Successful Polymer Selection for CEOR: Brine Hardness and Mechanical Degradation Considerations

Abstract The selection of the right polymer chemistry in chemical enhanced oil recovery processes is key to successfully increase oil recovery. To start the screening process, it is necessary to look at minimum 3 parameters including brine composition, reservoir temperature and permeability. In addition to simple rheological tests, it is mandatory to evaluate the long-term and shear stabilities of the polymer candidates to ensure that viscosity is maintained over time at an economical concentration. In few cases, the polymer that yields the best results (especially viscosity yield) at an instant t is not always the best compromise when considering long-term stability or shear sensitivity. This paper aims at providing some guidelines to select the optimum chemistry for a wide range of field conditions. The resistance of different acrylamide and ATBS-based polymers to salinity and shear is evaluated through viscosity measurements over a wide range of brine compositions. A parameter called R+, that reflects brine hardness, is introduced in this study. Brines considered are either with a constant Total Dissolved Salt (TDS) and varying R+ or different total salinities with a constant R+. This parameter is also used to compare shear resistance of the different polymers. For all the polymers, there is a threshold value of R+ beyond which viscosity remains constant. Interestingly, this threshold is reached for lower value of R+ for polymers containing ATBS, a monomer also well-known to provide calcium tolerance. Increasing the amount of ATBS yields better tolerance to divalent cations and provides shear resistance. A minimum amount of sulfonated monomer is required to improve the overall stability in complex brines. Some important guidelines are summarized in this paper that can help screen products among a wide range of reservoir conditions in order to find the most suitable candidate for a chemical enhanced oil recovery project.