Capillary resistance and trapping of hydrocarbons: a laboratory experiment

Abstract

Low permeability cap rocks retain oil by capillary forces when the pore throats of the seals are sufficiently small to prevent a flux of oil into the cap rock. In order to investigate the influence of aquifer overpressures on oil retention, water pressure was applied to a water-wet, highly permeable (1988 mD) core sample, which was oil-saturated to irreducible water saturation S wi and mounted with a low-permeability and water-wet membrane at the outlet. A water pressure difference of 0.5 MPa was applied across the core. This pressure was high enough to ensure fluid flow through the sample. The experiment was designed to see whether the water pressure would force oil through the membrane or if capillary forces at the sandstone–membrane interface would retain the oil, in which case water flow might take place in the (residual) water in the core and through the membrane. The experiment showed that oil was kept in place by capillary forces while water flowed through the core and the membrane. Accordingly, residual water can move through sandstones that are saturated to S wi . The experiments also demonstrated that the permeability associated with this residual water is high enough to prevent overpressures in the aquifer below the oil–water contact from pushing oil through a membrane seal. Thus, even for this highly permeable sandstone, the overpressure in the aquifer will not cause capillary seal failure.

Relative permeability to wetting‐phase water in oil reservoirs

“…The second test was performed at a different laboratory (Statoil Research Centre in Trondheim, Norway). The results of the first test gave a calculated permeability to the wetting-phase water of 0.74 Â 10 À3 mD (almost identical to the value reported by Teige et al [2005]). The results of the second test gave a calculated wetting-phase water permeability of 0.29 Â 10 À3 mD.…”

“…[12] However, the experiments reported by Teige et al [2005] were only conducted on one highly permeable ($1,990 mD) sandstone plug. The relevance of their results to less permeable rocks thus remained uncertain.…”

“…Also, these authors did not address the influence of varying capillary pressures, and so the hydrocarbon column heights, on the permeability to the wetting-phase water. Rodgers' [1999] general concern about the permeability to the wetting-phase water, the magnitude of the DP term, and accordingly the significance of reservoir overpressures to hydrocarbon exploration, was therefore not fully addressed by the Teige et al [2005] investigation.…”

Petroleum Migration

“…As an oil or gas lag starts to accumulate near the top of the structure, the permeability with respect to water flow is reduced further. If permeability to water is nevertheless not zero, while the migration of oil may be prevented by capillary forces water may in some cases slowly seep through the cap rock even if the permeability is very low (Teige et al 2005).…”

“…In the laboratory, water has been shown to flow though a cap rock while oil has been retained by the capillary pressures (Teige et al 2005). …”