Page 184 - Origin and Prediction of Abnormal Formation Pressures

DRILLING PARAMETERS 159

Belotti and Gerard (1976) discussed a similar system, which was developed in
Europe and was successfully tested in more than 40 wells. The system consisted of
a set of sensors which acquire necessary information and transmit it to an on-site
data unit, where the information is scaled, displayed, recorded and processed by the
mini-computer. Storage on magnetic tape cassettes allowed playback of magnitude
of overpressure, porosity, and geologic information at desired depth intervals for
comparative studies with other individual pressure indicators, such as well logs.
Herbert and Young (1972), using historical field data from several Louisiana Gulf
Coast wells, developed equations based on regression analysis for predicting pore
pressures. When the results of this analysis are applied to drilling data, the transition
from normal pore pressures to overpressures can be predicted. This, however, can be
done only on a geographically-regional basis.
Correlations between the well log data and rock drillability have been developed
by Gstalder and Raynal (1966) and E1-Hadidi (1970). Acoustic transit time data from
geophysical well logs can be used to predict rock drillability, provided the lithology is
known.
The basic concepts of the SNAP log (Lutze et al., 1972) apply the vibrations from
the tricone bit, as measured at the Kelly, to give an instantaneous log of the formation
characteristics while it is being drilled.

LOGGING WHILE DRILLING

Methods have been developed for recording the formation, mud, and bit data at the
bottom of the borehole, and then transmitting these data to the surface. The great need
for the development of additional energy resources and ever-increasing costs of drilling
and exploration, created an incentive for developing methods providing downhole
real-time measurements. Proposed methods and granted patents are numerous.
Logging-while-drilling measurement systems essentially perform only two basic
functions: (1) recording of the desired parameters at the bottom of the wellbore,
and (2) data transmission to the surface. Downhole measurements comprise: (1) well
control information, (2) directional drilling control, (3) drilling optimization, and
(4) formation evaluation. Many different logging-while-drilling systems have been
developed. Basically, there are four different types of data telemetry: (1) mud-pressure
pulses, (2) insulated conductor or cable, (3) electromagnetic methods, and (4) acoustic
methods.

TORQUE
Torque variations are continually monitored at the drilling rigs. Torque usually
increases gradually with depth due to increased wall-to-wall contact of drillpipe
and wellbore. In the presence of underbalance (i.e., negative differential pressure),
overpressured shales tend to flow or heave into the borehole. Hence, a drastic increase
in torque may serve as an additional pressure indicator.