Abstract: There is start pressure gradient when oil/water flow in low permeability reservoirs. When the pressure gradient is greater than the start pressure gradient, fluid can flow. Every layer has limit injection-production well spacing in heterogeneous multi-layered low permeability reservoirs. When the limit well spacing is larger than a real injection-production well spacing, the reserves in the layer can be produced, or can not. So with the full-consideration of the vertical heterogeneity in such reservoirs, the relationship between the producing degree of reserves and the injection-production well spacing in the reservoirs is established. The factors affecting the producing degree are analyzed. A new method for determination of reasonable injection-production well spacing according to the water-drive reserves producing degree in low permeability reservoirs is proposed.

Key words: low permeability reservoir, start pressure gradient, injection-production well spacing, water flood, reserves producing degree