Effects of Polymers with Different Anionic Groups in Drilling Fluids and Their Properties of Temperature and Salt Resistance

To investigate the performance characteristics of polymers with different functional groups in drilling fluid matrix (clay suspension), two anionic acrylamide-based polymers with molecular weights of approximately 1 million and the same monomer ratio, namelypoly(acrylamidoacrylate) (P(AM-AA)) and poly(acrylamidoallyl sulfonate) (P(AM-SAS)), were synthesized using a water-dispersion polymerization method. Using polyacrylamide (PAM) with the same molecular weight as a reference, thermogravimetric analysis, scanning electron microscopy, zeta potential analysis, particle size distribution test, and drilling fluid performance test were conducted to assess the thermal stability of the polymers as well as the microstructures and properties of the corresponding drilling fluids. The results demonstrated that both P(AM-AA) and P(AM-SAS) exhibited significant intercalation effects on clay in drilling fluids, leading to an increase in the zeta potential of clay particles and the formation of a bimodal particle size distribution. These polymers also effectively reduced filtration loss and altered the rheological properties of the drilling fluids. However, the influence of polymers on temperature and salt resistance did not conform to the traditional understanding that sulfonic acid groups offer better hydration and salt resistance compared to carboxyl groups. The performance characteristics of the polymers were influenced not only by their own structure, but also by the clay content, salt content, temperature in the drilling fluids. Specifically, in freshwater drilling fluids, P(AM-AA) exhibited a slightly lower but comparable reduction in filter loss compared to P(AM-SAS). Furthermore, P(AM-AA) was less affected by clay content than P(AM-SAS) and showed stronger filter loss reduction in brine drilling fluids with NaCl content <= 3 wt%, while exhibiting less sensitivity to variations in NaCl content. Conversely, P(AM-SAS) demonstrated stronger filter loss reduction and lower sensitivity to NaCl content in brine drilling fluids with NaCl content >3 wt%. The influence of high-temperature aging (120-180 degrees C) on filter loss reduction function was similar for both P(AM-AA) and P(AM-SAS). The rheology of the formulated drilling fluid was less sensitive to clay content when utilizing P(AM-AA) compared to P(AM-SAS). Moreover, the apparent viscosity of the drilling fluid with P(AM-AA) was more impacted by NaCl when the NaCl content was below 5 wt%, showing greater sensitivity than P(AM-SAS). However, there was no significant difference in rheological stability between P(AM-AA) and P(AM-SAS) under high-temperature aging conditions.