Imaging from Nearly Horizontal Wells Under Complex Overburden Conditions
Willis, Mark¹; Peterman, Paul²; Pei, Donghong¹; Zhou, Ran¹; Cornish, Bruce¹
¹Halliburton, Houston, TX.
²Halliburton, Highlands Ranch, CO.
In areas with complicated overburden issues, interpreting images created using conventional reflection surface seismic techniques can be challenging. Features such as gas clouds will dim out P wave reflection events (e.g., Bohai Bay, China). Transition zone acquisition in an active deltaic system with rapid sediment deposition and oceanic tides (e.g., Mahakam Delta, Indonesia) will give rise to structural distortions and time variant statics. Seismic data collected in an inclined or nearly horizontal deep borehole using surface seismic sources (VSPs) may provide an option to obtain an uplift in reflection image quality. Traditional VSP migration imaging methods can create good images below the borehole if the velocity field is well- known from the surface down through the reservoir depth. However, we show that the distorting overburden effects can be removed using interferometric processing techniques without having to derive or know the properties of these complicating structures. Since the imaging process uses the reference point of the borehole, only the relative effects away from the borehole need to be considered and not the full travel path through the distorting overburden strata. Images below the borehole are created from direct energy traveling downward past the borehole which acts as if it is a virtual source within the borehole. Images shallower than the borehole are created using energy from reflections deeper than the borehole that travel upward past by the borehole and therefore act as an upward directed virtual source in the borehole. Comparisons are made between a conventional migration result and the interferometric images created above and below the borehole. Images below the borehole are more easily obtained as they are formed with primary (reflected) energy. Surprisingly, the lateral extent of the images formed above the borehole is large, but they are more difficult to obtain since they are created from "double bounce" interbed multiple reflections. We show that a reasonably good image is possible to obtain, even with a small number of shots (less than 20).
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012