Experimental study on inhibition of hydrate in oil-based drilling fluid for deepwater drilling

Oil-based drilling fluids(OBDF) were used in deepwater drilling of South China Sea.The formation characteristics of gas hydrate in oil-based drilling fluid were investigated under the conditions of 4℃ and 20 MPa by using the experimental system of gas hydrate integrative simulation.The experiments showed that the induction time of hydrate formation in OBDF is less than that in water-based drilling fluid because natural gas solubility in oil is greatly higher than that in water,and OBDF is dispersive emulsion.It means that the formation of gas hydrate in the oil-based drilling fluid was easier and quicker than that in the water-based drilling fluid under the condition of deep water(high pressure and low temperature).The higher content of water in oil-based drilling fluid could generate large amount of hydrate.In the deepwater drilling process,the content of water in the oil-based drilling fluid should be reduced,and the density of drilling fluid should increase and prevent the formation water and gas from entering into well bore.High-dosage ethylene glycol can effectively inhibit gas hydrate from forming in the oil-based drilling fluid.Addition of proper polyol or inorganic salts such as polyethylene glycol and NaCl can effectively inhibit hydrate from forming in the oil-based drilling fluid.