Abstract

The flocculation of aqueous-based drilling fluid, particularly at high temperature or in a high-salinity environment, is a common problem. The aggregation and settling of solid particles (e.g. manganese tetraoxide) causes operational problems, such as stuck pipe, well control, false bit gain, and scale problems. The flocculation of manganese tetraoxide particles becomes more severe at high density with the presence of clays such as bentonite and common fluid contaminants, such as cement and divalent salts. This affects the rheological behavior and sagging tendency of the drilling fluid adversely. Dispersants called deflocculants, thinner or dispersing agents, are used in drilling fluids to prevent solid flocculation. To minimize such flocculation problems, a thorough study was undertaken to identify aggregation of Mn3O4 particles and appropriate dispersants for water–based fluids weighted with Mn3O4.

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. A TGA was used to assess the thermal stability. The compatibility of over 50 dispersants with aqueous solutions was examined using a variable speed mixer. Viscosity and sagging tendency were evaluated using a Fan 35A viscometer and density variation measurements. The effect of CaCl2 was considered in the experimental study. The drilling fluids were aged using a hot rolling oven up to 400°F.

Linear and other structures of particle aggregations were identified in Mn3O4-based filter cake. Dispersants examined were categorized into groups A, B, and C. Groups A and B dispersants showed settling times of 24 hours and 1–3 hours, respectively. More than 25 dispersants were assigned to groups A and B. Laser particle analysis showed that aggregation of particles was reduced in the presence of a lignosulfonate-based dispersant. Lignosulfonate and acrylate-based dispersants were compatible with water-based fluids and thermally stable up to 400°F and in the presence of CaCl2. Both dispersants reduced the sag factor and fluid loss for the drilling fluids in the presence of cement contamination. The shear stress/rate relationship and viscosity measurements showed that the rheological behavior of drilling fluids dispersed with either dispersants was improved. The dispersants chosen were non-toxic to aquatic organisms and biodegradable. The examined drilling fluids were high-density fluids up to 20 lbm/gal appropriate for deep drilling.

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