Page 180 - Origin and Prediction of Abnormal Formation Pressures
P. 180
DRILLING PARAMETERS 155
m
0
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Or-
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0.5 1.0 1.5 0.5 1.0 1.5 2.0
d - exponent do - exponent
Fig. 6-4. Comparison of plots of depth versus d-exponent and dc-exponent based on drilling data averaged
over 50-ft intervals; Louisiana well, U.S.A. (After Fontenot and Berry, 1975, fig. 6, p. 129. In Chilingarian
and Vorabutr, 1981, fig. 16-4, p. 588; Courtesy of Oil and Gas Journal.)
Effect of drill bits
When button and diamond bits are used instead of standard rock bits, an empirical
correction is required, i.e., a 1-inch reduction in the bit diameter is used in calculations.
Reliability of this concept in actual field applications, however, is debatable.
There is a decrease in dc-values when a button or diamond bit is used. It should
be assumed that, at the point of a bit change, the first shale layer drilled with the
button or diamond bit has the same pore pressure as the last shale layer drilled with
a rock bit. Hence, a shift of the normal compaction trend line in the de-versus-depth
plot is required to account for the change in bit type. This shift is similar to the one
required due to the change in bit size. The effect of bit size change is shown in Fig. 6-1.
Consequently, trendline shifts become necessary when the (weight-on-bit)/(bit size)
ratio is significantly changed, regardless of the type of the bit.