Akpan, EU 
ORCID: https://orcid.org/0000-0001-5589-2462
2019,
Water-based drilling fluids for high temperature and dispersible shale formation applications
, PhD thesis, University of Salford.
Abstract
The negative impacts of drilling fluids, most especially oil-based muds, on the environment are a source of concern to regulators and governments all over the world. Consequently, regulations on the discharge and disposal of drilling fluids worldwide have become stricter. The recent research in drilling fluids formulation is focussed on the design of high-performance water-based drilling fluids, which are environmentally friendly, remain stable when exposed to high temperatures, and prevent problems associated with reactive shale. Even though water-based fluid systems are environmentally-friendly, their instabilities when exposed to high temperatures remain a big challenge. Biopolymers are commonly used in water-based muds to provide viscosity but degrade at high temperatures.
In this study, an attempt has been made to investigate whether the combination of anti-oxidants, formate salts, pH control agents, and polyethylene glycol could increase the stability temperatures of biopolymers in water-based drilling fluids and inhibit shale dispersion at high temperatures. The rheological properties of the drilling fluid formulations with pH 8-10 were measured using Models 800 and 1100 viscometers before and after aging dynamically in a roller oven for sixteen hours. The stability temperature of drilling fluid formulation was defined as the temperature at which it retains 50% of its original viscosity after aging for 16-hours. Shale rock samples were characterised using scanning electron microscope photos; X-ray diffraction analysis was used to identify the mineral contents of the shale samples. Shale dispersion tests were carried out by aging shale cuttings in drilling fluid formulation and in freshwater, dynamically in a roller oven for 16 hours at 120°C. The percentage recovery of shale rocks after dynamic aging was determined. Experimental data indicated that the stability temperatures of diutan gum, konjac gum, and xanthan gum in bentonite water-suspension after aging for 16 hours were 115°C, 65°C, and 100°C respectively. The mud formulations with the additives - sodium erythorbate, potassium formate, and polyethylene glycol – retained at least 50% of their viscosities up to 232°C. The additives, therefore, significantly retarded the degradation of the biopolymers and other mud additives up to 232°C. The result from the shale dispersion test showed that the shale cuttings recovered from freshwater was 78%; with drilling fluids formulated with the additives, the shale cuttings recovered were 100%. When a shale plug (14.99g) was exposed to freshwater, it was found that 79% of the shale plug was recovered. When another shale plug (13.77g) interacted with an inhibitive mud formulation, it was found that 100% of the shale rock was recovered. This new fluid systems which are stable at high temperatures and inhibits shale dispersion can meet high temperature and shale formation drilling requirements.
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