Acquiring and processing deep dual-well DAS walkaway VSP in an onshore desert environment

Abstract

This study describes the first successful dual-well distributed acoustic sensing (DAS) vertical seismic profiling (VSP) walkaway acquisition in two 4 km deep wells in an onshore desert environment in the Middle East. A dual-fiber interrogator enabled efficient recording of high-channel-count walkaway VSP that was suitable for robust velocity model building and imaging. A good signal-to-noise ratio (S/N) was achieved due to three main factors. First, the fiber cable that was deployed outside the tubing provided good receiver coupling. Second, a gauge length of 24 m was used as a natural receiver array, boosting the S/N. Finally, 16 sweeps per shotpoint were performed using two vibrators in the source array. The selection of optimal acquisition parameters enabled on-tubing fibers to record high-quality seismic signals down to about 4 km. Each shot was recorded by two fibers deployed in wells that are 1.5 km apart. Even with high-quality casing cementation, DAS records exhibited reduced sensitivity/coupling in the shallower section, requiring low-pass filtering for robust first-break picking. The corridor stacks at the two wells show an excellent tie to the surface seismic and agree with the zero-offset VSP geophone corridor stack at one of the wells. A massive ensemble of first-arrival picks enabled multioffset traveltime inversion to reconstruct a reliable velocity profile, approaching the sonic log and overcoming the lower sensitivity of DAS measurement at higher well deviation angles. We used the inverted velocity model to migrate the upgoing reflection response at the two deep onshore wells. The results demonstrate the ability of the dual-well DAS recording to obtain a high-resolution VSP subsurface migrated section with an estimated vertical resolution of about 15 m of reflections down to about 4 km.