The performance characteristics of a new commercial additive for low-toxicity oil-based drilling fluids were evaluated to establish recommended applications of the product. The increases in thixotropy and gel strengths claimed by the supplier upon addition of their amide resin to mineral oil muds offered the possibility of solving barite settling problems common with these systems. Microscope observations and rheology measurements of formulated fluids show that the amide resin rapidly disperses the organophilic clay in mineral oil at low mixing energy, rather than acting to extend the organophilic clay. The dispersed platelets are then free to interact and build thixotropy to prevent solids settling. The best application of the new product is in mud mixing plants with mineral oil muds.
Oil-based muds are finding increasingly greater applications in drilling troublesome shales, hot holes, sour and sweet gas environments, and highly deviated wells. A problem encountered with application of certain oil muds is their inability to support weighting agents and cuttings, particularly in deviated holes. Solids settling also occurs with the same mud systems when transported from the mixing plant to the rig. These problems are related to insufficient building of thixotropy in the oil mud by organophilic clays used in certain mineral oils. The objective of this research was to evaluate a new commercial oil mud additive (amide resin) which is advertised to promote thixotropy in oil-based drilling fluids by "beneficiating" the organophilic clays. The intent was to establish a recommendation for proper application of theadditive in Amoco operations.
In recent years, the base oils for invert emulsion muds used in offshore locations and in other environmentally sensitive areas have changed from diesel fuel to low-toxicity refined mineral oils.2 The lower aromatic content of the mineral oils reduces the potential toxicity of oil muds to marine life in tests specified by government agencies. Unfortunately, the switch to mineral oil in oil mud formulations has changed the rheological behavior of formulated muds.
Organophilic clay is added to oil muds to build gel structure that keeps barite and drill cuttings suspended.3 Generally, there is a decrease in the gel building characteristics of the most common organophilic clays (with tallow-based quaternary amines) in mineral oils relative to diesel oils.3 As a consequence. higher concentrations of organophilic clay are used to achieve desired field points and gel strengths3. Some mud service companies have used basically the same emulsifier packages and organophilic clays with mineral oils as they used with diesel oil. Other mud companies have modified their products specifically for use in mineral oil to gain more effective oil mud properties.
Amoco has experienced several operations problems with mineral oil muds as a result of the poor suspending characteristics of the organophi lic clays used. Barite settling with weighted muds, especially in deviated wellbores, has resulted in serious lost return problems when breaking circulation of muds left in the hole. Barite settling has also occurred during shipping of the mud from the mixing plant to the rig.
