Improved Oil Recovery using Thermally and Chemically Protected Compositions Based on co- and ter-polymers Containing Acrylamide

Abstract

Abstract The change in properties of several anionic polyacrylamides that were used in various EOR projects around the world is discussed. Due to mechanical, radical and/or thermal degradations reported, the molecular weight, anionicity and viscosity of polymer change from the time it is injected until breakthrough when it is produced. Strategies for optimizing the long term stability of EOR polymers have been developed for various polymer types combined with protective additives. Polymers investigated are acrylamide (AMD) based co- and ter-polymers functionalized with monomers such as sodium acrylate (AA), sodium acrylamido-tertiary-butyl sulfonate (ATBS) or N-vinyl pyrrolidone (NVP). The additives belong to five families: free radical absorber, oxygen scavenger, metal precipitant, chelating chemicals, and sacrificial agent. The additives have been selected in addition to their chemical properties for their physical and toxicological characteristics to allow to be formulated as one single formulation under the ready-to-use powder or inverse emulsion form. Chemical and thermal stability for polymer solutions containing 50 to 500 ppb of oxygen have been evaluated using a new test apparatus that allows precise control of the oxygen level. The "as is" stabilized polymer compositions can be used for polymer, surfactant polymer and alkaline surfactant polymer floods. With this technology, polymer viscosity is stable while it propagates through the reservoir and a favorable mobility ratio is maintained from injectors to producers. This enables better sweep efficiency and more efficient oil recovery. Laboratory data shows that new acrylamide based polymer compositions maintain viscosity in the presence of oxygen and iron at high temperature and salinity. ATBS, NVP containing polyacrylamides with protective additives are shown to have the best resistance to radical and thermal degradation. The viscosity of polymer solutions aged up to 120°C can typically reach 60 - 110 % of initial viscosity after a few months compared to 10-40% of previous systems. The polymer and additive combination must be optimized in order to achieve long term thermal and chemical stability at specific reservoir conditions. Results show that polymer flooding can be successful in reservoirs with relatively high temperature and poor water quality. Improved polymer compositions stability leads to increased oil recovery from chemical floods used in EOR.