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

A highly efficient and accurate tool for predicting the seismic response of reservoir fluid flow has been developed which integrates the finite-differ ence injection method with a reservoir simulator and a petrophysical model. Finite-difference methods allow for the full response to he synthesized as the wavefield interacts with a seismic model. This includes wave propagati on 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 lo cal alterations to the model. Thus, it is ideally suited for time-lapse sei smic studies. The modeling methodology is demonstrated on a case study from the Gullfaks held in the North Sea. Six complete marine seismic surveys over the reservo ir 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 f or 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 end migrated. In a noise-free environment the replacement of oil by water at a constant pressure caused visible changes in the synthe tic seismic response that closely correspond to the impedance changes in th e reservoir because of fluid flow. Downhole permanent sensor or vertical seismic profiling configurations were also considered: they provided a particularly suitable acquisition geometr y for time-lapse seismic monitoring. The recorded wavefields during and bef ore production were greatly different (comparable to the magnitude of the w avefield itself). Moreover, multicomponent measurements may allow for elimi nation of changes attributable to environmental effects in the overburden a nd sourer characteristics. The study also indicates that monitoring the pha se change of a reflector below a reservoir may provide a fluid flow indicat or. The simulation technique thus provides an important tool for designing downhole surveys and deploying permanent sensors.