Multiphase Flow During Underbalanced Drilling

In underbalanced drilling, the pressure in the wellbore is lower than in the formation. The low-density drilling fluids used for underbalanced drilling are air, gas, foam, aerated drilling fluid, and low-density drilling fluid. The advantages of underbalanced drilling are: improving the ROP, reducing lost circulation, extending the bit life, improving the formation evaluation, reducing formation damage, and finding the oil and gas early. The technical and economic limitation of underbalanced drilling include: poor wellbore stability; easy entry of formation fluids; difficulties in drilling directional wells; safety factors; and economic factors. This chapter focuses on the problems related to the wellbore multiphase flow. There are three underbalanced drilling methods relevant to the multiphase flow: gas drilling, drill pipe injection aerated drilling, and annulus injection aerated drilling. The chapter presents case studies that simulate and analyze the multiphase flow of these methods.