Experimental study on the gas tightness of a mined cavern with groundwater

The gas tightness of underground rock cavern tanks installed for the storage of gas and oil is preserved by sealing the rock joints around the cavern tanks with groundwater. This storage system is efficient and cost effective, because utilizing groundwater around the rock cavern tanks negates the need for expensive steel liner plates. However, the mechanisms and principles of this gas sealing method are not entirely clear. Two conditions are important for a groundwater sealing system. The first condition is that gas does not exude into the rock joints from the cavern surface. The second condition is that gas bubbles must be retained in the rock joints, even if exuding into the joints. Ensuring higher water pressure than gas pressure satisfies the first condition. The second condition is thought to be governed by bubble buoyancy, capillary pressure, and drag force, however the effect of these factors on the second condition is not clearly understood. Therefore, we performed laboratory experiments to elucidate the mechanisms of securing gas tightness in rock joints with groundwater. The experimental model was comprised parallel glass plates imitating an actual underground rock joint, and we observed the behavior of air bubbles injected into the gap filled with water. The width of the gap between the parallel glass plates and the hydraulic gradient were varied parametrically. This paper shows the methods and results of the experiments and theoretical speculations of the reliance of gas tightness on the buoyancy, surface tension, and drag force.