Synthetic Seismic Response Research Articles

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‐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.

P‐to‐S conversion for a thin anisotropic zone produced by vertical fracturing

Point‐source synthetic seismic responses for a thin, fractured bed are generated, interpreted and processed. The synthesis is carried out for a compressional source and multicomponent surface receivers. The anisotropy considered has hexagonal symmetry, with a horizontal symmetry axis, and represents oil‐ and gas‐filled, aligned vertical fractures for a broad range of fracture densities and aspect ratios. P‐to‐S reflected conversions recorded on the horizontal geophones show both kinematic and dynamic anomalies that increase with increasing fracture density and are only weakly dependent on aspect ratio. In contrast, the vertical component P‐wave reflections provide a much poorer diagnostic of fracturing. Analytic expressions for the eigenvalues and eigenvectors of a vertically fractured system are presented, that have the same simplicity as those for transverse isotropy. New linearized expressions for mode‐converted amplitudes are developed for small angles of incidence and are used to interpret the synthetic response.

Stratigraphic Framework and Well-Spacing"Controls on Reservoir Heterogeneity, Volumetric Calculations, and Synthetic-Seismic Response In a 3-D "Reservoir"Model: McKittrick Canyon, West Texas and New Mexico: ABSTRACTS"

Stratigraphic Framework and Well-Spacing"Controls on Reservoir Heterogeneity, Volumetric Calculations, and Synthetic-Seismic Response In a 3-D "Reservoir"Model: McKittrick Canyon, West Texas and New Mexico: ABSTRACTS"

Crustal structure and reflectivity of the Swiss Alps from three‐dimensional seismic modeling: 1. Helvetic Nappes

Forward modeling based on combined geologic surface data, borehole and laboratory data on velocities, and seismic reflection data aid in understanding the seismic response in a transect through complex Alpine nappe structures. The synthetic seismic response from the model compares well with the observed data. The results suggest, however, that strong three‐dimensional effects result in out‐of‐plane reflections and diffractions, as well as defocussing and scattering of seismic rays, thus rendering the use of two‐dimensional migration techniques questionable. The model shows an unexpected rapid change in internal structure of the Helvetic nappes between the seismic line and outcrops located 6–8 km farther west. Moreover, the basement‐cover interface on the northern flank of the Aar Massif, a basement uplift with a heave of 8 km, does not rise regularly in the hanging wall of a single major thrust fault, but rises in a series of steps due to several thrusts and folds. The northernmost structures appear to be fault‐related open folds, whereas tight fold structures appear to dominate in the south.