Impact of Undrained Pore Pressure Response on Expected Failure Stress in Anisotropic Shales

Abstract

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.

The Anisotropic Behavior of a Clay Shale: Strength, Hydro‐Mechanical Couplings and Failure Processes

“…Similar conclusions have been drawn by Duda et al. (2022) who accounted for the anisotropic pore pressure parameters resulting in substantial differences in modeled shear strength.…”

“…Therefore, when performing undrained tests, the different effective stress paths and failure stresses do not permit model fittings unless a considerable amount of tests have been performed under various effective consolidation stresses resulting in shear stresses at equal mean effective stresses for different loading constellations. Similar conclusions have been drawn by Duda et al (2022) who accounted for the anisotropic pore pressure parameters resulting in substantial differences in modeled shear strength.…”

“…For shales and similar rocks, many laboratory studies have investigated the anisotropic mechanical and hydro-mechanical properties including the elastic constants (e.g., Kim et al, 2012;Sarout et al, 2017), the poroelastic behavior (Duda et al, 2021;Horsrud et al, 1998;Islam & Skalle, 2013), and the compressive shear strength (Ambrose et al, 2014;Attewell & Sandford, 1974;Bonnelye et al, 2017b;Chenevert & Gatlin, 1965;Horsrud et al, 1998;Ibanez & Kronenberg, 1993;Islam & Skalle, 2013;McLamore & Gray, 1967;Minardi et al, 2021;Niandou et al, 1997;Wild & Amann, 2018a). Many previous studies conducted uniaxial compressive and indirect tensile strength tests under various loading configurations (e.g., Ajalloeian & Lashkaripour, 2000;Cho et al, 2012;Debecker & Vervoort, 2009;Karakul et al, 2010) and have shown that rocks exhibit strength anisotropies.…”

Anisotropic Poroelastic Modelling of Depletion-Induced Pore Pressure Changes in Valhall Overburden

“…For other overburden shales, less pronounced dominance of the poroelastic pore pressure parameters B 33 over its in-plane counterpart B 11 may also play a role in decreasing the quality of approximation. However, the comparison of Lista shale with other overburden shales described by Holt et al (2017Holt et al ( , 2018a, Lozovyi and Bauer (2019), Soldal et al (2021) and Duda et al (2022) suggests that Lista's pore pressure parameters lie within a range typical for relatively soft caprock shales (in terms of static moduli and strength). In the case of Lista shale, parameter B 11 = 0.53, parameter B 33 = 1.51 and the ratio between them B 11 /B 33 = 0.35.…”

“…Consequently, the distance between the Mohr circle and a potential failure envelope can be smaller than anticipated, as shown schematically in Fig. 2 (for more details see Duda et al 2022). This can strongly affect stability assessment of media of interest and in a field scenario can potentially lead to increased problems during drilling and with the integrity of existing wells, or to fault reactivation.…”