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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: DREAM™ - Drilling and Real Time Migration:
A New Method for the Geophysical Monitoring of Wells
By
Agip S.p.a., Milan, Italy
Interpreted seismic sections and derived
maps in time and/or depth are normally
used to locate wells. The same data
are subsequently
modified and reinterpreted on the
basis of final well results. In the drilling
phase, there is a sort of blackout in the use
of geophysical information. The "migration
while drilling continuously" (MWDC©)
method is a new approach to the while drilling
geophysical monitoring of a well.
The approach makes it possible to answer
two basic questions: Where is the bit
in relation to the seismic section? Is the
geological prognosis coherent with the
while-drilling information?
Starting from a depth reference section
obtained with an initial velocity
field, the
method allows the operator to continuously
update the
velocity
field while drilling, and
thus to refine the depth model and iterate
the depth migration. The seismic velocities
obtained by recording systems (i.e.,
Seisbit, or Tomex), which exploit the noise
generated by the bit as a source of acoustic
energy, are the most suitable to integrate
well and migration velocities. Other velocities
may also be derived from logging-while-drilling (LWD) measurements. One
of the main advantages of seismic-while-drilling
(SWD) tools compared to the other
possible sources of
data
is the capability
of prediction "ahead of the bit." As the
original depth seismic image is continually
updated, integrating the new well
velocity
information on the initial model, the
seismic image becomes more and more
reliable. With this more reliable image,
important decisions can be made, such as
redefining the well program, picking casing
points, predicting over-pressured
zones, etc.
Agip and Western Atlas are developing a
new product that combines MWDC© principles
and software from Western Atlas.
The software package (DREAM™) is
structured into four main modules: a
project database, a while-drilling info package
(Well Data
Analysis
), a modeling module,
and the depth-processing package. The
prototype actually handles 2-D seismic
data
, but extensions to 3-D
data
sets are
presently being developed. The core of the
DREAM™ package is the integration
between surface
velocity
data
(geophysical
time domain) and well
velocity
data
(geological depth domain) that have a different
physical meaning and, therefore,
may vary a great deal. Those
velocity
functions
are processed by an interactive module
(Well
Data
Analysis
) that calculates,
using linear and non-linear regression
techniques, the new
velocity
profile. This
new
velocity
information is added to the
flow of operations so that the initial
velocity
model can be changed laterally
and vertically using also the section's
structural information. The resulting
model makes it possible to obtain a new
depth (and time) seismic image using migration
or conversion algorithms. Both
MWDC© principle and DREAM™ prototypes
have been practically applied on synthetic
data
sets and real cases. Examples
will be shown concerning an exploration
well located in the Po River plain.
MWDC© - Agip patent
DREAM™ - Western Atlas patent
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