MULTILEVEL GEOPHONE TOOL STRINGS IMPROVE WELL SEISMIC REJECTS

William Barry CGG Logging Aberdeen, Scotland Glides Omnes CGG Massy, France Multilevel strings of geophone tools are changing the economics of well seismic because rig downtime is divided by almost the number of levels while data quality is considerably improved. Since the introduction of well seismic methods, check shots, vertical seismic profiles, and walkaway VSPs, the cost of rig downtime has been a major consideration. It has certainly limited the use of well seismic, particularly

Jan. 14, 1991

6 min read

William Barry
CGG Logging
Aberdeen, Scotland

Glides Omnes
CGG
Massy, France

Multilevel strings of geophone tools are changing the economics of well seismic because rig downtime is divided by almost the number of levels while data quality is considerably improved.

Since the introduction of well seismic methods, check shots, vertical seismic profiles, and walkaway VSPs, the cost of rig downtime has been a major consideration.

It has certainly limited the use of well seismic, particularly offshore.

Multilevel geophone tools were initially developed with the reduction of rig downtime in mind.1

Single level tools are now starting to be replaced by strings of tools.

Putting tools on a string offered an opportunity to improve their mechanical characteristics. Some of these improvements resulted from design, while others were due simply to the fact that a string of tools operates simultaneously at more than one level.

MULTILEVEL TOOL DESIGN

Multilock(TM) is an example of a three component, multilevel geophone tool (Fig. 1). It comprises:

A downhole telemetry unit (DTU) fitted on a hydraulic locking device.
It can handle a total of 12 or six channels (sampling interval 2 or 1 ms, respectively).
Light (33 kg) and short (go cm) tools called satellites, that are suspended beneath the locking devices and are equipped with three component geophones and a locking arm (anchoring force 49.8 kg in a 9.4 in. hole).

The tools can withstand pressures as high as 17,400 psi and temperatures as high as 350 F.

The specific characteristics of the strings of tools and the associated advantages in comparison with single level tools are summarized (Table 1).

The advantages of downhole digital telemetry are obvious, and other points are described in the following paragraphs.

RIG DOWNTIME REDUCTION

Comparison of the examples in Table 2 with single tool runs and zero offset multitool runs demonstrates some interesting rules of thumb, based essentially on experience from the North Sea.

Satellite tools multiply the speed of acquisition.
First, a rate of 12 levels/hr is typical when recording offshore with a single tool and source position on a VSP run of 70 levels or more.
With a four level Multilock, the rate reaches about 25 levels/hr, although the extra preparation and descent time that are required by the Multilock tool means that the rates become similar below 50 levels.
However, with one or more extra source positions, the acquisition rate actually increases. in the second example, a total of 400 levels was acquired in 10 hr even though the vessel had to reposition for each Multilock station.
Satellite tools improve quality control.
A major downhole problem for seismic recording is recognizing noise sources. Is the tool well clamped? Is the casing ringing?
Is the rig heaving, or is someone rearranging the pipe deck?
A geophone array quickly answers these questions, which often require minutes of waiting, reclamping, and moving a single tool. Noise from above can be seen to arrive later at the bottom of the array, noise moving with one tool in the array points suspicion at that tool, and so on.
Satellite tools improve cost-effectiveness.
A walkaway VSP line generally requires between eight and 12 geophone tool levels to image an area of interest.
To shoot each of these lines can require as long as 2 hr.
A four level tool needs only two or three runs, and the rig and boat time saved is often of greater value than the cost of the survey.

SUPERIOR COUPLING

When individual geophone tools form part of a string, it is possible to group the preamplifiers and the telemetry system in the uppermost container and thus to carry the geophones in shorter and lighter tools.

This results in superior coupling. The response curves of the components are flat and practically identical within the 10-1 25 Hz band (Fig. 2).

High quality seismograms can be produced in which the elastic propagation of seismic waves can be fully observed.

Consider two seismograms corresponding to an offset VSP (Fig. 3). Both the pressure waves and P-S converted waves are clearly visible and form a fine network of line-ups with different slopes (steeper for the P) corresponding to downgoing wavetrains (down-dipping to the right) and upgoing reflections (downdipping to the left).

With this quality of three component data it is possible to produce P and P-S wave images of the subsurface 3 and derive additional information concerning lithology, particularly azimuthal anisotropy associated with fractures.

ACCURATE TRANSIT TIMES

Because the distance between the individual geophone tools is fixed and accurately known and transit times can be measured on records generated by a common shot, the measured transit times are affected neither by depth inaccuracies nor by slight shifts of the time origin.

Transit times picked between elements of a string are compared with transit times picked on much higher frequency signal., 2-25 kHz vs. 10-100 Hz, 5 Its sampling rate vs. 2 ms (Fig. 4).

The overlaps corresponding to measurements repeated with different string elements show repeatability that is always better than 0.2 ms. When the string remains fixed, repeated shots show that transit times can be picked with an accuracy of 0.1 Ms.

NOISE SCREENING

Four mini-VSPs were recorded in a well with a four level string (Fig. 5).

The P wave first arrival is about 150 ms after the time origin. These arrivals are almost identical on all four tools.

The tube waves arrive at about 600 ms. They are obviously much stronger on the uppermost tool, and their amplitude decreases steadily from the second (S1) to the fourth (S3) tool.

This shielding effect is confirmed by a test in which no seismic source was activated, and the average noise levels in the different tools were compared (Table 3).

The results show that even when unlocked, the uppermost element shields the lower satellites from noise transmitted along the wire line.

CONCLUSIONS

As initially planned, multilevel strings of geophones divided rig downtime for well seismic acquisition by almost the number of levels.

In addition, the properties of multilevel designs greatly enhance the data quality:

Transit times can be determined with accuracy of 0.01 ms even with a 2 ms sampling interval.
The uppermost container protects the individual geophone sondes from tube waves and noise bursts transmitted by the wire line.
The geophone sondes can be made lighter and shorter; consequently coupling is superior.

REFERENCES

Hardage, B.A. Vertical Seismic Profiling. Part A: Principles, Geophysical Press, 1983.
Cliet, C., Laurent, Jr., Michin, D., Ollivier, Y., and Omnes, G., Properties of a string of downhole geophones other than faster data acquisition, expanded abstract, 59th annual international SEG meeting, 1989.
Omnes, G., Blanco, J., Renoux, P., Analysis and Applications of a Three Component Offset VSP in the North Sea, expanded abstract, 58th annual international SEG meeting, 1988.