Study on the flow behavior of underbalanced circulative micro-foam drilling fluid

Underbalanced drilling is a new method for the exploratory development of low pressure and permeability reservoirs; circulative micro-foam drilling fluid is a new technology which is developed for realizing near-balanced drilling and underbalanced drilling. The flow behavior of circulative micro-foam drilling fluid in wellbore was researched by applying BPHT experiment apparatus. It is concluded that the flow behavior parameters of circulative micro-foam drilling fluid is only related to temperature but not to pressure; the constitutive equation accords with the rheological law of power-law fluid, the expressions of consistency coefficient and liquidity index were obtained through analyzing the flow behavior experiment data under the condition of HTHP. The density of circulative micro-foam drilling fluid increases as the increase of pressure and decreases as the increase of temperature, but in wellbore the rate of increase as pressure is greater than that of decrease as temperature, so the density of drilling fluid in wellbore is greater than that under ground condition. The fluid drag force of micro-foam drilling fluid in annulus were analyzed theoretically and the pressure distribution formulas of micro-foam drilling fluid in wellbore were given.