The effects of chemical adsorption on wellbore stability in transversely isotropic shale formations

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 the case of reactive shale formations, it has been shown that clay minerals have a tendency to adsorb water from an aqueous fluid. There are some experimental studies in the literature that focused on the adsorption phenomena in shale rocks, while few attempts have been made to model the phenomenon. In this study, the moisture adsorption to shale surfaces is investigated to identify the proper isotherm type curve. The experimental results show 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. It is found that the adsorption parameter can be introduced as an index to characterize different shale formations.;Using a concentrated drilling fluid, available transport models that are based on the ideal solution assumption fail to properly address the true shale-drilling fluid interaction. This study shows that the adsorption theory can be used to generalize the transport equations in order 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 wellbore stability in shale formations. Having developed a transient model that predicts the instantaneous moisture content around the wellbore, it is possible to update the compressive strength of rock 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 the moisture adsorption. Comparisons of several compressive failure criteria indicate that stability reduction of the wellbore due to moisture transport is a common pattern regardless of the selected criteria.;The interactions between shale and aqueous fluid are investigated experimentally through Shale-Fluid Interaction Testing Cell which enabled us to observe the pore pressure response of shale samples once they were exposed to various aqueous solutions. Altogether, three types of shale and eight types of fluids were tested. In addition, our experimental investigation led us to formulate an aqueous fluid to be used for drilling operations through shale and clay-bearing formations. The aqueous fluid consists of pore plugging agents which are chemically active. The experimental results indicate that these agents reduce the permeability of the shale samples significantly compared with the conventional additives. It is also believed that the inhibitive mechanism is a combination of plugging the clay spacing and adsorption of these agents to the clay minerals which hinders the pore pressure propagation. A.;Chemo-poro-elastic wellbore stability model considering both isotropic and anisotropic rock strength is also developed. Our analysis indicates that the in-situ stress regime, weak plane orientation, adsorption tendency of shale and transport coefficients carry the most important roles in determining the mud weight window. The results of this investigation assist in drilling fluid design and address wellbore stability issues in troublesome shale formations.