Page 179 - Origin and Prediction of Abnormal Formation Pressures

154 W.H. FERTL, G.V. CHILINGAR AND J.O. ROBERTSON JR.
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d- exponent do - exponent

ACTUAL MUD WEIGHT, Ib/gal

Fig. 6-3. Comparison of plots of depth versus d-exponent and de-exponent based on drill bit data, Louisiana
well, U.S.A. (Modified after Fontenot and Berry, 1975, fig. 3, p. 127. In Chilingarian and Vorabutr, 1981,
fig. 16-3, p. 587" Courtesy of Oil and Gas Journal.)

Drilling in areas where overpressured zones are encountered in shallow, uncon-
solidated elastic formations, indicates the increased importance of proper hydraulics
programs in achieving effective penetration rates in these soft formations. The latter
are drilled utilizing a combination of tooth cutting and jetting action. As a result,
increased jetting action will increase penetration rate which, in turn, will result in de-
creasing values of the de-exponent. This gives an appearance of a non-existent transition
zone. Furthermore, maintaining a constant circulating rate when a transition zone is
anticipated in unconsolidated sediments will minimize effects of hydraulics.
Transition zones at depths as shallow as 1500 ft have been successfully defined by
.~ontrolled drilling conditions (i.e., maintaining constant bit weight, rotary speed, mud
weight, and circulation rate). The fast penetration rate may be measured even by timing
with a stop watch the drill joints lowered during drilling.