HIGH-RESOLUTION CROSSWELL IMAGING OF A WEST TEXAS CARBONATE RESERVOIR .3. WAVE-FIELD SEPARATION OF REFLECTIONS

Using crosswell data collected at a depth of about 3000 ft (900 m) in West Texas carbonates, one of the first well-to-well reflection images of an oil reservoir was produced. The P and S brute stack reflection images created after wavefield separation tied the sonic logs and exhibited a vertical resolution that was comparable to well log resolution. Both brute stacks demonstrated continuity of several reflectors known to be continuous from log control and also imaged an angular unconformity that was not detected in log correlations or in surface seismic profiling. The brute stacks, particularly the S-wave reflection image, also exhibited imaging artifacts. We found that multichannel wavefield separation filters that attenuated interfering wavemodes were a critical component in producing high-resolution reflection images. In this study, the most important elements for an effective wavefield separation were the time-alignment of seismic arrivals prior to filter application and the implementation of wavefield-separation filters in multiple domains, particularly in common offset domain. The effectiveness of the multichannel filtering was enhanced through the use of extremely fine wellbore sampling intervals. In this study, 2.5 ft (0.76 m) vertical sampling intervals for both source and receiver were used, whereas most previous crosswell data sets were collected with much coarser sampling intervals, resulting in spatial aliasing and limiting the utility of the data for reflection processing. The wavefield separation techniques employed in this study used data volumes and associated filtering operations that were several orders of magnitude larger than those encountered in conventional VSP data analysis.