Waterflooding in fractured reservoirs

an important way to develop low permeability reservoirs

can effectively improve the flow field effectiveness and increase oil discharge area. Rapid and accurate prediction of production of low permeability reservoirs with water injection can establish a foundation for optimal design of development. However

the flow field of water injecting into low permeability reservoirs presents unsteady and nonlinear flow characteristics

so the calculation methods based on Darcy's law are not applicable. After the analysis of flow fields

the equivalent flow units are divided. Taking the threshold pressure gradient of oil-water two-phase flow into consideration

we established the analytical calculation method for unsteady oil production with injection in different fracturing modes

based on the flow line integral method. And the method is simpler and faster compared with physical and numerical simulations

so it is applicable for optimizing fracturing injection and production well patterns and water injection development strategies in low permeability reservoirs. Besides

this method was used to calculate and analyze the effects of threshold pressure gradient

fracturing injection and production methods and fracture lengths on the productivity of oil wells. The results show that (1) the greater threshold pressure gradient leads to larger seepage resistance

so the well productivity is lower than that without considering the threshold pressure gradient; (2) the well productivity is controlled by flow units

so with different fracturing injection and production modes

the effects of production increase and the time of water breakthrough are completely different. Although the increase of production with simultaneous fracturing injection and production is the best

being 3.1 times that of other modes

the water breakthrough time is just 24 months; (3) with the increase of fracture length

the oil production increases

whereas the effect of increased oil production is not obvious any more when the fracture length exceeds the optimum length.