Page 222 - Air and gas Drilling Field Guide 3rd Edition

CHAPTER

9
Aerated Fluids Drilling

The term aerated fluids describes the broad category of drilling fluids that are
basically incompressible fluids injected with compressed air or other gases. Aer-
ated drilling fluids have been used to drill both shallow and deep boreholes since
the advent of air and gas drilling technology in the mid-1930s. The first engineer-
ing discussion of an aerated drilling mud project was given in 1953 [1]. Aerated
drilling fluids were initially used to drill through rock formations that had fracture
and/or pore systems that could drain the incompressible drilling fluids (e.g., fresh
water, water- and oil-based drilling muds, formation water, and formation crude
oil) from the annulus. These borehole drilling fluid theft rock formations are
called lost circulation sections. The injection of air into drilling muds has been
considered an important technological tool in countering the detrimental effects
of lost circulation sections. The injection of air into drilling mud creates bubbles
in the mud and, because of the surface tension properties of the bubbles relative
to the properties of rock and drilling mud, the bubbles tend to fill in the fracture
or pore openings in the borehole wall as the aerated mud attempts to flow to the
thief fractures and pores [2]. This bubble blockage restricts the flow of the dril-
ling mud into these lost circulation sections and thereby allows the drilling opera-
tions to progress safely. Aerated fluids have been used to avoid lost circulation in
shallow water well drilling, geotechnical drilling, mining drilling, and in deep oil
and natural gas recovery drilling operations. Aerated fluids drilling operations are
nearly always direct circulation operations.
Since the late 1980s another important application for aerated fluids drilling
operations has emerged. This is underbalanced drilling applied to oil and natural
gas recovery operations. Over the past two decades practical field research has
demonstrated that most oil and natural gas bearing rock formations can be pro-
duced more efficiently if they are drilled with drilling fluids that have flowing bot-
tom hole pressures that are slightly less than the pore pressures of the potential
producing rock formations being drilled. Underbalanced drilling operations allow
the oil or natural gas to be produced into the annulus as the drilling operation pro-
gresses. The underbalanced drilling operation allows the natural fracture and pore
systems to be kept clear of rock cutting fines and drilling mud filter cake, thereby
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