High molecular weight copolymers as rheology modifier and fluid loss additive for water-based drilling fluids

In this work, the impact of molecular structure such as the degree of hydrolysis (DOH) and nature of comonomers in acrylamide-based copolymers on the rheological and filtration properties of bentonite dispersions was assessed using three different water-soluble polymers for water-based drilling fluids. These polymers are (1) copolymer of acrylamide and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) with 10% DOH (P1), (2) copolymer of acrylamide and 2-acrylamido-2-methylpropane sulfonic acid (P2) with 25% DOH, and (3) copolymer of acrylamide and acrylic acid (P3) with 15% DOH. Steady shear rheology data was fitted to the Herschel-Bulkley model and flowability test was performed at 25 °C and 85 °C. Aging of bentonite and bentonite/polymer dispersions was performed at 90 °C for 16 h. Results showed that the incorporation of polymers in bentonite enhanced the filtration and rheological properties. The bentonite/P3 dispersion showed better rheological properties compared to other bentonite/polymer dispersions. The better rheological properties of bentonite/P3 dispersion suggest that acrylamide-based copolymers containing smaller comonomers such as acrylic acid are better compared to the bulky comonomers such as AMPS for drilling fluids applications. The filtration properties are less affected by the DOH and nature of comonomers, nevertheless, all polymer/bentonite dispersions showed better filtration properties compared to bentonite dispersion. The use of this high molecular weight polymer provides high thermal stability, superior rheological properties and better filtration characteristics for water-based drilling fluids.