BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
tested. To do this it is necessary to perform a displacement of the drilling fluid with a solids-free
completion brine, sometimes viscosified with a water soluble polymer, e.g. xanthan gum
derivative. Concern about the physical erosion of the filter cake with the completion fluid is also
always an issue. That is, the filter cake should be durable and stable enough to permit the
completion or other operation to take place and protect the well bore during the entire operation.
The ideal drilling fluid or drill-in fluid would mechanically seal all pore openings exposed to the
wellbore, stay intact during completion operations, then be easily removed by production of oil and
gas. Problems arise in designing these fluids because production zones vary in pressure,
permeability, porosity, and formation configuration. Generally, fluids used to control fluid leak-off
in permeable formations require an initial high pressure spike before removal can begin - from
about 300 to 500 psi. This pressure spike is indicative of damage to the original permeability of the
permeable formation. It would be desirable if fluids could be devised which would easily form an
impermeable filter cake to prevent the loss of expensive completion fluids to the formations and
which effectively protect the original permeable formation during various completion operations
such as a gravel packing or wellbore workovers. At the same time, however, it is also highly
desirable for the filter cake to be easily removable at the beginning of production, causing little or
no damage to the formation.
Water-soluble organic polymers, such as HEC, have been used to slow the leak-off rate of a clear
fluid into a permeable formation. However, these polymers will not effectively control the leak-off
rate. This can only be controlled by bridging the pore opening with rigid or semi-rigid particles of
sufficient size and number. Bridging materials used for this purpose (e.g., oil-soluble resins, gel
pills, sized salt, and benzoic acid) rely upon breakers or dissolution to remove them. Unfortunately,
these methods have not always been effective, resulting in considerable formation damage or
unsatisfactory leak-off control.
PERFFLOW ® System
Baker Hughes Drilling Fluids has developed systems based upon a bridging technique which not
only positively controls leak-off of the fluid into the formation, but also provides a bridging zone
that can be easily and effectively removed by the produced fluid. PERFFLOW systems provide
effective leak-off control over a wide range of permeabilities. Depending on the type of completion
programmed for the well, usually no breakers or fluids to dissolve the bridging solids are required;
removal is accomplished by flowing the well.
PERFFLOW 's fluid loss control mechanism involves the mechanical bridging of properly sized
particles on pore throat openings. The graph below illustrates the particle size distribution of the
graded calcium carbonate (MIL-CARB) used in PERFFLOW. Note the broad particle size
distribution available to bridge a wide range of permeability, from a few milli-Darcys to over 10
Darcys
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-22