LAUSR

Abstract Oil production by spontaneous imbibition from matrix block can occur in both co- and counter-current modes when matrix block was partially covered by water. In this paper, oil recovery from partially water-covered matrix blocks with different boundary conditions was studied by numerical calculation. The boundary conditions were categorized and named according to the number of water-cover- and oil-covered-faces and relative position between water-covered- and oil-covered-faces. The numerical models with different boundary conditions were established. The water saturation distribution and direction of fluid flow in the matrix blocks were presented. In addition, the imbibition recovery curves of oil recovery versus imbibition time for different boundary conditions were presented and the effect of different factors on the rate of oil production was discussed. The calculated results showed that the area of water-covered-face has more significant effect on the rate of oil production than that of oil-covered-face. The distance travelled by fluids from water-covered-face to oil-covered-face is another important factor that affects the rate of oil production. The water imbibed from a certain water-covered-face preferentially displaces oil towards the closest oil-covered-face. Accordingly, three criteria for calculation of characteristic length for co-current imbibition is proposed and the close correlation of imbibition curves was obtained in the early stage of imbibition process by use of the proposed characteristic length to calculate the dimensionless time.