Pore-scale Characteristics Of Entrapped Air In Quasi-saturated Porous Media

The presence of entrapped air in groundwater is called quasi-saturated state.The existence of entrapped air will deeply affect the hydraulic properties of the aquifer.At present,the formation and evolution of quasi-saturated entrapped air has not fully understood.It has great scientific and practical significance to study the affecting factors,formation process and escape process of entrapped air in quasi-saturated medium from a microscopic scale.This article uses X-CT technology to carry out microscopic features of entrapped air,comparison of capillary and quasi-saturated air phases,and microscopic observations of entrapped air escape.Aim to understand the affecting factors,formation process,escape conditions and process of entrapped air.The paper mainly achieved the following results:(1)By observing the observation of three samples with different particle sizes and particle sizes in quasi-saturated,the particle size of the medium is good and the particle size is small,which is conducive to the formation of trapped gas in a disconnected manner,and the number of entrapped air is greater and the size distribution of the bubbles is wider.The medium with a large pore coordination number,a small pore throat diameter,and a large pore cavity throat is easier to form entrapped air in a "snap-off" manner,and the trapped gas saturation is greater.(2)Air microscopic studies of quasi-saturated and capillary zones indicate,the capillary zone are the entrapped air front in the water level rises,and most of the entrapped air in the quasi-saturated zone was formed during capillary rise.(3)Under different vacuum degrees,during the escape of entrapped air,the small bubbles in the pores escape earlier than the larger ones,and some bubbles will break up and form multiple small bubbles un the escape,and a few bubbles coalesce larger bubbles.The extraction pressure when bubbles begin to escape in large quantities needs to reach 80 Kpa,and when the extraction pressure reaches 95 Kpa,there are still a large number of bubbles that have not escaped from the pores.In addition,the calculation of the critical capillary force for the escape of bubbles shows that the escape resistance of the trapped gas is 3-5 times the maximum capillary force of the entrapped air.