BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
The effects of several drilling fluid types on the return permeability of a Cook Inlet Alaska core
from the Middle Ground Shoal Field are given in Figure 11 - 11. Note that the oil fluid gave a very
good return permeability as did the polymer-base fluid with 3% KCl. Since the formation contained
a significant amount of expandable clays, the performance of these two fluids can most likely be
attributed to their swelling inhibiting properties.
Figure 11 - 11
Return Permeability Data on Cook Inlet Cores
In other formations, particle dispersion and movement may be the primary source of damage.
Lignosulphonates are excellent deflocculating agents, and thus produce extremely fine particle-size
dispersion, particularly in high pH fluids. This can result in considerable particle transport into the
core or particle movement within the core. In formations where this condition exists,
lignosulphonates fluids can produce considerable damage with return permeabilities below 50%.
Sandstones containing kaolinite packets are particularly susceptible to particle plugging. The
packets can become dispersed from the quartz surface and migrate to plug a pore throat. Fluids that
enhance dispersion thus increase the plugging effect.
In general, with all other factors equal, the lower the permeability of the formation, the more
subject it is to damage. This, no doubt, is due to the smaller pore openings of the lower permeable
rock. Rock factors such as grain mineralogy, grain size, grain shape, and grain packing all exert an
influence on the size of the pore throats. There is no specific permeability that the problem of pore
plugging becomes acute, but certainly rocks with permeabilities below 100 md can be highly
susceptible.
In summary, both the horizontal and vertical permeabilities of the formation are important in a
well. Since the vertical permeability of the rock is normally lower than its horizontal permeability,
significant differences may occur in their susceptibility to damage. Thus, when analyzing cores
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-23