Summary A particle‐based model for the simulation of wave propagation is presented. The model is based on solid‐state physics principles and considers a piece of rock to be a Hookean material composed of discrete particles representing fundamental intact rock units. These particles interact at their contact points and experience reversible elastic forces proportional to their displacement from equilibrium. Particles are followed through space by numerically solving their equations of motion. We demonstrate that a numerical implementation of this scheme is capable of modelling the propagation of elastic waves through heterogeneous isotropic media. The results obtained are compared with a high‐order finite difference solution to the wave equation. The method is found to be accurate, and thus offers an alternative to traditional continuum‐based wave simulators.

[1] In the upper 5 -10 km of the crust fractures may remain partially open, exerting a strong influence over the response of their host material to seismic waves due to their superior deformability. This raises the possibility of using seismic data to determine not only the location but also the properties of fracture networks. We present a particle-based numerical scheme for modeling rock deformation and wave propagation and show that it accurately captures wave propagation across fractures. We investigate the effect of fracture tensile properties (i.e. fracture cohesion) on the wavefield and demonstrate that seismic data can, in theory, be used to distinguish fracture tensile and compressional properties.

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Imaging and characterizing time-lapse changes in reservoir properties rely on accurate estimation of the corresponding time shifts and amplitude changes. We evaluated the accuracy of competing time-shift extraction algorithms — cross correlation, nonlinear inversion, dynamic warping, the correlated-leakage method, and the optical-flow-warping method — for estimating 4D time shifts, particularly from subsalt reservoirs. A synthetic model of a Gulf of Mexico field was used for testing these methods. The model contains both subsalt and extra-salt reservoirs. We imposed 4D velocity changes within both reservoirs and in the overburden of the subsalt reservoir. Each of the time-shift extraction methods was evaluated, and each was found to have various strengths and weaknesses. The correlated-leakage and optical-flow-warping methods reproduced the model time shifts most accurately and with the best resolution.

Pressure depletion during production from a reservoir can cause geomechanical changes in both the reservoir and overburden. Dropping the pore pressure in the reservoir increases the load on the rock matrix, resulting in compaction. In most cases, the magnitude of this compaction is negligible; however, when depletion pressures are large or when the reservoir rock is highly compressible, the compaction may be significant. As the reservoir pulls away from the surface of the earth, a stress arch forms in the overburden, with the vertical stress decreasing directly above the reservoir and increasing in the pillars of the stress arch. These stress changes can perturb the seismic velocities, which, combined with the changes in path length to the reservoir, can cause traveltime changes in the overburden. The unloading of the overburden directly above the reservoir shields the reservoir from seeing the full change in pore-fluid pressures and thus can have a significant impact on the amplitude changes in the reservoir between repeat seismic surveys. We describe a workflow for rapidly modeling these geomechanical changes and their associated seismic signatures. Using a set of simple synthetic reservoir models, we demonstrate the impact of reservoir aspect ratio (thickness to diameter ratio) and dip on the magnitude of the stress arch. Finally, we present a case study from the Gulf of Mexico that demonstrates the importance of including geomechanical changes in 4D modeling and interpretation.