Leakage through non-fractured shaly caprock sequences is often envisioned as fluid percolation through water-wet pore networks. Leakage by this process requires that the buoyancy of the hydrocarbons or CO2 column overcomes the capillary entry pressure of the caprock pores. If it does not, then leakage through such caprocks depends on the extremely slow diffusion process. In the latter case, fluid residence time would be almost infinite. Experimental research that have addressed wettability alterations and pore-scale fluid flow in the presence of capillary sealing have triggered fresh thoughts that conclude with modifications to above mentioned leakage model. The main premise of this model is that residual water in hydrocarbon-filled reservoirs can be both continuous and mobile, and can also extend into overlying membrane seals whereas oil is retained in the reservoirs by capillary forces is. We suggest that oil-wet flow paths can be established in membrane seals due to wettability changes as a result of residual water movement that supplies acidic components to the seals. Such wettability alterations allow seals to leak by Darcy flow and small pore throats will promote leakage instead of inhibiting it. Capillary sealing is not so ever-lasting under such conditions.


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