Application of Hydrophobically Associating Water-soluble Polymer for Polymer Flooding in China Offshore Heavy Oilfield

Abstract

Abstract Polymer flooding is one of the most important EOR technologies currently producing substantial oil increment in China onshore oilfields. The application of polymer flooding in the offshore environment is more complex and face to many technical challenges due to the absence of the fresh water source and limited space on the platform. Compared to regular anion polyacrylamide mostly used in onshore oilfields, hydrophobically associating water-soluble polymer (HAWP) could keep relatively high viscosity in the high salinity brine. A kind of HAWP was introduced in the first pilot test of polymer flooding conducted in SZ36-1 oilfield of Bohai Bay, China. The single injector pilot test was applied successfully, obtaining 2,5000m3 incremental oil in the corresponding production well and water cut decreased from 95% to 54%. It indicates the technical effectivity of HAWP used as a driving agent in the high salinity heavy oilfield. A scale up test has been conducted in the well group and the same polymer has been employed. The corresponding wells have showed a good response and produced 12,000m3 incremental oil up to the Feb. 2007. Meanwhile, the produced fluid containing trace of HAWP has been collected for evaluation of its influence on the offshore production system. The tests showed the feasibility of HAWP for offshore polymer flooding and provided experience for its further application. Instruction Oil production in China offshore oilfields increased significantly with the more and deeper research on exploration. It is estimated that around 70% of oil reserve in China offshore oilfields are heavy oil, and 64% of them are discovered in Bohai bay. Currently the oil recovery by water flooding is only 18 - 25%, which is far below than that of the onshore the average oil recovery of 32 -40%. Assuming that recovery rate for Bohai heavy oil reservoir with 2.4×109t reserve improves 1% shows that there is approximately an additional hundred million-class oil reservoir to be produced, which will make a significant contribution to the longevity of the Bohai oilfields, and realize the low input and high rewards of development of oilfields. Polymer flooding for enhanced oil recovery has been proven both technical and economic feasibilities in China1. It is used to reduce the local remaining oil saturation by changing the fractional flow curve and to improve sweep in the less permeable zones. As a driving agent, polymer reduces the mobility of aquifer phase to force the remaining heavy oil. Moreover, it has the advantage that it is not require complex and additional surface equipments for polymer flooding in the relatively limited platform area. However, for a successful polymer flooding, it faces five main challenges for its application to offshore heavy oilfields.High salinity and hardness of the make-up water (Ca2+; and Mg2+: 600 - 1,200mg/L), and high viscosity of oil (in average of 70mPa·s), which require the polymer to be salt resistant and good viscosification.Limited spacing of platform does not allow big volume installation for polymer resolving, so development of fast-resolving polymer is necessary.Drilling and completion by wire wrapped screen gravel-packed sand control and large displacement of offshore oilfields require strong anti-mechanical degradation ability of polymer.Long-term stability of polymer in the formation is required because of the large well spacing and thick pay zone.Treatment of produced fluid to meet the environmental requirements should fast and complete in the platform. So the effect of polymer to the produced fluid should be considered in the polymer design.