4D seismic monitoring of the Girassol field (Angola): enhanced interpretation of production mechanisms through impedance inversion

Abstract

4D-seismic is regarded with increasing interest as a strategic source of dynamic information, and as an added value monitoring tool to optimise reservoir management decisions. The actual value of the 4D-seismic information relies on the possibility to bring it early enough in the project life to impact the development plan, and to participate in the construction of a reliable Reservoir Model. Two seismic monitors have been acquired on Girassol- Jasmim, as part of a seismic monitoring project. The 4D processing and interpretation workflow we have developed has allowed us to start exploiting the essential of 4D information very early after the monitor acquisition. This key dynamic information has been used to constrain the reservoir model history match. The integration of 4D information has impacted fundamental decisions, like the drilling of a development well on a poorly drained area identified on 4D data. Girassol-Jasmim 4D experience proves that time-lapse seismic brings added-value information all along the project life. 4D can effectively help optimising the development wells locations. The additional reserves associated to wells targeting undrained panels, the optimisation of development well locations and the increase in reliability of the Reservoir Model (basis for the reservoir management decisions) largely justify 4D expenses. The main goal of this work is to deconvolve, at least partially, pressure and saturation changes from 4D data. To advance in this direction, we have inverted seismic traces to extract reliable P and S time-lapse impedance changes through 4D-seismic multi-offset simultaneous inversion. Combined with the results of time shift inversion process we have been able to split the P Impedance changes into Velocity and density parts. As Pressure does not impact the density in first approximation it is theoretically possible to approach pressure and saturation evolutions within the reservoir. A first step of well inversion tie have been performed and shows very consistent results..