Abstract: To study seal ability of mudstone caprock to gas in waterdissolving phase, based on seal mechanism analysis of the mudstone caprocks to gas in waterdissolving phase, a research method of calculating relative percolation velocity, by Darch's law, of the gas in waterdissolving phase through mudstone caprocks under unit pressure difference was proposed. Based on the relative percolation velocity of gas in waterdissolving phase through mudstone caprocks, a comprehensive evaluation index (CSWI)for seal ability of mudstone caprock to gas in waterdissolving phase was established. CSWI can reflect not only the influence of the characteristics of mudstone caprock itself on its seal ability to gas in waterdissolving phase, but also the influence from the characteristics of the formation water to the seal ability of mudstone caprock to gas in waterdissolving phase. It is a comprehensive evaluation index for the seal ability of mudstone caprock to gas in waterdissolving phase. It was applied in evaluating comprehensively the seal ability of Kq₁₊₂ mudstone caprocks to gas in waterdissolving phase to the east of the Daqing Pacanticline. The result reveals a good seal ability of Kq₁₊₂ mudstone caprocks in waterdissolving phase in the west of research region, with a decreasing trendency in seal ability from west to east. The evaluation results are in accord with the actual geological conditions of gas accumulation in the west of research region, revealing the effectiveness of the evaluation.

Key words: mudstone caprock, waterdissolving phase, seal ability, comprehensive evalution Kq₁₊₂, Daqing