A failure criterion for shale considering the anisotropy and hydration based on the shear slide failure model

A failure criterion fully considering the anisotropy and hydration of shale is essential for shale formation stability evaluation.Thus,a novel failure criterion for hydration shale is developed by using Jaeger’s shear failure criterion to describe the anisotropy and using the shear strength reduction caused by clay minerals hydration to evaluate the hydration.This failure criterion is defined with four parameters in Jaeger’s shear failure criterion(S_(1),S_(2),a andφ),three hydration parameters(k,ω_(sh)andσ_(s))and two material size parameters(d and l0).The physical meanings and determining procedures of these parameters are described.The accuracy and applicability of this failure criterion are examined using the published experimental data,showing a cohesive agreement between the predicted values and the testing results,R^(2)=0.916 and AAREP(average absolute relative error percentage)of 9.260%.The error(|D_(p)|)is then discussed considering the effects ofβ(angle between bedding plane versus axial loading),moisture content and confining pressure,presenting that|Dp|increases whenβis closer to 30°,and|D_(p)|decreases with decreasing moisture content and with increasing confining pressure.Moreover,|D_(p)|is demonstrated as being sensitive to S1and being steady with decrease in the data set whenβis 0°,30°,45°and 90°.

A review of the shale wellbore stability mechanism based on mechanicalechemical coupling theories

Wellbore instability in hard brittle shale is a critical topic related to the effective exploitation of shale gas resources.This review first introduces the physicalechemical coupling theories applied in shale wellbore stability research,including total water absorption method,equivalent pore pressure method,elasticity incremental method of total water potential and non-equilibrium thermodynamic method.Second,the influences of water activity,membrane efficiency,clay content and drilling fluid on shale wellbore instability are summarized.Results demonstrate that shale and drilling fluid interactions can be the critical factors affecting shale wellbore stability.The effects of thermodynamics and electrochemistry may also be considered in the future,especially the microscopic reaction of shale and drilling fluid interactions.An example of this reaction is the chemical reaction between shale components and drilling fluid.