Slippage Effect and Quasi Starting Pressure in Low Permeability Water- bearing Gas Reservoirs

摘要

Lab tests were carried out to study the gas slippage and quasi starting pressure in water-bearing gas reservoirs of low permeability. The permeability of the testing cores is mainly lower than 1 mD. The testing results indicate that: for similar water saturation, the lower the permeability is, the more serious the gas slippage is. For similar permeability, with the increasing of water saturation, the gas slippage effect increases first and then decreases. The turning point is called critical water saturation (Sw)c1. The relationship between the critical water saturation and the core coefficient is binomial, where the core coefficient is the ratio between permeability and porosity. By curve fitting, it was found that, when the water saturation is lower than a critical value, gas slip factor is a logarithmic function of the ratio of core coefficient and water saturation; when water saturation is higher than the critical value, the gas slip factor is a logarithmic function of the multiplication of core coefficient and water saturation. The quasi starting pressure gradient may exist when the gas flow in porous media that contains water. The reason is that the increase of capillary resistance is larger than gas slippage as the water saturation rises. In further, it was concluded that there is another critical water saturation (Sw)c2. When the water saturation is larger than the critical value (Sw)c2, the quasi starting pressure gradient exists. The relationship between the quasi starting pressure gradient and the ratio of core coefficient and water saturation is a power function. Further more, the relationship between the critical water saturation and absolute permeability is also a power function.