Page 151 - Geothermal Energy Systems Exploration, Development, and Utilization

3.3 Drilling Mud 127
borehole may occur. If this is encountered it can cause severe problems (‘‘buried’’
bit or drillstring) because of ‘‘overload’’ of the carrying capacity of the air column
in the borehole.
Possible advantage of air drilling is a generally higher drilling progress (ROP)
due to the pressure relief on bottom. Thus, the ROP with air as drilling fluid may
reachupto10times theROP of a‘‘conventional’’ mud.
The use of air as drilling mud in geothermal wells is clearly a desirable alternative
to minimize formation damage, but not practical in all circumstances (e.g., due to
borehole stability reasons).

3.3.2
The Importance of Mud Technology in Certain Geological Environments

3.3.2.1 Drilling through Plastic/Creeping Formations (Salt, Clay)
Soft plastic formations like clay, clay-rich marl, shale, and salt rock pose a special
challenge in the drilling and completion processes. Creeping formations tend
to decrease the caliper during drilling when a very low mud pressure is used.
Washouts represent caliper increases where a very high mud flow is used. Although
soft formations entice to drill with a high ROP, a moderate penetration combined
with moderate WOB allows stress releases in plastic formations and prevents
abnormal creeping during and specially after the drilling operation. In clay-rich
formations inhibitors can minimize the swelling process whereas in salt rock
overgauge or washouts (especially in potassic salt) this can almost be not avoided.
Besides the mud technology, special emphasis should be given to straighten the
selection of the drill bit and bit hydraulics. As stress increases with well bore depth,
a formation becomes more plastic and requires different bit cutting mechanics to
cut or to shear efficiently. Special emphasis on sustainable wellbore stability should
be given on plastic formations that can exert abnormal stresses at the borehole wall.

3.3.2.2 Formation Pressure and Formation Damage (Hydrostatic Head, ECD)
The formation pressure is the pressure that is exerted by the formation fluids
within the pores of the rock and is a key issue in well integrity (e.g., well control
during operation or best casing shoe depth selection). Usually the formation
pressure can be assumed as hydrostatic or as pressure exerted by a column of water
from the formation’s depth to ground level. During circulation, the downhole
annulus pressure is always higher than that of the static column. The equivalent
mud weight corresponding to this circulation pressure is called the equivalent
circulation density (ECD). ECD can be interpreted as the density of a hypothetical
fluid, which in static conditions and at any depth produces the same pressure as a
given drilling mud in dynamic conditions. It can be calculated as follows:

ECD = ρ d + P a / g × L/100
where
−1
ρ d (kg l ) = density of the mud containing cuttings (mud in annulus),
P a (bar) = circulation pressure loss in the annulus,