In-Situ Rheology Investigation of a Potential ATBS-Based Polymer in Carbonates Under Harsh Conditions

ATBS-based polymers are now a practical choice for polymer flooding projects in the carbonate reservoirs of the Middle East, thanks to their ability to withstand extreme conditions of high temperature and salinity. As a result, it is essential to identify and evaluate other polymers with similar capabilities to perform well in these challenging environments. This paper investigates polymer in-situ rheology under harsh carbonate conditions using a novel ATBS-based polymer comprising two different molecular weights. Polymer solutions with concentrations of 1000 and 1200 ppm were used for three single-phase and one two-phase injectivity studies. Two 3-inch and one 12-inch Indiana limestone outcrops (single-phase) and a 3-inch reservoir core (two-phase) with an absolute permeability ranging from 47 to 726 mD were utilized. Experiments were carried out with synthetic formation water (243,000 ppm) at a moderate temperature of 50 °C. A multi-tap pressure configuration was used to measure the pressure along the 12-inch core. The injectivity studies consisted of four stages: brine pre-flush, polymer injection, polymer tapering, and brine post-flush. Based on the findings, polymer solutions exhibited shear-thickening behavior in porous media, with onsets occurring below 2.5 ft/day. In lower-permeability single-phase experiments (<171 mD) using low molecular weight polymers, the shear thickening onset occurred at lower velocities compared to a high-permeability (726 mD) single-phase experiment with a high molecular weight polymer. Therefore, in this study, the permeability effect dominates that of molecular weight on polymer viscoelastic behavior. Compared to single-phase studies, a clear enhancement in injectivity was observed in the presence of oil. Based on pressure stabilization data, high and low molecular weight polymers demonstrated excellent stability across all tested velocities. These findings were further supported through RRF calculations. The measured residual resistance factor (RRF) was found to be below 3 for all experiments except for the one conducted in the presence of oil. In-situ rheology tests showed different behavior across the experiments due to variations in polymer concentrations, MW, filtration, oil presence, and the permeability of the cores. Findings in the 12-inch core showed that the first two sections of the core experienced the most significant permeability reduction despite their high permeability (>1000 mD). This research investigates the in-situ rheology of a novel ATBS-based polymer encompassing two molecular weights in carbonate reservoirs under harsh conditions. The results highlight the added advantage of using longer cores and multi-tap pressure coreflooding systems for better monitoring of polymer flooding studies. This study is one of the few that explores the in-situ rheological properties of novel ATBS-based polymers with different molecular weights.