Development and applications of solids-free oil-in-water drilling fluids

The increasing application of near balanced drilling technology to low-pressure and depleted fractured reservoirs requires the use of low-density drilling fluids to avoid formation damage.Solidsfree oil-in-water (O/W) emulsion drilling fluid is one type of low-density drilling fluid suitable for depleted fractured reservoirs.In this paper,the solids-free O/W drilling fluid was developed and has been successfully used in the Bozhong 28-1 oil and gas field,by which lost circulation,a severe problem occurred previously when drilling into fractured reservoir beds,was controlled,thereby minimizing formation damage.The O/W emulsion drilling fluid was prepared by adding 20% (by volume) No.5 mineral oil (with high flash point,as dispersed phase) into seawater (as continuous phase).Surfactant HTO-1 (as a primary emulsifier) and non-ionic surfactant HTO-2 (as a secondary emulsifier) were added into the drilling fluid system to stabilize the emulsion; and YJD polymer was also added to seawater to improve the viscosity of the continuous phase (seawater).The drilling fluid was characterized by high flash point,good thermal stability and high stability to crude oil contamination.