While polymer flooding has proven to be an effective technique to improve oil recovery from mature reservoirs, the new objective pursued by the main stakeholders consists now in optimizing the overall costs and decrease the risks associated to the implementation of chemical enhanced oil recovery methods. This is particularly true for reservoir conditions with high temperatures (above 85°C) using injection brine with high hardness.
In such conditions, using specific robust polymers is necessary to ensure stability over months during propagation in reservoirs. These polymers containing Acrylamido-Tert-Butyl-Sulfonate (ATBS) and N- Vinyl Pyrrolidone (NVP) are more expensive than regular HPAM and have to be overdosed to reach target viscosities since their molecular weights are low. In extreme cases, the project can be aborted for economic reasons. However, many fields, especially in South America, have access to very soft injection brines with a total dissolved salt lower than 3000 ppm in many cases. Using such brine is a tremendous advantage for polymer flooding since less robust polymers can be selected and target viscosity can be achieved with low dosages.
This paper describes the selection and evaluation of polymers dedicated to high temperature reservoir conditions (from 85°C to 140°C) using very low salinity brine in comparison with harder brine. A series of rheological, shear and thermal stability tests have been performed to select the most appropriate polymer for each case. The impact of the chemical composition and the microstructure shows that the incorporation of NVP is not necessarily required to ensure stability over 6 months above 100°C in soft brines. Results show as well that ATBS improves shear and thermal stability in both soft and hard brine conditions. Besides, the incorporation of thermo-responsive moieties in the polymer improves viscosity properties resulting in a lower dosage even at high temperature to reach target viscosity.
The objective of the study is also to demonstrate the possibility to develop innovative and cost-effective polymers for each reservoir condition. It encompasses an early and particularly close cooperation between the polymer manufacturer and the company willing to improve oil recovery from its reservoirs.