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The AAPG/Datapages Combined Publications Database
AAPG Special Volumes
Abstract
from:
Chapter 16
Pore Pressure ahead of the Bit: An Integrated Approach
Nader C. Dutta
WesternGeco
Houston, Texas
William H. Borland
Schlumberger Wireline and Testing Services
Gatwick, United Kingdom
W. Scott Leaney
Schlumberger Wireline and Testing Services
Gatwick, United Kingdom
Richard Meehan
Schlumberger Wireline and Testing Services
Sugar Land, Texas
W. Les Nutt
Schlumberger Wireline and Testing Services
Fuchinobe, Japan
ABSTRACT
Undercompacted shales generally have a lower acoustic impedance (product of density and velocity) than those that follow a normal compaction trend. Departure from the normal compaction trend may indicate potential drilling hazards due to overpressure. Techniques that can monitor acoustic impedance can be used to indicate the existence of such potential hazards, and thereby, help in designing the casing and mud program.
Prediction of pressure ahead of the bit starts with the best predrill model. In frontier wells, commonly
seismic
data are the only data available.
Seismic
velocities from analysis of stacking velocities
and impedances from reflection sequence analyses, in conjunction with a predrill rock model, can be
used to develop a predrill pressure vs. depth profile. This has been used with considerable success in
deep-water wells. The limitations, however, are the lack of resolution in the reflection
seismic
data and
uncalibrated velocity models. Thus, a strategy is developed that can update this so-called static model
in real time using borehole data.
Conventional wire-line vertical seismic
profile (VSP) measurements are commonly used to provide
high-quality reflection data within and below the bottom of the well. Inversion of VSP data for acoustic
impedance has been demonstrated to be a reliable way to accurately predict acoustic impedance below
the bit, with more resolution than the conventional velocity data from stacking-velocity analyses. This
has been found to yield pressure vs. depth profiles, at the bit level, with more resolution. Downtime
on the rig is required to acquire the wire-line data.
Vertical seismic
profile inversion allows the location of the overpressured zone to be accurately
determined in two-way traveltime. This time estimate can be converted to depth if the formation acoustic
velocity ahead of the bit is known. The drill bit
seismic
technique, which uses a working drill bit as
the
seismic
source, provides continuous time to depth information. These data can be used to estimate
the formation acoustic velocity continuously, in real time, and to calibrate
seismic
velocity at the bit
End_Page 165------------------------
level and hence allow an accurate, continuously updated, prediction of the depth to the overpressure hazard in real time at the well site.
In this chapter we present a methodology to quantify and predict overpressure hazards ahead of
the bit using surface seismic
, VSP, and drill bit
seismic
.
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