Walkaway VSP in ultra-shallow water images deep Paleozoic targets, offshore UAE

Abstract

In 2019, ADNOC decided to record walkabove and walkaway VSP surveys in a deep deviated well, in ultra-shallow waters offshore Abu Dhabi. The field is part of a major gas and condensate development project in UAE. The primary objectives included vertical travel times and velocities, robust well to seismic tie, and higher-resolution VSP images of the Palaeozoic-age targets. Other objectives included acoustic impedance inversion and anisotropy calibration. This was the first time a walkaway was acquired with only 2 m of water depth and in a deep hostile downhole environment. This study showed the importance of pre-survey modelling to achieve the VSP objectives. The data were acquired in continuous mode with GPS time stamping. Good data quality was recorded, despite high temperature, deep targets, multiple casings and a small volume seismic source deployed at 1 m depth. Wideband 5–90 Hz data were retrieved across 6 km of complex overburden, resulting in images with three-fold resolution improvement compared to the surface seismic data. The VSP also provided acoustic impedance sections of the deeper targets and measurements to optimize surface seismic processing. The results validate the viability of 3DVSP surveys with similar shallow-water source and downhole receiver array for reservoir characterization and well placement.