Wafers are the new plugs: a novel technique to obtain strength parameters in shale under drained conditions

Argillaceous rocks are a highly effective geological barrier for the long-term containment of hydrocarbons and, more recently, CO2 and radioactive waste. In these scenarios, the long-term sealing integrity of argillaceous formations is of direct concern, as pore pressure changes in the underlying reservoir may result in mechanical damage of the caprock, due to a delayed response of low-permeability material to such pressure changes. In order to be able to model the slow drainage, and hence change of the effective stress state, mechanical parameters under drained conditions are needed. In general, such mechanical parameters are obtained during triaxial compression experiments under in-situ conditions. However, in the case of low-permeability rocks, such experiments are frustrated by slow drainage upon deformation induced pore pressure changes, leading to a heterogeneous internal effective stress state and only partially drained mechanical behavior. Though part of this problem can be solved using smaller samples (i.e. shorter drainage paths) and/or longer experimental times (days – weeks – months), this is not always a viable route. In an attempt to improve sample drainage during deformation, we developed a novel technique employing very thin intact rock wafers (1-2 mm thick), which tremendously improves drainage times. Subsequently, these wafers can be deformed under in-situ conditions, using a direct-shear configuration. We tested this new method on wafers of Opalinus Shale (κ < 10-20 m2, Peff = 15-50 MPa, T = 20°C, dry or wet with Pp = 10 MPa, ė ~ 5 • 10-4 s-1). Comparison of our strength data with published values obtained on plugs showed good agreement and that Terzaghi’s principle applies to our wet experiments, i.e. the material was fully drained. Therefore, we inferred that strength parameters obtained using this new method can be used to reliably delineate the effective stress conditions under which dry and wet, intact shale caprock will be stable versus unstable: so called failure envelopes. Such drained failure envelopes can be employed in mechanical modeling efforts to predict the deformation behavior of low-permeability rocks under naturally imposed strain rates (< 10-10 s-1).