Impact of Acrylate and 2-Acrylamido-Tertiary-Butyl Sulfonic Acid Content on the Enhanced Oil Recovery Performance of Synthetic Polymers

Polymer flooding is a mature enhanced oil recovery (EOR) technology that has seen increasing interest over the past decade. Copolymers of acrylamide (AMD) and acrylic acid (AA) have been the most prominent chemicals to be applied, whereas sulfonated polymers containing 2-acrylamido-tertiary-butyl sulfonic acid (ATBS) have been used for higher temperature and/or salinity conditions. The objective of this study was to generate guidelines to aid in the selection of appropriate polyacrylamide chemistry for each field case. Our focus was in sandstone fields operating at the upper end of AA-AMD temperature tolerance, where there is a decision as to whether sulfonation is required. The performance of the polymer throughout the whole residence time in the reservoir was considered because the macromolecule can undergo some changes over this period. Several key properties of nine distinct polymer species were investigated. The polymers consisted of AA-AMD copolymers, AMD-ATBS copolymers, and AMD-AA-ATBS terpolymers (up to 15 mol% ATBS). The polymer solutions were studied both in their original state as they would be during the injection (initial viscosity, initial adsorption, and in-situ rheology), as well as in the state in which they are expected to be after the polymer has aged in the reservoir (i.e., in a different state of hydrolysis with corresponding changes in viscosity retention and adsorption after aging for various time periods). We note that the combination of viscosity retention and adsorption during the in-situ aging process has not been typically investigated in previous literature, and this is a key novel feature of this work. Each of the above parameters has an impact on the effectiveness and the economic efficiency of a polymer flooding project. The majority of the work was carried out in seawater (SW) at a temperature of 58ºC. Under these conditions, AMD-AA samples showed similar solution viscosity at 5 to 30% AA. When the AA-AMD polymer solutions were aged at elevated temperature, the AA content steadily increased because of hydrolysis reactions. When the AA content was 30 mol% or higher, the viscosity started to decrease, and the adsorption started to increase as the polymer solution was aged further. Thermal stability improved when ATBS was included in the polymer structure. In addition, sulfonated polyacrylamide samples showed constant initial viscosity yields and decreasing initial adsorption with increasing ATBS content. The samples showed that the maximum observed apparent in-situ viscosity increased when the bulk viscosity and relaxation time of the solution increased. The information generated in this study can be used to aid in the selection of the most optimal polyacrylamide chemistry, which may not necessarily be the standard 30% AA and 70% AMD copolymer, for sandstone fields operating with moderate/high salinity brines at the upper end of AA-AMD temperature tolerance.