Stress and pore pressure changes due to depletion of or injection into a reservoir are key elements in stability analysis of overburden shales. However, the undrained pore pressure response in shales is often neglected but needs to be considered because of their low permeability. Due to the anisotropic nature of shales, the orientation of both rock and stresses should be considered. To account for misalignment of the medium and the stress tensor, we used anisotropic poroelasticity theory to derive an angle-dependent expression for the pore pressure changes in transversely isotropic media under true-triaxial stress conditions. We experimentally estimated poroelastic pore pressure parameters of a shale from the Lista formation at the Valhall field. We combined the experimental results with finite element modelling to estimate the pore pressure development in the Valhall overburden over a period of nearly 40 years. The results indicate non-negligible pore pressure changes several hundred meters above the reservoir, as well as significant differences between pore pressure and effective stress estimates obtained using isotropic and anisotropic pore pressure parameters. We formulate a simple model approximating the undrained pore pressure response in low permeable overburden. Our results suggest that in the proximity of the reservoir the amplitude of the undrained pore pressure changes may be comparable to effective stresses. Combined with the findings of joint analysis of locations of casing deformation and total and effective stresses, the results suggest that pore pressure modelling may become an important element of casing collapse and caprock failure risk analysis and mitigation.

Undrained pore pressure response to stress changes in shales may have impact on drilling problems, fault reactivation and microseismicity observed at significant distances from hydrocarbon reservoirs and injection zones. This mechanism has not been thoroughly studied and is usually disregarded in geomechanical modelling workflows. We examine to what extent the inclusion of the pore pressure response affects the amplitude of total stress changes expected to cause shear failure in several overburden and outcrop shales. Poroelastic pore pressure coefficients and Mohr-Coulomb failure envelope parameters are estimated through triaxial laboratory experiments and are combined to model the total stress increases at failure for a wide range of loading scenarios and rock orientations. Differences in modeling output are explored on the basis of fully anisotropic and simplified isotropic pore pressure parameters as well as a case where pore pressure changes are neglected altogether. As a result, shear failure in shales is expected at significantly lower stresses changes and is plausible in a much wider range of loading paths once the undrained pressure response is considered. Therefore, we postulate that it should be taken into account during injection or production operations safety assessment.

Stresses and pore pressures change around depleting hydrocarbon reservoirs or fluid injection sites as result of reservoir pore pressure change. Such changes influence the stability of new wells drilled and may lead to fault activation, resulting in induced seismicity and possibly to casing damage in existing wells. Forecasting requires reliable geomechanical modeling of the reservoir and its surroundings. Classical analytical solutions are too simple, so numerical models are required. The rocks surrounding reservoirs are often anisotropic (like overburden shale), but their anisotropic elasticity is normally neglected, either because of model limitations, or in lack of input data. Here, however, the impact of anisotropy along with the elastic contrast between undrained surroundings and drained (isotropic) reservoir rock is addressed through a series of numerical scenarios. It is shown that isotropic models are unable to reproduce the stress changes predicted by the anisotropic models. Pore pressure changes may be underestimated by neglecting the anisotropic nature of the poroelastic Skempton parameters.