Curing minor leaks and restoring the integrity of a wellbore in a safe and economical way is always challenging in oil and gas production. In this work, a composite pressure-activated sealant, combined with liquid and solid sealing materials, was prepared via the demulsification approach. The structure, morphology, and size distribution of key particulates in the sealant were examined, and the in-situ self-adaptive sealing property was examined with a specially design dynamic sealing detector. The results indicated that the pressure-activated sealant was a multi-dispersed phase system, and the dispersed colloid particles were regular in shape and had a narrow size distribution of 300–400 μm. The solid sealing materials were introduced to construct a composite pressure-activated sealant, and the sealing capability can be markedly reinforced by cooperativity of liquid and solid sealing materials. A mechanochemical coupling model was put forward to rationalize the dynamic sealing process. Finally, such sealant system was employed in a certain offshore gas well with sustained casing pressure to verify its applicability in minor defect repairs.