As waterflooding further advances,a new situation including the rise of water-bearing layer,the declining of oil production and the unbalance between injection and production has appeared in the production of the reservoirs in the Sanjianfang Formation.The oil wells located at the main river channel sandbodies are characterized not only by thick sandstone and better physical properties,but also by high initial production of single well,high recovery percentage of reserves and big decrease of reservoir pressure.The injected water flows towards the wells which have good physical properties,and the reservoir pressure decreases mainly along the extensional direction of the sandbodies.Most of the first and second order reservoirs,controlled by sedimentary cycle,developed in the middle and lower parts of reservoir group,that is to say,in the third or fourth subbeds.These subbeds have high fluid production and injectivity,which results in the water rising mostly in the single layer or direction.The variation of reservoir fluid production is closely related to water flood response,and the rise of ground water is the fundamental cause leading to the decrease of oil production in the main oil-producing areas.
