A common practice in the oil and gas industry is to calculate horizontal stress magnitudes based on elastic properties derived from acoustic logs or seismic velocities. The derived stress variations are then used, among other things, for sweet spot identification, hydraulic fracture modeling and packer placement decisions in horizontal wells. However, the validity of the models depends on the applicability of the underlying assumptions, which include that the earth is a one-dimensional, horizontally layered elastic medium subject to constant lateral strain with properties that do not change with time. In this paper we use finite element analysis to quantify the stresses that develop for perfectly elastic time-invariant layered materials under gravitational loading with zero lateral strain, and investigate the effect on the calculated stresses of laterally varying mechanical properties and of tilted layers. We find that the predictions of the simple model are significantly influenced both by layer tilt and lateral elastic properties variations. Titled layers have smoother vertical stress profiles compared to the profiles of perfectly flat layers. Layers with laterally varying mechanical properties also have laterally varying stresses, but the simple model over-predicts these variations and in some cases lateral variations in the ratios of least to greatest stress trend opposite to estimates based on elastic properties alone. Because even the smallest departure from the underlying assumptions results in errors in predictions, we conclude that in most cases it is risky to quantify in-situ stress magnitudes based on elastic properties.
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