Environmentally Friendly Dispersants for High Temperature Invert-Emulsion Drilling Fluids Weighted by Manganese Tetraoxide

This paper evaluates different environmentally friendly dispersants in invert-emulsion drilling fluids that can be used to drill HP/HT wells. The drilling fluid is weighted by manganese tetraoxide (Mn₃O₄) particles, which have a specific gravity of 4.8 and a mean particle diameter of 1 µm. Manganese tetraoxide has different wetting properties and surface chemistry than other weighting agents. Hence, there is a need to find dispersants for manganese tetraoxide that give reduced sag, reduced rheological properties, and low fluid-loss at HP/HT conditions. This is particularly important for deep wells with narrow operating windows between pore-pressure and fracture pressure gradients.

The stricter global environmental regulations mandate the dispersants to be environmentally friendly, e.g. within OCNS group D or E.

First, oil compatibility tests and particle settling time experiments were conducted on several dispersants. From the experiments, we identified 3 oil-compatible dispersants that gave the longest settling time in base oil and belonged to OCNS group D. We investigated the effectiveness of the selected chemicals in dispersing manganese tetraoxide at HP/HT conditions. Samples of 1.9 and 2.4 SG drilling fluids were prepared and tested. Drilling fluid samples were statically aged at 400°F and 500 psi for 16 hours. Sag and rheological measurements were taken before and after aging to determine the effect of HP/HT conditions on fluid properties. Sag tendency was measured by viscometer sag shoe test (VSST). Then, HP/HT dynamic filtration tests were done at 500 psi differential pressure and 300°F to determine HP/HT dynamic fluid-loss.

We have found that one of the dispersants (nonionic) with a concentration of 8 and 16 g/l for 1.9 and 2.4 SG, respectively, gives low rheological properties and reduced sag before and after static aging. It also gives the lowest fluid-loss of the selected dispersants. For 2.4 SG fluid, 10-minute gel strength was reduced from 50 to 32 lb/100ft², plastic viscosity from 37 to 25 cp, sag from 0.249 to 0.135 lbm/gal, and the fluid-loss at 300°F was reduced from 44.4 to 39.6 cm³, with the addition of the dispersant. This dispersant prevents agglomeration of particles, thereby reducing fluid rheological properties, sag, and fluid-loss.