Dynamic thermal aging of water-based drilling fluids with different types of low-rank coals as environmental friendly shear thinning additives

This paper focuses on the development of a drilling fluid system that satisfies both environmental and technical requirements, acting as an environmentally appropriate alternative to oil or synthetic-based drilling fluids. The effectiveness of two types of low-rank coals (leonardite & lignite) as shear-thinning additives on the rheological behaviour and filtration control in drilling fluids after dynamic thermal aging is investigated. The most suitable model that best describes their rheology is the Herschel-Buckley model over the entire range of shear rates and all temperature levels. Dynamic thermal aging influences the yield stress at zero shear rates, the consistency index, the flow behaviour index of the Herschel-Buckley model as well as the yield point and plastic viscosity of the Bingham-Plastic model. Despite their geochemical differences, the addition of both low-rank coals in the drilling fluids improves the yield stress and yield point build-up with temperature increase and maintains them within acceptable values compared with coal-free drilling fluids. The highest aging temperature under dynamic conditions deteriorates significantly the rheological and filtration control parameters. For the specific concentration of additives and dynamic thermal aging, the temperature of 177 °C appears to be an upper thermal bound which the drilling fluids lose their design purpose. It is also demonstrated that considerable energy is required to change both the plastic viscosity and the yield point of the proposed drilling fluids with both low rank coals showing their effectiveness as shear thinning agents to maintain the desired low shear strength and yield point after dynamic thermal aging.