A finite‐difference injection approach to modeling seismic fluid flow monitoring

Abstract

A highly efficient and accurate tool for predicting the seismic response of reservoir fluid flow has been developed which integrates the finite‐difference injection method with a reservoir simulator and a petrophysical model. Finite‐difference methods allow for the full response to be synthesized as the wavefield interacts with a seismic model. This includes wave propagation in arbitrary heterogeneous anisotropic and anelastic media, scattering, and mode conversions. The finite‐difference injection method, in turn, can be used to efficiently synthesize the seismic response from models after local alterations to the model. Thus, it is ideally suited for time‐lapse seismic studies. The modeling methodology is demonstrated on a case study from the Gullfaks field in the North Sea. Six complete marine seismic surveys over the reservoir at different stages during waterflood oil production were synthesized. A total of 180 shot gathers were synthesized with computational savings of a factor of 54 after one single full simulation. The computational savings for the analogous 3-D study are 370 or greater after the initial simulation. The surface seismic response acquired along a towed streamer was processed through to stack and migrated. In a noise‐free environment the replacement of oil by water at a constant pressure caused visible changes in the synthetic seismic response that closely correspond to the impedance changes in the reservoir because of fluid flow. Downhole permanent sensor or vertical seismic profiling configurations were also considered; they provided a particularly suitable acquisition geometry for time‐lapse seismic monitoring. The recorded wavefields during and before production were greatly different (comparable to the magnitude of the wavefield itself). Moreover, multicomponent measurements may allow for elimination of changes attributable to environmental effects in the overburden and source characteristics. The study also indicates that monitoring the phase change of a reflector below a reservoir may provide a fluid flow indicator. The simulation technique thus provides an important tool for designing downhole surveys and deploying permanent sensors.