Experimental study of the breakthrough pressure of capillaries and the sealing ability of shale

Several experiments were made with mono-capillary-tubes to study how breakthrough pressure of mudstone cap rock changes when there are multiple two-phase interfaces in the flow path. The results indicated that breakthrough pressure is related to diameter but not the length of the capillary tube when there is only liquid phase in the tube. However, with two mixed phases in the tube, the breakthrough pressure slightly increases with the number of two-phase interfaces, and the increase becomes more remarkable when the wettability of tube surface changes from hydrophilic to a mix of hydrophilic and hydrophobic. The results indicated that the sealing ability of black shale could be improved by the presence of organic matter and hydrocarbon generation. Additionally, for shales with two phases in pore system, breakthrough pressure has a positive relation with the thickness of cap rock. This interpretation is different from previous ones with respect to breakthrough pressure being related to the thickness of cap rock. Mixed phases of oil/gas and water is also an important explanation why fluids can only be mobilized when the driving pressure gradient reaches a critical value, which is called starting pressure gradient in low permeability pools.