more sensitive than the P-wave velocity. At higher pore pressures, the influence of pore pressure on P-wave velocity is greater, and the influence on S-wave velocity is smaller. The experimental results for low-porosity sandstone are significantly influenced by pore pressure, whereas the attenuation of P and S waves in high-porosity sandstone is controlled by porosity and less sensitive to changes in pore pressure. The presence of fractures has a significant impact on velocity and amplitude, and the impact of fractures is no longer significant when the pore pressure is high. Injecting a small amount of gas into water-saturated rocks can cause a strong attenuation of wave amplitude, indicating that the influence of mixed fluids on amplitude is more significant than other factors. This study provides a theoretical explanation for gas chimney formations in shallow loose rocks and helps analysis of seismic wave propagation properties in the field.