BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
Changes in Cuttings
Shale cuttings from an abnormally pressured zone are larger than those from a normally
pressured zone. They are characterized by sharp and angular edges, while normal pressure
shale cuttings are generally small and flat with rounded edges.
The variables which determine the size and shape of shale cuttings are:
• mineralogical, chemical, and physical properties
• type of drilling fluid system
• circulation rate
• hole geometry
• downhole agitation.
Gas or saltwater or a combination of the two in the over-pressured shale helps support some of
the overburden. Hottman and Johnson proposed one method to explain this. Over-pressure in
the shale, if not offset by equal or greater hydrostatic pressure from the drilling fluid, will
create a condition which aids the spalling (sloughing) action of the shale.
H.C.H. Darley and Chenevert found that shales develop pressure if certain fluids are allowed
into the pores. The abnormally pressured shales then act much like popcorn – exploding into
the wellbore – if forces across the face of the wellbore are not equalized.
Shale Density
Shales which are normally pressured have undergone normal compaction and their densities
increase uniformly with depth. This uniform increase allows shale density to be predicted. Any
reduction from the extrapolation can be interpreted as a zone of higher pore pressure. The
primary problem of relying on shale density is that the shale cuttings must be circulated to the
surface before measurements can be made.
Flowline Temperature
Due to abnormal pressures exerted on fluids in a transition zone, above normal temperatures
occur. A normal trend of flowline temperatures can be plotted. A change of 2° to 6°F above
this trend can be an indication of a transition zone. In addition to indicating a pore pressure
change, changes in flowline temperature can also be attributed to:
• an increase in circulation rate
• a change in solids content of the drilling fluid
• a change in drilling practices
• an increase in bit torque.
The temperature curve, although not definitive, is still an additional indicator to help make a
decision to stop drilling or increase existing fluid weights.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-15