Borehole Acoustic Imaging Using 3d Stc And Ray Tracing To Determine Far-Field Reflector Dip And Azimuth
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A new sonic imaging technique uses azimuthal receivers to determine individual reflector locations and attributes such as the dip and azimuth of formation layer boundaries, fractures, and faults. From the filtered waveform measurements, an automatic time pick and event localization procedure is used to collect possible reflected arrival events. An automatic ray tracing and 3D slowness time coherence (STC) procedure is used to determine the ray path type of the arrival event and the reflector azimuth. The angle of incidence of the reflected arrival is related to the relative dip, and the moveout in 3D across the individual sensors is related to the azimuthal orientation of the reflector. This information is then used to produce a 3D structural map of the reflector which can be readily used for further geomodeling.
This new technique addresses several shortcomings in the current state-of-the-art sonic imaging services within the industry. Similar to seismic processing, the current sonic imaging workflow consists of iteratively testing migration parameters to obtain a 2D image representing a plane in line with the desired receiver array. The image is then interpreted for features, which is often subjective in nature and does not directly provide quantitative results for the discrete reflections. The technique presented here, besides providing appropriate parameter values for the migration workflow, further complements the migration image by providing dip and azimuth for each event that can be used in further downstream boundary or discontinuity characterization.
A field example is presented from the Middle East in which a carbonate reservoir was examined using this technique and subsequently integrated with wellbore images to provide insight to the structural geological setting, which was lacking seismic data due to surface constraints. Structural dips were picked in the lower zone of the main hole and used to update the orientation of stratigraphic well tops along the well trajectory. 3D surfaces were then created and projected from the main hole to the sidetrack to check for structural conformity. One of the projected surfaces from the main hole matched the expected depth of the well top in the sidetrack but two were offset due to the possible presence of a fault. This was confirmed by parallel evaluation of the azimuthal sonic imaging data acquired in the main hole that showed an abrupt change in the relative dip of reflectors above and below the possible fault plane using the 3D STC and ray tracing. Dip patterns from both wells showed a drag effect around the offset well tops, further confirming the presence of a fault. A comparison of the acquired borehole images pinpointed the depth and orientation of the fault cutting both wells to explain the depth offset of the projected 3D well top surfaces.
Nicholas Bennett is currently a Principal Research Scientist at Schlumberger-Doll Research Center, Cambridge, Massachusetts where he has been working since completing his Ph.D. in Mathematics from Yale University in 1997 with Professor Ronald Coifman. Nick’s main activities have involved acoustics, particularly sonic imaging, LWD conveyance of nuclear magnetic resonance measurements, and imaging using LWD electromagnetic measurements.