HIGH-RESOLUTION CROSSWELL SEISMIC EXPERIMENT WITH A LARGE INTERWELL SPACING IN A WEST TEXAS CARBONATE FIELD

A high-frequency crosswell seismic dataset acquired in a west Texas ca rbonate field has demonstrated the feasibility of the technique with a large interwell spacing. Two crosswell profiles were acquired with a well spacing of 1500 ft (460 m) and over a depth interval from 7700 ft (2350 m) to 9600 ft (2930 m) using a piezoelectric bender source. The data quality is profile and depth dependent, with the ambient noise l evel at the receiver position being the most important factor. Noise l evels and noise characteristics among three wells were significantly d ifferent. Tube waves and gas- and fluid-movement in the borehole are t he dominant noise sources found in the data set. Two lithologic proper ties, attenuation and transmission loss controlled the data quality. G ood quality and high frequency (>1000 Hz) data were acquired over most of the survey interval which contains massive limestones. However, we could not acquire any useful data within the shale layers. Transmissi on losses and the effects of the source radiation pattern that occurre d at interfaces with large impedance contrasts limited the aperture of the useful data. There were two critical issues encountered during. t he reflection imaging process: (1) sparse trace spacing and poor coher ency of the reflection events in common-source gathers degraded the im age in a region near the receiver well; and (2) possible lateral veloc ity heterogeneity in the medium and limited aperture made it difficult to build an appropriate velocity model for reflection imaging.