G. M. G. Teige1*, C. Hermanrud1,2 and H. G. Rueslåtten3

1 Statoil Research Centre, Rotvoll, N7005, Trondheim, Norway.

2 Also at:  University of Bergen, N-5007, Bergen, Norway.

3 Numerical Rocks, Trondheim, Norway.

*author for correspondence:

Leakage through non-fractured shaly caprock sequences is often envisioned as hydrocarbon percolation through water-wet pore networks. Leakage by this process requires that the buoyancy of the hydrocarbon column overcomes the capillary entry pressure of the caprock pores. If it does not, then leakage through caprocks that are not hydrofractured depends on diffusion of hydrocarbons, which is an extremely slow process. As a result, the hydrocarbon residence time would be almost endless. 

We suggest a novel model for leakage through non-fractured caprock shales. Two lines of laboratory-based research motivate our suggestion. One of these addresses wettability alterations, while the other addresses pore-scale fluid flow in the presence of capillary sealing. The latter approach has demonstrated that water can flow vertically through water-wet oil reservoirs and further through caprocks, whereas the oil remains in the reservoir. This recognition is a consequence of the observation that residual water in the reservoir can be both continuous and mobile.

We suggest that such flow of residual water through reservoirs and into caprocks can lead to the establishment of oil-wet flow paths in membrane seals. This process takes place in a sequence of events that leads to wettability alterations in the seals. Such wettability alterations allow seals to leak by multiphase Darcy flow, which would hardly happen if the seals were water wet and with sufficiently small pore throats.

One implication of this model is that capillary sealing will be effective only for a limited period of time. The model also predicts that residual oil saturation in the shales as a whole remains low, as the suggested flow paths only develop locally and develop narrow leakage pathways.

JPG Home (opens in this window)