Polymer Injectivity in Low Permeability Carbonate Cores: Impacts of Polymer Filtration, Mechanical Shearing, and Oil Presence

Abstract

Abstract Polymer injectivity into porous media is one of the main issues that is 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 pre-degradation, and oil presence on in-situ rheology and injectivity of an ATBS-based polymer in low-permeability carbonate cores. In this work, an ATBS polymer of 1000 ppm concentration was used, and various pre-treatment approaches were adopted to improve polymer injectivity, including pre-filtration, pre-shearing, 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 21 and 85 mD. For the two-phase studies, the cores were aged at irreducible water saturation and 120 °C for 14 days, then flooded with glycerol followed by brine to achieve a representative immobile Sor. The corefloods were conducted at 50°C in high salinity water of 243,000 ppm. Bulk rheological studies have confirmed that the polymer can withstand high salinity and temperature. However, achieving polymer injectivity in low permeability core samples without 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. Pre-shearing the polymer by 40% and subsequently filtering it through 3, 1.2, 0.8, and 0.45 µm filter membranes improved its injectivity. With this filtration process, the polymer successfully propagated through a core plug of 63 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 36 mD, without pre-degradation, using a 1.2 µm filter. In-situ rheology tests demonstrated a pronounced impact of oil presence. A near-Newtonian behavior at representative reservoir flow rates was noted. However, at higher injection rates, a shear thickening behavior was evident. The permeability reduction factor determined from the successful corefloods was between 2 and 4. Additionally, it was found that the residual resistance factor was lower when oil was present, suggesting less polymer retention and less damage to the formation. Different approaches are reported in the literature to evaluate and improve polymer injectivity; however, there is a lack of research that combines pre-shear degradation, permeability, and oil presence effects. This study is distinctive in its evaluation of the impact of pre-shearing and pre-filtration on enhancing the injectivity of an ATBS polymer in low-permeability carbonate rock. The results emphasize the importance of conducting polymer injectivity tests in the presence of oil to achieve more accurate outcomes.