Managing the solid flocculation in high-density drilling fluids is a challenge in drilling operations. A weighting material such as manganese tetroxide is used to formulate drilling fluids at density requirements for deep drilling because of its high specific gravity (i.e. 4.8). However, the microsized particles, surface charge, and presence of biopolymers increase the tendency to agglomerate and, thus, promote sagging. Developing a good dispersant is critical for a successful drilling operation, especially in HP/HT conditions.
The dispersion of Mn3O4 particles in aqueous solutions was evaluated through visual transparency testing and a PC-2200 Spectrek laser particle analyzer. A Mastersizer 2000 was used to obtain the particle size distribution and specific surface area of Mn3O4 particles. TGA and variable speed mixers were used to assess the thermal stability and compatibility. Rheology and sagging tendency were evaluated using a Fan 35A viscometer and density variation measurements. The microstructure of Mn3O4-based filter cake was interpreted through SEM to identify the aggregation of solid particles. The effect of CaCl2 on the solubility and dispersion of Mn3O4 particles in aqueous dispersant solutions was considered in the experimental study. The drilling fluids were aged at 400°F.
Several dispersants showed high potential to disperse manganese tetroxide particles in water-based drilling fluids. The dispersants examined showed particle settling times from 0.03 to 24 hours. Zeta potential measurements showed a region of dispersion stability at a pH from 6 to 11 for lignosulfonate and sulfonic acid-based dispersants. Zeta potential and particle settling tests indicated that dispersion with sulfonic based copolymer is more effective than that of benzene sulfonic acid. Zeta potential values increased with temperature in sulfonic acid-based dispersant (i.e., 77 and 120°F). Zeta potential and transparency experiments showed that the optimum dispersant concentration was below 1 wt% and nearly 0.5 wt%. TGA analysis showed that the acrylates- and lignosulfonate-based dispersants were thermally stable between 300 and 500°F. SEM images of filter cake showed that aggregation of particles was reduced when acrylic based dispersant was used in clay contaminated drilling fluid. In most cases, the rheological behavior (i.e, shear stresses and viscosities) of clay contaminated and non-contaminated drilling fluids (17.5 and 20 lbm/gal) before/after heat aging at 400°F was lower in the presence of dispersants. Dispersant solutions with NaCl had varying compatibilities and particle settling interactions. The settling times of manganese tetroxide particles were lower, higher, or did not change for several dispersant solutions at 5 and 10 wt% NaCl. The sagging tendency of clay contaminated and non-contaminated drilling fluids was improved in the presence of dispersants. For instance, the sag factor was reduced from 0.55 to 0.51 in clay and rock salt contaminated drilling fluids when acrylic based copolymer was used.