The Impact of Layering and Heterogeneity on Stresses around Boreholes

An accurate understanding of orientation and magnitude of the stresses surrounding a borehole is decisive for the identification of a stable well path and the successful design of completions and stimulation measures including fracking stages. However, measurement of the in situ stress is challenging: Current engineering practice favours two approaches: 1) borehole break-out – drilling-induced fracture interpretation complemented by extended leak-off tests; 2) stress estimation from shear-wave slowness measured with advanced wireline tools. Both methods rely on the applicability of Kirsch’s (1898) stress perturbation equations and laboratory measurements of elastic moduli that are correlated with rock properties that can be logged. Method (2) can be applied where breakouts are absent. Yet little is known how lithologic layering and spatial variations in the mechanical properties of the rock affect its results and the detection limit of stress anisotropy. Here we present finite-element simulations of borehole-related stress perturbations in multi-layer composites with realistic scattering and spatial variations in elastic moduli. Using stress magnitude-shear wave correlations from the literature, travel times are calculated for borehole-parallel trajectories. These results are interpreted in terms of the minimum differential stress that needs to exist in order to be able to detect stress directions. The uncertainty of measurements of stress magnitudes obtained with this method is analysed as well. Our results show that the stress field around boreholes is strongly affected by lithological variations. “Ideal” Kirsch-compatible conditions where the well is aligned with one of the eigenvectors of the stress field, layers are perpendicular to the well, and far-field stresses are Andersonian is rare. More common scenarios are going to be illustrated with a series of simulations including deviated wells. These will also elucidate how rock stress responds to fluid pressure changes in the well.