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A laboratory investigation was made of the effects of isotropic unloading on the mechanical behaviour of cemented sandstones, modelling in a simple way the influence of sampling on hydrocarbon reservoir sandstones that have been cemented at depth. Artificial samples of sandstone were created by allowing a cement to hydrate while a sand sample was under a high confining stress in a triaxial apparatus. Comparisons were then made between the behaviour of samples that had been unloaded to zero confining stress with that of samples tested at stress levels around that during cementing. It was found that the isotropic unloading damaged the cement, which had a significant effect on the small-strain and yielding behaviour but not on the strength and large-strain behaviour. Comparisons were also made with tests on natural samples of reservoir and analogue sandstones, and it was found that the artificial sandstone had reproduced the key elements of their behaviour. The intrinsic properties of all of the sandstones tested were found to be similar, although there were significant differences in the peak strengths and gross yielding envelopes. Cement content was found not to be the sole factor that influenced the peak and gross yield envelopes, and significant anisotropy was also revealed in one sandstone.

An analysis approach is described which includes the effects of rock fabric strength anisotropy on wellbore instability. When drilling in geologically complex conditions - shown to be commonplace in the Andean foothills of South America - bedding-plane slippage, resulting from an unfavourable interaction between in-situ stresses, the well trajectory and bedding, is shown to cause severe instability for certain well trajectories. Case-history examples from the Pedernales field (Venezuela) and Cusiana field (Colombia) are presented which confirm the analytical predictions.