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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
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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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