A capillarity-advective model for gas break-through in clays

Laboratory testing has investigated how gases can break through compacted specimens of illite, bentonite, and sand–illite or sand–bentonite mixtures. Specimens were formed with a wide range of initial clay densities, water contents and degrees of saturation. Tests were done using two different test procedures. In one, equal increments of gas pressure were applied at constant time intervals until break-through was observed. In the second, the pressure was held constant, and the time required for break-through recorded. s show that the pressure at break-through increases with clay density and decreases with degree of saturation. When the degree of saturation is below about 85% in illite and clay–illite, and 93% in bentonite and sand–bentonite, there is only a small resistance to gas migration. Above these degrees of saturation, break-through pressures rise sharply. In an approach that differs from some others that have been reported, it is postulated that gas migration is only possible when its pressure is higher than a Gas Entry Value (GEV) that is related to capillarity effects in the largest pores of the material. Thereafter, the rate of advance of the gas–water interface depends on advective flow, that is, on the pressure (hydraulic) gradient across the specimen. Analysis shows that times to break-through should decrease inversely with pressure increase and this was observed in the experiments. Tests were also done on specimens made with non-polar paraffin instead of water. This inhibited the development of bound water in diffuse double layers (DDLs) and led to break-through at much lower pressures.