Page 185 - Origin and Prediction of Abnormal Formation Pressures

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

DRAG

Drag is defined as the excess in hook load over the free handling load. Drastic
increase in drag may signal the presence of an overpressured formation. In the presence
of a gradual pore pressure increase, however, as in very long transition zones or in
the case of drilling from floating vessels, this pressure indicator becomes questionable.
Furthermore, increase in drag may be caused by bit balling, severe dog legs, deviated
holes, differential sticking, and extra volume of cuttings influx into the wellbore while
drilling through transition zones.

DRILLING MUD PARAMETERS

Mud-gas cutting
Mud logging aids in formation evaluation and detection of overpressured zones.
As early as 1945, the use of mud-gas logging was recommended as an overpressure
indicator and as a warning of impending blowouts (Pixler, 1945). Similarly, Rochon
(1968) proposed mud-gas anomalies as an aid in controlling drilling mud hydrostatic
head-pore pressure relationships. If formation permeabilities are extremely low, the
degree of gas cutting can be roughly correlated with the amount of underbalance
(Goldsmith, 1972). Generally speaking, however, mud-gas cutting may or may not be
directly related to the increased formation pressures, because it is greatly affected by the
geological environment penetrated and the drilling techniques used in the subject well
(Fertl, 1973; Daw et al., 1977).
Several factors which may affect mud-gas logs and, thus, complicate their use for
pressure detection include (Fertl, 1973): (1) potential pay zones, (2) connection gas,
(3) Kelly air, (4) downtime, (5) gas composition, (6) presence of lignite and (7) coal
seams, (8) degradation of mud additives, (9) gas flushing, (10) volcanic material, (11)
deep-seated mud volcanoes, (12) faults, (13) shale gas, (14) thermodynamic processes,
and (15) recycled gas.
Equipment for gas detection and analysis of gas is readily available.

Flowline specific weight of drilling fluids

Reduction in the specific weight of the drilling fluid at the mud flowline can be used
as an additional indicator for gas cutting and the possible presence of overpressured
formations. Consequently, continuous mud weight indicators have become an integral
part of many on-site data collection and analysis units.
Issenmann and Lucon (1971) discussed a continuous mud weight recorder, which
basically consists of a constant-height column through which mud from the wellhead
outlet is circulated by a special pump at a constant flow rate. The weight of the mud is
measured by a pressure gauge located at the bottom of the column and then transmitted
to a recorder.
Another high-resolution drilling fluid monitor system, which consists of pressure and
density sensors, has been developed by Goddard et al. (1973). The measurements are