This thesis provides a detailed evaluation of different environmentally friendly dispersants in invert-emulsion drilling fluids that can be used to drill wells under difficult conditions such as HPHT. The drilling fluid is weighted by manganese tetraoxide (Mn3O4) particles, which have a specific gravity of 4.8 and a mean particle diameter of ca1 micrometers. 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 rheology, and low fluid-loss at HPHT conditions. This is particularly important for deep wells with narrow operating windows between pore-pressure and fracture pressure gradients. The stricter global environmental regulations mandated 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 31 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 selected chemicals in dispersing manganese tetraoxide at HPHT conditions. 1.95 and 2.4 S.G. drilling fluid samples were first prepared and tested without any contaminant and then in the presence of rev dust and cement as contaminants. Drilling fluid samples were statically aged at 400 degrees F and 500 psi for 16 hours. Sag and rheological measurements were taken before and after aging to determine the effect of HPHT conditions on fluid properties. Then, HPHT dynamic filtration tests were done at 500 psi differential pressure and 300 degrees F to determine HPHT dynamic fluid-loss. We have found that one of the dispersants (nonionic) gives low rheology and reduced sag before and after static aging. It also gives the lowest fluid-loss of the selected dispersants. For 2.4 S.G. fluid without contaminants, 10-minute gel strength was reduced from 50 to 32 lb/100 ft^2, plastic viscosity from 37 to 25 cp, sag from 0.249 to 0.135 lbm/gal, and fluid-loss was reduced from 44.4 to 39.6 cm^3 with the addition of dispersant. This dispersant prevents agglomeration of particles, thereby reducing fluid rheology, sag, and fluid-loss.

Three effective solutions to dissolve the filter cake created by water-based drilling fluids weighted with Mn3O4 particles were developed. Hydrochloric acid at concentration lower than 5 wt% can dissolve most of Mn3O4-based filter cake. Dissolving the filter cake in two-stage treatment of enzyme and organic acid was effective and eliminated the associated drawbacks of using HCl. Finally, combining low and safe concentration of HCl with an organic acid in one-stage treatment was very effective. Hydrochloric acid (10-wt%) dissolved 78 wt% of Mn3O4-based filter cake at 250°F after 28 hours soaking time. However, Chlorine gas was detected during the reaction of 5 to 15-wt% HCl with Mn3O4 particles. At 190°F, 1- and 4-wt% HCl dissolved most Mn3O4 particles (up to 70-wt% solubility). Their reactions with Mn3O4 particles followed Eq. 8 at 190°F, which further confirmed the absence of chlorine gas production at HCl concentrations lower than 5-wt%. EDTA and DTPA at high pH (12) and acetic, propionic, butyric, and gluconic acids at low pH (3-5) showed very low solubilities of Mn3O4 particles. GLDA, citric, oxalic, and tartaric acids produced large amount of white precipitation upon the reactions with Mn3O4 particles. Similarly, DTPA will produce damaging material if used to dissolve Mn3O4-based filter cake in sandstone formation. At 4-wt% acid concentration, lactic, glycolic, and formic acids dissolved Mn3O4 particles up to 76 wt% solubility at 190°F. Malonic acid at lower concentration (2-wt%) dissolved 54 wt% of Mn3O4 particles at 190°F. Manganese tetraoxide particles were covered with polymeric material (starch), which significantly reduced the solubility of filter cake in organic acids. Therefore, there was a need to remove Mn3O4-based filter cake in two-stage treatment. Enzyme-A (10-wt%) and Precursor of lactic acid (12.5-wt%) dissolved 84 wt% of the filter cake. An innovative approach led to complete solubility of Mn3O4 particles when low and safe concentration of HCl (1-wt%) combined with 4-wt% lactic acid at 190°F. HCl (1-wt%) combined with lactic acid (4-wt%), dissolved 85 wt% of the Mn3O4-based filter cake after 18-22 hours soaking time at 250°F in one stage treatment.