BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
environment, tends to lose much of this property. The water-soluble organic polymers provide a
lubricating and shearable surface. As described above, the natural lubricity of the partiallyhydrolyzed
polyacrylamide is a factor in imparting lubricity to the fluid, but this property also
carries over into the filter cake surface. The polymer maintains considerable water in the cake
surface, even in the presence of contaminating electrolytes, and produces a much slicker and more
lubricating surface than the clay-based systems.
One facet of torque in a horizontal hole that cannot be easily answered is the relationship of the
drillstring against the inevitable cuttings bed that is formed. Questions can be asked as to what
extent does the drillstring extend into this bed, and to what degree is the bed compacted? The
answers become even more elusive in the steeper angles of the hole, where the cuttings bed is more
dynamic and transitional. Regardless of the final effect of all the parameters of the cutting bed
issue, it would appear that the lubricating properties of the drilling fluid remain important.
Formation Damage
The obvious reason for drilling a horizontal well is to expose more of the producing zone to the
wellbore for increased production. Thus, the importance of preventing formation damage during
drilling and completion of the horizontal well is just as important as it is in its vertical counterpart.
In fact, it may be more important since the increased cost of drilling the horizontal well must show
additional production for its justification. A 50% damage or permeability loss in a potential 500
bbl/day horizontal well represents a considerably greater loss in production than a 50% damage in a
100 bbl/day vertical hole through the same reservoir.
Sandstone production is by far the most susceptible reservoir rock to damage or permeability loss
during drilling and completion operations. Though formation damage may be caused by poor
drilling and completion practices, one of the primary causes is due to exposure to the drilling and
completion fluid.
This loss, attributable to the fluid, can be caused by one or more of the following occurrences:
• Particle transport from the drilling fluid into the production zone, subsequently plugging pore
throats.
• Fluid filtrate reacting with expandable clays in the rock to decrease pore throat diameters.
• Particle movement within the permeable rock due to dispersion of clays and other minerals
from quartz surfaces.
• Wettability changes of the formation from exposure to the drilling fluid filtrate.
• Interaction of fluid filtrate with formation fluids to form water - insoluble precipitants.
The effect of these actions on the permeability of sandstones has been the subject of a considerable
investigation. In almost all cases, some damage is caused by the drilling and completion fluid. The
primary objective is to limit this damage as much as possible. Consequently, selection of the
drilling and completion fluid becomes an extremely important function in drilling the horizontal
well.
Examining the Formation
Where possible, a thorough examination of the formation should be made to determine its
susceptibility to damage and to identify the mechanisms of damage. This analysis is of
considerable value in selecting the drilling or completion fluid for a particular formation.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-21