Structural uncertainty without surface seismic data

Surface seismic surveys had not been conducted in a Middle East field because of surface constraints.  This lack of data created uncertainties in the structural setting for defining the appraisal target in the carbonate reservoir.

Automated time picking, ray tracing, and 3D STC

FMI microimager logs in the main well and sidetrack showed the depth and orientation of a possible fault cutting across both wells. To pinpoint the position of the fault away from the wells and deep into the reservoir, Schlumberger proposed conducting 3D far-field sonic service. The service applies a patented end-to-end workflow to data obtained with eight azimuthal sensors at 13 receiver stations of the Sonic Scanner platform.

The 3D far-field sonic service automates time picking to rapidly and consistently analyze thousands of shot gathers to identify reflection events. Ray tracing inversion of the time picks and 3D slowness-time-coherence (STC) analysis of the underlying arrival event determine each event’s type and position to efficiently guide the smart migration workflow. The resulting 3D sonic image produced  in the Techlog wellbore software platform is consistent with the event type and orientation of the true reflections, with their direct association providing strong quality control.

Fault projection from the wells across the reservoir

The results of 3D far-field sonic service and the near-wellbore image logs were integrated to trace the possible fault across the reservoir. One of the projected surfaces from the main well matched  the expected depth of the formation top in the sidetrack but two were offset due to the possible  presence of the fault.

The fault’s presence was confirmed by parallel evaluation of the azimuthal sonic-imaging data acquired in the main well that showed an abrupt change in the relative dip of reflectors above  and below the possible fault plane using the service’s 3D STC analysis and ray tracing. Dip  patterns from both wells also showed a drag effect around the offset formation tops, providing further confirmation.

With the fault’s position determined, the uncertainty in the structural model could be resolved in the Petrel E&P software platform.

For additional details, see N. Bennett et al., “Borehole Acoustic Imaging Using 3D STC  and Ray Tracing to Determine Far-Field Reflector Dip and Azimuth,” Petrophysics 60, no. 2 (April 2019), 335–347.