ATBS Polymer Injectivity in 22–86 md Carbonate Cores: Impacts of Polymer Filtration, Mechanical Shearing, and Oil Presence

Summary Polymer injectivity into porous media is one of the main issues crucial for the success of a polymer flood project, especially in low-permeability carbonates. Most studies on polymer flooding have focused on high-permeability carbonate cores, with few studies examining permeabilities below 100 md. This paper investigates the impact of filtration, mechanical predegradation, and oil presence on in-situ rheology and injectivity of an acrylamido-tertiary-butyl sulfonate (ATBS)-based polymer in 22–86 md carbonate cores. In this work, an ATBS polymer of 1,000 ppm concentration was used, and various pretreatment approaches were adopted to improve polymer injectivity, including prefiltration, preshearing, and their combination. Polymer injectivity and in-situ polymer rheology evaluations were performed in the absence and presence of oil using carbonate core samples with absolute permeabilities between 22 md and 86 md. For the two-phase studies, the cores were aged at irreducible water saturation and 120°C for 14 days and then flooded with glycerol followed by brine to achieve a representative immobile residual oil saturation (Sor). The corefloods were conducted at 50°C in high salinity water of 243,000 ppm. The resistance factor (RF) was calculated using water permeability at Sor to present the impact of oil presence on polymer rheology. Bulk rheological studies have confirmed that the polymer can withstand high salinity and temperature. However, achieving polymer injectivity in low permeability core samples in the absence of oil has been challenging, with a continuous increase in pressure drop. Various filtration schemes were tested in combination with shear degradation through multiple coreflooding experiments. Preshearing the polymer by 40% and subsequently filtering it through 3-µm, 1.2-µm, 0.8-µm, and 0.45-µm filter membranes improved its injectivity. With this filtration process, the polymer successfully propagated through a core plug of 64 md in the absence of oil. Experiments with oil showed improved injectivity in low-permeability core plugs. The polymer was successfully injected in samples with permeability as low as 26 md, without predegradation, using a 1.2-µm filter. Interpreting the injectivity behavior without an internal pressure tab system was challenging. At representative reservoir flow rates, near-Newtonian behavior was observed. However, there was evidence of shear thickening behavior at higher injection rates. The permeability reduction factor determined from the successful corefloods was between 2 and 4. However, it could not be verified due to the absence of polymer retention data and an internal pressure tab system. Nevertheless, it was found that the residual resistance factor (RRF) was lower when oil was present. Different approaches are reported in the literature to evaluate and improve polymer injectivity; however, there is a lack of research that combines preshear degradation, permeability, and oil presence effects. This study is distinctive in its evaluation of the impact of preshearing and prefiltration on enhancing the injectivity of an ATBS polymer in low-permeability carbonate rock. Furthermore, this study is one of the few to demonstrate the evaluation of promising ATBS-based polymer propagation through 22–86 md carbonate core plugs in the absence and presence of oil.