Page 181 - Origin and Prediction of Abnormal Formation Pressures

156 W.H. FERTL, G.V. CHILINGAR AND J.O. ROBERTSON JR.

Bit wear will cause a decrease in penetration rate and, thus, an increase in the
dc-exponent. Consequently, dull bits can mask the presence of a transition zone.
Consideration of bit wear in graphical and computerized techniques will eliminate this
limitation.
An alternative solution is to study the offset well data and previous bit performance in
order to determine when to pull a bit prior to drilling through transition zones. Although
this approach may result in an additional trip, it will greatly reduce the hazards of a well
kick and time required to kill it, or even of a blowout, should the downhole mechanical
problems occur.

Overbalance

The dc-exponent is also affected by the difference between hydrostatic pressure of the
mud column and formation pressure (i.e., amount of overbalance). A linear, instead of
the required hyperbolic, correction will introduce a serious error into calculations with
increasing overbalance. In other words, arbitrary and 'emotional' mud weight increases
should be avoided.
Knowledge of formation pressure while drilling is important for achieving safe and
economic operations and in the selection of proper casing seats. The dc-exponent,
supplemented with several other pressure indicators, provides the drilling engineer with
the required information for making proper decisions.

DRILLING RATE EQUATIONS
The application of proper drilling rate equations in order to establish correlations
between the formation and borehole pressures requires instrumentation at the rig site to
record simultaneously several drilling parameters.
For example, a general equation for penetration rate in shales has been developed
by Combs (1968) using data from six Louisiana wells in a regression-type analysis.
Combs' correlation shows that penetration rate (R) is proportional to the weight on the
bit (W), rotary speed (N), and bit hydraulics, each raised to a fixed power:

R- Ro ~ -~ 31~D, f (Po)f (T) (6-3)

where Dh is borehole diameter (inches), Dn is bit nozzle diameter (inches), f is function
of, n l, n2 and n3 is weight, speed and hydraulics exponents, respectively; Combs'
recommendations: nl - 1.0, n2 = 0.6, n3 -- 3; N is rotary speed (rpm), Q is flow rate
(gal/min), Pa is differential pressure (lb gal -l 1000 ft-I), R is penetration rate (ft/h),
R0 is shale drillability with sharp bit at zero differential pressure (ft/h), T is bit wear
index-equivalent rotating index, and W is weight on bit (in 1000 lb).
According to Combs (1968), penetration can be predicted from Eq. 6-3 with a
standard deviation of approximately 30%, whereas pore pressure can be predicted with
a standard deviation of about 1.0 lb/gal equivalent pore pressure.
In connection with computerized drilling control, Young (1968) expressed penetration