Application a novel thermo-sensitive copolymer as a potential rheological modifier for deepwater water-based drilling fluids

In deepwater drilling, rheological properties of water-base drilling fluids (WBDF) varied remarkably within the wide temperatures range, causing problems including equivalent circulating densities control, lost circulation, etc. To solve these problems, the key was how to reduce the temperature dependence of rheological properties. In this study, the combination of thermo-sensitive polymer and clay particles was used to solve this problem due to the distinct thermo-rheological properties of these two components. In detail, a novel thermo-sensitive copolymer (PANA) of acrylamide (AM), sodium 2-acrylamido-2-methylpropane sulfonate (NaAMPS) and N-vinylcaprolactam (NVCL) was synthesized by free radical polymerization under optimal conditions. And the structure of PANA was characterized by FTIR, elemental analysis, and gel permeation chromatographic measurements. The thermo-rheological properties of PANA in solution and slurry were investigated through viscometer. Results from rheological tests illustrated that PANA showed certain thermo-thickening phenomenon in the temperature range of 4–95 °C due to the thermo-associative behavior of thermo-responsive caprolactam (CL) moieties. And the temperature dependence of rheology of PANA based drilling fluids was further studied over the temperature range of 4–75 °C according to the American Petroleum Institute (API) standard. Results illustrated that the rheology of PANA based fluids were found to be more stable than those of typical WBDF. The change rate of main rheological parameters of PANA fluids, such as, apparent viscosity (AV), yield value (YP), and low shear rate viscosity (LSRV), was less than 15%, while parameters of typical WBDFs varied dramatically (near or over 30%). Therefore, the relatively stable rheological properties of fluids with temperature changes could be achieved through the synergistic effect between the novel thermo-sensitive copolymer and bentonite particles. In addition, the rheological control mechanism of PANA was further investigated through transmittance tests, Environmental Scanning Electron Microscope (ESEM), zeta potential experiments, and particle size distribution measurements.