Abstract: The maximum horizontal principal-stress orientation of Paleogene low permeability reservoir of Anpeng oilfield in Biyang sag is in dispute. The available core observation and orientational coring data indicate that there exist crossed two-group natural fractures in this reservoir, one trend ranges from 20°to 40°, the other trend from 80°to100°, with shear or extension-shear mechanical property. It is inferred that the maximum horizontal principal-stress orientation of the fractures when formed was about N60°E. But affected by the idea of being EW trend of the orientation, the present maximum principal-stress orientation has become the direction of injected water breakthrough, causing quickly raising of water cut in production wells. This paper presents the measures for improving the waterflood sweep efficiency as follows: converting production well to injection well or adding new wells allow the original EW array of water injection wells to become NE one.

Key words: Biyang sag, low permeability reservoir, fracture, stress, water flood