In order to ensure safe drilling in deep water and marine gas hydrate bearing sediments, the needed characteristics of drilling fluid system were analyzed. Moreover, the effect of different agents on hydrate formation and the low-temperature rheology of designed polyalcohol drilling fluid were tested, respectively. The results show that clay can promote gas hydrate growth, while modified starch and polyalcohol can inhibit hydrate formation to some extent, and PVP K90 has a good performance on hydrate inhibition. The influence of clay on low-temperature rheology of polyglycols drilling fluid is notable. Therefore, the clay-free polyalcohol drilling fluid is suitable for deep water and marine gas hydrate drilling under optimal conditions.
As the oil and gas industries continue to increase their activity in deep water, gas hydrate hazards will become more serious and challenging, both at present and in the future. Accurate predictions of the hydrate-free zone and the suitable addition of salts and/or alcohols in preparing drilling fluids are particularly important both in preventing hydrate problems and decreasing the cost of drilling operations. In this paper, we compared several empirical correlations commonly used to estimate the hydrate inhibition effect of aqueous organic and electrolyte solutions using experiments with ethylene glycol （EG） as a hydrate inhibitor. The results show that the Najibi et al. correlation （for single and mixed thermodynamic inhibitors） and the Ostergaard et al. empirical correlation （for single thermodynamic inhibitors） are suitable for estimating the hydrate safety margin of oil-based drilling fluids （OBDFs） in the presence of thermodynamic hydrate inhibitors. According to the two correlations, the OBDF, composed of 1.6 L vaporizing oil, 2% emulsifying agent, 1% organobentonite, 0.5% SP-1, 1% LP-1, 10% water and 40% EG, can be safely used at a water depth of up to 1900 m. However, for more accurate predictions for drilling fluids, the effects of the solid phase, especially bentonite, on hydrate inhibition need to be considered and included in the application of these two empirical correlations.
Fulong NingLing ZhangGuosheng JiangYunzhong TuXiang W uYibing Yu
