Abstract

Onshore seismic monitoring for CO2 injection in carbonate reservoirs in the Middle East is a major challenge for many reasons. The 4D signal is generally much smaller than that of clastic or chalk reservoirs due to the high bulk moduli of the rocks and the relatively small fluid effect. In addition, seismic data are characterized by low signal-to-noise ratio (S/N) due to poor signal penetration below high-contrast near-surface layers, poorly consolidated materials at the surface, conversion of source energy into surface waves and trapped modes, and scattering from near-surface complexities. Seasonal variations in surface conditions combined with low S/N make acquisition and processing of 4D seismic data some of the greatest challenges in geophysics today. We show results from feasibility and field pilot seismic programs for enhanced oil recovery (EOR) monitoring that resulted in successful imaging of CO2 injection in such a challenging onshore environment and achieved 4D metrics comparable with offshore seismic monitoring. The final acquisition choice included a hybrid surface source/buried receiver system with point sensors and sources based on cost and effectiveness of the various technologies tested. Acquisition included continuous monitoring with a full 3D survey acquired once every four weeks for a period of several years. Burying sparser receivers with dense source coverage minimized 4D noise to acceptable levels (< 5% normalized root mean square) and allowed fluid saturation changes from CO2 EOR to be observed.

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