Twelve Years Field Applications of Offshore Heavy Oil Polymer Flooding from Continuous Injection to Alternate Injection of Polymer-Water

Abstract

Abstract South part of NN reservoir, located at Bohai Bay of China, has reservoir characteristics of complex oil-water contact system, thin single pay zones of 6~8 m and heavy oil viscosity with 413~741mPa·s. At initial natural energy production stage, daily oil of horizontal well reached 21~ 35m3/d. Due to complex oil-water relationship and high formation oil viscosity, oil production rate of whole reservoir decreased from 0.6% to 0.3% after one year. Considering increasing viscosity of injected fluid and reducing the difference of water-oil mobility ratio, polymer flooding with continuous injection mode was carried out since 2008, and its initial good application effect demonstrated polymer flooding was suitable for the thin heavy oil reservoir. However, the problem of gradual decreasing injection ability and oil increment occurred after eight years of continuous injection. A series of physical experiments indicated that profile inversion was inevitable for long PV continuous injection, which results from the difference in the variation of resistance coefficients in high and low permeability zones. The phenomenon of profile inversion was also found in field application by water profile test data over the years. It became critical to inhibit profile inversion for the following injection stage. Two kind of modes including continuous and polymer-water alternate after long PV continuous injection were considered by double tube contrast experiments. Results showed that alternating injection performance was better than continuous injection, low permeability tube had higher oil recovery and larger flow fraction in alternating injection, which demonstrated profile inversion was inhibited in alternating injection mode and more fluid flew in the low permeability tube. After alternate injection was carried out in target reservoir in 2016, the injection rate was increased by 20% because of lower phase viscosity. The average resistance coefficient of alternating mode reached as high as 1.74, which was similar to continuous injection process (2.06). Around the three injection wells, there are nine wells increasing the oil production, and incremental oil production for single well has reached 0.45×104m3~2.12×104m3. The total daily oil production was enhanced by 30%, and the water cut was decreased by 8%. During alternating injection process, polymer agent dosage was reduced by 50%, and operation cost of barrel oil was reduced by 85%, which showed good economic benefit. Polymer flooding has been considered a suitable method for reservoirs with viscosities up to 150 mPa·s. Our practice confirmed again such alternating injection of polymer-water development techniques were also effective and economical for heavy oil fields, even in the offshore oil fields. Polymer flooding pilot of initial continuous and then alternate can provide certain technical reference for similar reservoirs.