The role of moisture adsorption in wellbore stability of shale formations: Mechanism and modeling

Understanding the interaction of drilling fluid with a shale formation is a critical step to properly describe pore pressure distribution, which directly affects wellbore stability. In this study, the moisture adsorption to shale surfaces is investigated to identify the proper isotherm type curve. It is shown that the moisture content of shale is correlated with water activity using a multilayer adsorption theory. It is found that the GAB model (developed by Guggenheim, Anderson and De Boer) fairly well describes the adsorption process for the selected shale types. Accordingly, the moisture content is correlated with water activity through the GAB model, which is incorporated into the transport equations. The adsorption parameter signifies the characteristic of a specific shale that interacts with aqueous fluids and can be introduced as an index to characterize different shale formations. In this study, the constitutive equations are generalized to consider the case of non-ideal solutions. The coupled transport equations are solved using a finite difference method and numerical computations are conducted to predict the stability of the wellbore. Having developed a transient model that predicts the instantaneous moisture content around the wellbore, it is possible to update the rock compressive strength as a function of its moisture content using available empirical correlations in the literature. The results of this analysis indicate that the range of safe mud weight reduces substantially due to moisture adsorption. Comparisons of several compressive failure criteria indicate that stability reduction of the wellbore due to moisture transport is a common pattern based on the selected criteria. The results of this investigation assist in drilling fluid optimization and address wellbore stability issues in troublesome shale formations. (C) 2015 Elsevier B.V. All rights reserved.