Calculation and Design of Flow Parameters for Two-Phase Flow in Aerated Drilling
Authors: Zhou Kaiji , Wang Jin , Hao Junfang 
Published: December 2007

The flow characteristics and dynamics parameters of two-phase flow inside the pipe and within annulus is very important for design and operation in aerated mud drilling. Under the constrain condition that proper gas/liquid ratio for given formation pressure of the well to be planned should be selected and two-phase flow pattern inside drillpipe and within annulus should be controlled, this paper applied multi-phase flow theory to calculate and control injecting pressure, injecting flow rate, friction pressure drop inside drillpipe and within annulus, pressure drop in nozzles and dynamic pressure along the wellbore in aerated mud drilling. Rheology of aerated mud, casing program, gas/liquid ratio, mud density, wellbore back pressure, drilling procedures were also analyzed. 5 experimental wells were drilled by using aerated mud in oilfield, Drilling operation show the theoretical model and calculating method are reliable, and the pressure of the aerated mud column can be reduced efficiently. The density of aerated mud ranges from 0.45g/cm3 to 1.2g/cm3, It can be used to explore and exploit low pressure low permeability reservoir and meet the requirements of underbalanced and/or balanced drilling. Aerated mud has good cuttings-carrying capability. Good wellbore stability was also obtained. Aerated mud drilling is a new technology that is worthy of paying attention and speeding.

Introduction

It is well known that balanced pressure and unbalanced pressure drilling can greatly promote penetration rate and prevent formation formation damage. Aerated mud could make the real balanced pressure drilling be realization. That means, under the condition of zero pressure difference between effective hydraulic column pressure and formation pressure, the drilling operation is still safely put into practice, even unbalanced pressure difference (Pme - Pp = - P). Besides, in general, the pressure factor of low pressure low permeability reservoir bed and the exploited long time reservoir is low. Obviously, these reservoir can not be exploited with conventional drilling fluid, whereas aerated mud two phase drilling fluid is very idea choose to these formation economically and safely.

Now, the aerated mud density is able to be controlled to the rang of 0.45 1.2g/cm3, not only it can decrease the hydraulic column pressure in brothel, but also it has a good cuttings-carrying capability. Like regular mud, aerated mud returning back to surface and after degassed and removed drilled solids in surface, could be reused. Apparently, using Aerated mud is more economic thin foam mud (a one-used drilling fluid), furthermore, using Aerated mud is safer than air drilling fluid, because the latter often results from explosion and fire. Finally, even if formation fluid gets into hole, aerated mud drilling is safely operation.

As a drilling circulation medium, aerated mud is different from pure drilling mud. Excepting that aerated mud drilling needs high pressure air compressor, rotating sealhead, gas/liquid separator, gas/liquid mixer, and wellhead nature gas detector and so on. P. 153