Mar 2004 Abstract

Unocal's 4-C Experience: The Good, The Bad, and the Ugly

Unocal, through its Corporate Technology Plan, had a program, during 1998 and 1999, to develop multi-component technology in the following areas:

The Attaka 4C-3D pilot survey was acquired over the Attaka Oil Field to test the application of multi-component technology to solve a gas cloud problem.  Discovered in 1971, the development of the Attaka field has been plagued by the gas cloud that covers the core field area.  The loss of a platform by a gas blowout in 1997 prompted the BU to acquire a pilot 4C-3D survey around the area of the planned replacement platform.  A 3D VSP was simultaneously acquired in a well within the full-fold coverage area using the 4C shots as sources.  At this point the results looked encouraging.  However, an in-depth look at smaller but significant faults located within the 4C imaged area show they are not imaged well.  The PS data set is significantly better than the PP data and the original marine streamer data but, at this time, it falls short of meeting our geophysical needs.  Therefore, using the technique is not commercially viable at this time.

The Thai 4C-2D survey was obtained to increase the seismic bandwidth to image thin reservoir sands, 10 to 25 ft. thick.  Conventional marine streamer seismic did not provide this resolution.  Additionally, the business unit expanded their plan to include shear wave recording to address the problem of low P-wave impedance contrasts between the sands and shales.  A four line 2D program was acquired to test different imaging problems that existed in the area.  Line 1 in the Dara area was shot to study a gas cloud problem.  Line 2 over Erawan looked at low impedance P-wave contrasts in the gray bed reservoir.  Lines 3 and 4 were shot at the location of the proposed Trat platform to test finer resolution of the of thin sand beds in red bed section.  A 3C-VSP was acquired to calibrate the Trat lines.  The PZ data is better or at least equal to the marine streamer data.  The PS data provide better imaging in both the red and gray bed sections.  However, the technique is not commercially viable because (1) there is a high drilling success rate, (2) very low cost drilling and (3) a short life cycle of the producing wells.  If and when any of these factors change, 4C acquisition may become the geophysical tool of choice.

The most challenging survey Unocal attempted was in the McArthur Field located in the Cook Inlet, Alaska (Fig. 12).  The goal was to image through a gas chimney that created a large time sag in the streamer data in addition to degrading the image quality.  A major logistical issue was dealing with the large tidal currents as great as 6 knots with 10 m tides that limited shoot to times of slack tide.  It obviously was a logistical challenge to hold our cable in place between slack tides as it took 3 to 4 slack tides to layout, shoot and pickup the cable.  Prior to the survey, a dual offset VSP was acquired in a deviated well bore.  The near offset survey had both upgoing and downgoing travel paths within the gas chimney.  The far offset VSP was designed to keep the downgoing travel paths mostly outside the gas cloud.  The plan was to use these two VSPs to help calibrate the 4C processing of the OBC dataset.  Processing of both the VSP and OBC data turned out to be challenging.  For the OBC data, a strongly sloping water bottom, very noisy data and large statics were obstacles that had to be overcome.  At best, the dominant frequency of the PS data is 30 hz in PP time.  The PP data has much better frequency content than the PS data set.  Why?  That question puzzles us to this day.  Coupling of the horizontal phones is the primary suspect as we are left with quite a lot of energy on the transverse phones even after rotation to the inline and crossline components.  At this time, the needs of the business unit were not meet.  Data quality would have to be improved to the level of the marine streamer data outside of the gas cloud to impact our current drilling program.

Conclusions

Biography