Xing 4-6 is a multilayer sandstone reservoir which has already experienced a secondary water flood in the entire reservoir and a tertiary polymer injection in certain formations. The polymer flooding began in 2001 as a typical polymer flooding field in Daqing. Currently,the Xing 4-6 is reaching the end of the designed polymer injection. In this paper, a cyclic water flood after the polymer injection is investigated in this field. The research is principally on the basis of reservoir simulations. After the history matching of the water flooding, a polymer flooding model describing target polymer flooding layer was extracted from the water flooding model to simulate the polymer injection process. By combining the development information and simulation results, the efficiency of the polymer flood was analyzed as the fundamental of cyclic injection investigation. Based on the plenty of geological study and multidisciplinary investigation, a cyclic flooding model describing Putaohua I formation was established.

Two cyclic injection cases were designed. Two cyclic units were defined in the model: the priority payzone P I33was considered as an independent cyclic unit due to its considerable proportion of the original oil in place (OOIP); other six layers were set as the second unit.In one case, the cyclic water was only conducted in the first unit; the other unit still employed continuous water injection. In the other case, two units were alternatingly involved in the cyclic injection. The predictions showed that the vertical sweep efficiency had been improved in upper zones which have vertical connectivity with lower layers due to the pressure pulsation and capillary pumping of the wetting phase in the low-permeability zones, especially the zones with high heterogeneities. Areally, the dead oils which were formed by the un-improvement of injection and production system were also mobilized. The cyclic water injection after polymer flooding improved the ultimate recovery. One more significant advantage was the extended use of the current well pattern. Comparing two cases, the case with only one cyclic unit involved had a better development efficiency in the priority payzone P I33 than that of the case with alternating cyclic injection, the incremental recovery in P I33 was 1.21%, which was 0.37% higher than that of the alternating case. Because the case with alternating cyclic injection took all zones into the consideration, the entire flooding efficiency was better than the other. The incremental recovery of alternating cyclic injection was 0.84% which was 0.32% higher than that of one cyclic unit injection on the reservoir scale. The detailed analysis in each case was also presented.

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