Effects of Concentration, Salinity, and Temperature on the Rheological Behavior of Diutan Gum in Aqueous Solution:...

Diutan gum (DG) is a microbial polysaccharide with great potential for application in activities associated with the deep drilling industry. This study aims to investigate the rheological behavior of diutan gum in aqueous solutions, focusing on its resistance to thermal and saline stresses for potential application in water-based drilling fluids. Thermogravimetric analysis (TGA) was performed to evaluate the thermal stability of the polymer. Two thermal events were observed from TGA: water desorption (approximately 100 °C) and polymer decomposition (230-274 °C, 40% mass loss). The high decomposition onset temperature (230 °C) exceeds typical drilling fluid requirements (<200 °C), confirming excellent thermal stability for high-temperature, high-pressure (HTHP) applications. The rheological behavior in aqueous solutions at different concentrations (0.75-1.75 g/L), salinities (3-90 g/L NaCl), and temperatures (25-80 °C) was studied and analyzed. Accelerated aging, static aging (SA), and dynamic aging (DA) tests (API RP 13B-1) on diutan gum solutions (1.75 g/L) were performed to evaluate salt tolerance (≤180,000 mg/L NaCl), thermal resilience (≤140 °C), and shear resistance (600 rpm) over 16 h, simulating field conditions. Pseudoplastic rheological behavior was observed for most samples, except for the sample subjected to DA at 140 °C and 600 rpm, which exhibited Newtonian behavior. Additionally, the elastic modulus (G' > G″) was more significant at low frequencies (0.1-10 Hz), indicating the potential ability of diutan to form more organized lattice structures, as observed under nearly all studied conditions. Rheological results from SA/DA tests confirmed Pseudoplastic behavior under all conditions, demonstrating the stability of diutan gum despite environmental variations. However, its thermomechanical resistance is limited under extreme conditions (140 °C, 600 rpm). The FTIR spectra confirmed the maintenance of the chemical structure of the diutan, which did not suffer from the loss of functional groups. This stability at high salinity and temperature can be attributed to the ability of diutan to form a double helix structure in aqueous media. This conformation drapes water in its interior and protects the main chain and its functional groups from the degradation imposed on the system. Based on its remarkable thermal and saline stability, diutan gum has emerged as a highly effective additive for water-based drilling fluids, improving wellbore stability, cutting suspension, and overall rheological performance under demanding conditions, including deepwater and presalt oil reservoirs.