Page 188 - Origin and Prediction of Abnormal Formation Pressures

DRILLING PARAMETERS 163

(1970). The increase or decrease in the chloride content of the drilling mud between the
inlet and outlet of the mud stream can be related to the drilling and pressure conditions.
In addition, the drilling rig data gathering units enable measuring and recording both
inlet and flowline drilling fluid resistivities.
Other techniques, such as variations in the contents of specific ions, redox and pH
measurements on the drilling mud stream, and various physical and chemical analyses
of drill cuttings, have been investigated by Fertl and Timko (1973a,b).

Pit level and total pit volume

Pit level indicators, which monitor variations in the total mud volume, may show
mud-volume reduction caused by lost circulation or increase in mud volume due to
the fluid entry into the wellbore as a result of unexpected high formation pressures.
An ultrasonic equipment exists to measure accurately drilling fluid levels in mud tanks
without having any contact with the mud (Dupin de Saint Cyr, 1973). Hence, this
method is particularly effective on floating, deep-water offshore drilling figs.

Hole fill-up

If the drillstring is pulled, the mud volume needed to fill the borehole should be
equal to the displaced pipe volume. Keeping the hole full is especially critical at the
time when drill collars are pulled, because on pulling the same length of collars as that
of the drillpipe, the level of drilling mud in the borehole will fall four to five times as
fast. Furthermore, if salt water, oil, or gas from the formation enters the well, the mud
volume required to fill the borehole will be less than the displaced volume of the pipe
pulled out. Consequently, this gives the first indication of a pressure kick. Measurement
of mud volume used to fill the borehole can be checked from changes in the pit mud
level.

Mud flow rate

Flow rate measurements are superior to pit level checks, because even small flows
can be detected before they become sufficiently large to show on any pit level measuring
device. More time is available, therefore, to take proper control measures.

SHALE CUTTINGS PARAMETERS

Shale bulk specific weight

Bulk specific weight of drill cuttings usually increases with depth. Measurement
techniques include (1) the high-pressure mercury pump technique, (2) the fluid density
gradient column, and (3) the mud balance method. Care must be taken in selecting and
preparing the drill cuttings for analysis. Multiple cuttings samples have to be tested due
to the variance in sample data. The average bulk specific weight value, for a given depth,