Accurate strength data are critical when determining safe mud weight windows during drilling, and also for understanding shale deformation mechanisms. 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.

Due to low shale permeability, pore pressure of high water content shales usually builds up during high axial loading. Pore pressure greatly influences shale strength. A theoretical analysis on the influence of confining and pore pressures on the deviatoric strength of shale is presented in this paper. Additionally, a model to predict pore pressure distribution within the shale sample 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.

On the other hand, certain rocks including low water content shales experience a strength increase at very high shear rates. This may be due to a form of micro-cracking that leads to dilatancy and thus reduction in pore pressure.

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.