Successful underbalanced operations demand a systematic approach with
integration of engineering, operations, production, reservoir, and geo
logy. There are significant advantages to underbalanced drilling (UD).
Formation damage can be minimized if proper circulating fluids and op
erating parameters are chosen; there is an opportunity for reservoir e
valuation and characterization while drilling; lost circulation and di
fferential sticking can be avoided, combined with improvements in dril
ling rate of penetration (ROP); and it also provides an alternative me
thod of well control. This paper discusses engineering challenges and
considerations for underbalanced operations using multiphase flow as w
ell as advances made during UD with rotary jointed pipe. Excluding air
, mist, and foam, underbalanced techniques with multiphase fluids are
reviewed. Much of the discussion and concepts also apply to coiled-tub
ing underbalanced operations.