Mechanism of enhanced oil recovery by discontinuous chemical flooding in Bohai oilfield
The water-flooding oilfield in Bohai Bay is dominated by the unconsolidated sandstone with high porosity and high permea-bility.As a result,the heterogeneity is further intensified from the stage of strong injection and production in long subsection to the high water cut stage.This leads to a series of problems,such as the difficulty in exploiting more sub-layers by water flooding and conventional polymer flooding and continuously and effectively expanding the swept volume.According to the differences in flooding control strength in chemical system,polymer gel,elastic dispersion fluid and polymer were defined as being of strong,medium and weak flooding control system.Based on the dynamic change rules of flow resistance during continuous chemical flooding(continuous injection in a single slug system),in combination with a test on the effective injection pressure of heterogeneous cores,a study was performed on the dynamic characteristics of expanding the swept volume by discontinuous chemical flooding(hereinafter referred to as DCF),composed of strong,medium,and weak flooding control systems.Then,the microscopic mechanism of the expanded swept volume was revealed by microfluidic experiment.The discontinuous chemical flooding slug combined injection mode was opti-mized by the parallel-cores flooding experiment.Finally,the feasibility of enhanced oil recovery by discontinuous chemical flooding was verified by field tests in offshore oilfields.The results show that the total mobility of oil and water phases during the continuous chemical flooding process is increased,the proportion of the total mobility of the high-permeability area in heterogeneous core is also increased,and the ability of expanding swept volume is restricted.Therefore,DCF substitution model for the slug combinations of different pharmaceutical systems can be used to effectively solve the problem of insufficient,excessive or uneven resistance increasing capacity from single slug injection.In addition,microfluidic experiments show that the"strong-weak"and"weak-strong"combina-tions of DCF can expand the swept volume by 29.2％and 14.0％as compare with the single continuous injection mode,respectively.Core flooding experiments show that the"medium-strong-weak-strong-weak"combination of DCF has a better EOR effect and can further improve oil recovery by 4.41％as compared with continuous polymer flooding.Thus,it can be seen that effective control of injection and production pressure difference and continuous expansion of microscopic heterogeneous remaining oil are the main mecha-nisms of EOR by non-continuous chemical flooding.The pilot test of DCF in 2 wells of BZ oilfield in Bohai Bay shows that the water cut of the flooding response well is reduced by up to 14％,and the oil production is increased to 6.97 × 104t,indicating a good effect of controlling water control and increasing oil production.
flow resistancediscontinuous chemical floodingswept volumeinjection and production pressure differenceslug combi-nation
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