Twenty Years’ Offshore Polymer Flooding Field Tests and Practices in Shallow Sea

Summary Due to the harsh reservoir conditions and environment as well as limited knowledge about the mechanisms, first polymer flooding field test in offshore reservoir in China was 7 years after the commercial application of polymer flooding in onshore reservoirs. The polymer flooding field test was gradually enlarged and expanded to six oilfields in Bohai. The water depth in Bohai oil fields were low. Among the six offshore polymer flooding projects, three were surfactants based (surfactant/polymer flooding). Different from onshore polymer flooding field practices in China, the associative polymer (AP) was injected in two offshore reservoirs to meet the requirement of high polymer solution viscosity in high salinity sea water. Because of the hydrophobic monomer in molecular structure, the AP has much better salinity-resistance performance and produced higher solution viscosity than conventional polymer partially hydrolyzed polyacrylamide (HPAM). The molecular size of AP was also larger than that of HPAM which may affect its formation transportation. The polymer flooding performances were different in terms of incremental oil recovery factor (IORF), injection pressure and produced fluids treatment. The limited space and processing time in offshore platforms constrained the selection of polymers and the injection schemes. The injection rate (<0.05 PV/a) in large well spacing offshore reservoirs was much lower than that for the onshore reservoirs (0.06–0.22PV/a). Although the performance of polymer flooding may be caused by the polymer type (HPAM vs AP), injection schemes may also account for the performance difference. The IORF in offshore reservoirs (5.4–7.8% OOIP) was much lower than that in onshore reservoirs (8–18% OOIP). The idea of injecting high viscosity polymer solution at very low water cut (around 10%) was proven not as good as expected. Reservoir pressure or energy keeping levels may account for it. It is remarkable that the oil production share from polymer flooding in offshore reservoirs (as high as 5 %) was much lower than onshore ones (as high as 30%). Formation blockage were observed in many wells in 4 of 6 offshore reservoirs while it remains unknown what caused the formation blockage. Practical challenges for offshore reservoirs include produced fluids treatment. The polymer flooding in offshore reservoirs can be as successful as in offshore reservoirs, given the advantages and disadvantages of APs were well studied and the reservoirs were well understood.