Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Well logging
Patent
1997-02-27
1998-09-22
Moskowitz, Nelson
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Well logging
367 53, 367 73, 364422, G01V 120, E21B 4709
Patent
active
058124934
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a method of representing the trajectory of at least one borehole in a space-time domain and more particularly a method of tying the trajectory of a side tracked borehole in a block of time-migrated seismic data.
Tying a borehole in the 2D or 3D seismic data is necessary when it is required to project onto a two-dimensional or three-dimensional seismic representation of the medium in which the borehole has been drilled all the information relating to diverse data recorded locally in or in the proximity of the borehole, usually called logs. This applies to information relating to sonic, density, resistivity, etc logs at the depth of the geological strata of said medium, at the biostrategraphic limits, at the dips of the reflectors defined in particular by the interface between two consecutive layers, at the azimuths or even at the images produced by the various borehole seismic survey techniques.
Tying is therefore effected at least
a) when, in processing seismic data, the borehole(s) are used as the reference in a zero-phase deconvolution or an AVO (Amplitude Versus Offset) calibration or when the velocities in the borehole are used in combination with the stacking velocities to construct migration models,
b) when the interpreter begins an interpretation to determine which seismic horizon corresponds to a given geological marker or reflector,
c) when the depth representations produced in a time-depth conversion must be tied to the borehole in terms of depth and dip.
Tying can be done in the depth domain, in the interval velocity domain or in the time domain.
Tying in the depth domain requires a depth model or a series of depth maps (or representations) consistent with the depth at the position of the borehole.
Tying in the interval velocity domain is required to obtain a velocity model that comprises interval velocities consistent with the interval velocities estimated for the borehole by a calibrated sonic log or any other method. This is generally the case for the models used for migration.
When the tying is done in the time domain, the trajectory of the borehole, which is defined in terms of depth, must be represented in the time domain. It is this type of tying that raises a real problem.
When it is desired to compare a series of attributes or of values to attach to each depthwise point of the trajectory of the borehole relating, for example, to the sonic, density, electrical logs at the limits of the strata, at the dips, at the azimuths, etc, the ideal would be to represent the seismic image in depth, i.e. to perform a 3D depth migration, and to do the tying in this "natural" domain. However, this requires a precise knowledge of the velocity field of the medium. In practice, the trajectory of the borehole is projected into the migrated time domain (X, Y, T). To do this, it is assumed that the migration has displaced the information to its correct X, Y position and that the migrated time is equal to a vertical traveltime. It follows that the log in the time domain is a vertically stretched version of the depth log, the depth-time relation being derived in general from integration of the calibrated sonic log by check-shot survey or by the first arrival times of a vertical seismic profile, for example. Unfortunately, the vertical stretching and the X, Y repositioning by the time migration are approximations that are too broad or coarse when a precise result is required (for example to delineate a reservoir) or in the case of a complex or tectonic environment.
It can be seen that an essential first operation is to produce a depth propagation velocity model.
Various techniques are used or have been proposed. Among those most frequently used, that relating to time-depth vertical conversion, that using migration along the image radius (the so-called HUBRAL correction), that using migration along the normal radius, that using tomography or including inversion of the stacking velocities and that using the so-called "coherency" method may be cited.
Time-depth vertical conversion an
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Kaculini Stefan
Robein Etienne
Elf Aquitaine Production
Moskowitz Nelson
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