BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
LOSS OF CIRCULATION
Reference is made in this section to: Prevention and Control of Lost Circulation: Best
Practices, Reference Manual 750-500-104, by Atlantic Richfield & Baker Hughes Drilling
Fluids, February 1999.
Loss of circulation is the loss of whole drilling fluid to exposed formations. This will occur where
there is a permeable zone with openings of sufficient size to permit entrance of whole fluid,
and/or the hydrostatic pressure is greater than the formation fracture pressure.
Improved drilling fluid technology, a greater understanding of hydraulics, and drilling practices
have greatly reduced the incidence of loss of circulation in most areas. However, in spite of
continued research to improve technology, loss of circulation problems still adversely affect
many drilling operations. Resulting treatments, extra casing strings, increased drilling time, and
drastically altered drilling programs greatly increase the overall drilling cost on these wells.
In general, the two classes of loss of circulation are,
• those inherent to the formation or natural loss of circulation
• those of a mechanical nature caused by poor drilling techniques
There are four types of loss of circulation zones in these classes.
1. Zones of normal strength which have been fractured by the use of high-density or highviscosity
drilling fluids or by pipe handling pressure surges
2. Zones that are naturally fractured or containing joints and fissures
3. Zones that are coarsely permeable, such as gravels and some conglomerates
4. Zones that are cavernous, such as limestones and dolomites, with vugular or “honeycomb”
porosity large enough to take whole drilling fluid
Loss of circulation into all of these zones can be precipitated by improper control of drilling fluid
density, flow properties, or pressure fluctuations. Field studies have shown that excessive drilling
fluid pressure fluctuations are often the main cause of loss of circulation. Factors that influence
the magnitude of pressure fluctuations include fluid rheological properties, speed of pipe
movement (surge pressure), relative diameter of the components (drillstring to borehole
diameter), and volumetric flow rate.
Pressure surges cannot be blamed for all loss of circulation, although this is a key variable. For
example, drilling fluid may be lost to fractures or cavernous formations that cannot support a
normal hydrostatic head. Conversely, in some areas high hydrostatic pressures that are necessary
to control high formation fluid pressures cause loss of circulation in adjacent formations.
Preventive measures must be tailored to the area and the nature of the loss.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-21