PREDICTION OF ELASTIC-WAVE VELOCITIES IN SANDSTONES USING STRUCTURAL MODELS

Elastic-wave propagation in fluid-saturated sandstones depends upon two sets of rock features: (I) the volume fractions and elastic constants of the rock constituents (quartz, clay, water, etc.) and (2) microstructural geometry (grain contacts, pore aspect ratios). While the former data are usually obtainable, the latter are relatively inaccessible. We present a new method for determining microstructural data using idealized but physically representative models of sandstone. The key to the method is the simulation of certain depositional and diagenetic processes in a manner that completely specifies the geometry of the resulting models. Hence, the geometric features of the grain space and void space required for various theories of elastic propagation can be calculated directly from the models. We find good agreement between predictions and measurements of compressional- and shear-wave velocities in both clean and clay-bearing saturated sandstones. In contrast with previous efforts at predicting velocities, we use no adjustable parameters and require no additional measurements on samples, such as dry velocities or analysis of thin-section images. The results suggest that it is feasible to predict elastic velocities directly from geological models in the absence of rock samples.