Interactions between aqueous drilling fluids and clay minerals have been identified as an important factor in wellbore instability of shale formations. Current wellbore stability models consider the interactions between aqueous drilling fluids and pore fluid but their interactions with shale matrix is neglected. This study provides a realistic method to incorporate the interaction mechanism into wellbore stability analysis through laboratory experiment and mathematical modeling. The adsorption isotherms of two shale rocks, Catoosa shale and Mancos shale are obtained. The adsorption isotherms of the selected shales are compared with those of other shale types in the literature. It is shown that the adsorption isotherms of various shale rocks can be scaled using their respective cation exchange capacity, into a single adsorption curve. The experimental results show that the moisture content of shale is correlated with water activity using a multilayer adsorption theory. The adsorption parameters can be suggested as an index to characterize different shale formations. This study shows that the adsorption theory can be used to generalize wellbore stability problem in order to consider the case of non-ideal drilling fluids. Furthermore, the adsorption model can be combined with empirical correlations to update the compressive strength of shale under downhole conditions. Accordingly, a chemo-poro-elastic wellbore stability simulator is developed to explore the stability of transversely isotropic shale formations. The coupled transport equations are solved using an implicit finite difference method. The results of this study indicate that the range of safe mud weight reduces due to the moisture adsorption phenomenon.
Shale rocks are often regarded as weak rocks in terms of compressive and tensile strength. It is known that presence of bedding planes and lamination imparts anisotropy to mechanical properties of rocks. The effect of bedding plane on rock failure has long been recognized [1-3]. Drilling through shale formations is often associated with borehole instability problems. York et al. (2009) reported that the wellbore instability issues in a well with 20,000 ft MD could at least cost 2.5 million dollars . Hamayun (2011) discussed the results of a survey that indicates about one third of Non-Productive Time (NPT) of drilling operations is spent on wellbore problems, of which a major portion is attributed to the wellbore instability issues . Hydration of clay minerals is recognized to be one of the important interaction processes between shale and drilling fluid during drilling process, which often leads to various operational problems such as shale swelling, stuck pipe, reduction of rate of penetration [6, 7]. Usually two kinds of clay swelling is realized, namely, the interacrystalline swelling due to the hydration of exchangeable cations and osmotic swelling which occurs due to a large difference between ion concentration (or water activity) of shale and aqueous fluids [8-10].