Polymer flooding is the most widely applied chemical flooding and plays an important role in EOR in Daqing oilfield since 1996. For polymer-flooded reservoirs, significant amount of polymer is still left behind in the reservoir(e.g. polymer-flooded NW block of Daqing, the polymer concentration in produced liquid is 100-900mg/L), the retained polymer is continuously produced from production wells, which, on the one hand, results in difficulties in treatment of produced liquid, on the other hand, affects the effect of successive waterflooding. Aimed at resolving this problem, laboratory and numerical simulation studies were conducted and a pilot test was carried out.

Laboratory study was used to investigate the mechanism of polymer adsorption-retention in various permeability cores, injection timing and amount of flocculating agent, residual resistance factor and plugging percentage. Reservoir simulation was applied to optimize injection parameters and to provide theoretical basis for pilot test. A pilot test, which includes 6 injectors and 13 producers with a well spacing of 237m, was performed in Daqing field. 6 injectors was in stage of succeeding waterflooding in May 2007, the injection of flocculating agent began on September 12, 2007, and was completed on December 1, 2007. On average, 5161.8m3 of flocculating agent solution per well was injected.

Response of oil increase is observed from all 13 producers, average oil increase per well is 3.1t/d, decrease of watercut is 3.11%, reduction of concentration of produced polymer reaches 105mg/L. The cumulative incremental oil is 4230.5t, the ratio of investment to revenue is 1:3.62.

This technology utilizes residual polymer in reservoir to carry out in-depth profile modification, so the cost of conformance control operation can be reduced significantly. Pilot test shows this novel technology is feasible and is a new approach to increasing oil recovery of waterflooding.

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