This study reveals the waterflooding pattern of ultra-low permeability reservoir after the analysis of waterflooding recovery characters of plenty ultra-low permeability oil reservoirs

with a view to further bettering the development of ultra-low permeability oil reservoirs and enhancing oil recovery.It can be concluded that fractures induced by water injection will open along the injection-production line in the direction of in-situ maximum horizontal principal stress.In addition

as the angle between the in-situ maximum principle stress and the injection-production line becomes smaller with the increase of water injection pressure

the induced fracture is more likely to open

resulting in an invalid circulation of injected water to injection-production wells and poor waterflooding effect.Moreover

the seepage resistance of linear lateral displacement of the fracture is less than that of the radial displacement calculated by equivalent percolation resistance method.In turn a well pattern conversion mode of "linear injection into dynamic fractures along the direction of in-situ maximum horizontal principal stress with matrix displacement of lateral side" is proposed.The optimized well pattern can improve water sweep efficiency by mitigating the effect of dynamic fractures on reservoir heterogeneity and reducing seepage resistance.In application

the productivity of single wells has doubled

the plane sweep coefficient has increased by 43.8%

and the waterflooding recovery ratio has increased by 19.3%.