Experimental and numerical study on the impact of strain rate on failure characteristics of shales

The determination of safe mud weight windows during drilling operations and the understanding of shale deformation mechanisms requires accurate knowledge of shale strength characteristics. A key factor in measuring the compressive strength of shale is the strain rate used during laboratory testing. Two phenomena are attributed to strain rate related strength alteration: pore pressure build-up and dilatancy hardening.One of the main factors that influence the compressive strength of shale is its pore pressure. Due to low shale permeability, pore pressure of high water content shales usually builds up during high axial loading. A theoretical analysis on the influence of confining and pore pressures on the deviatoric strength of shale is presented in this study. Additionally, a model to predict pore pressure distribution within shale samples during a typical triaxial compression test is developed. The effects of strain rate and permeability on pore pressure build-up, and thereby the compressive strength are assessed.It was also observed that low water content shales experience a strength increase at high shear rates. This may be due to the forming of micro-cracks within the shale network that leads to dilatancy and thus pore pressure reduction.Experimental results for two preserved shale samples obtained from the field are presented. It is shown that strain rates have different effects on the compressive strength for the two shale types. The deviatoric strength for the soft Pierre I shale decreases, while the strength for the highly compacted Arco shale increases with increasing strain rates. The reasons for these observed phenomena are analyzed, and their impacts on drilling operations are briefly discussed.