Shale instability involves fully coupled chemo-mechanical phenomena, in which fundamental intermolecular surface forces are acting between the clay surfaces. These forces are function of the composition, type, mount, and microfabric of the clay content in the rock, which are responsible for the global shale behavior. Thus, the knowledge of these features is fundamental to explain their swelling behavior. This paper presents the results of water distribution analyses of seven samples of shale from different oil fields along with their X-ray diffraction, in order to establish the differences in both composition and swelling behavior of such samples. The X-ray diffraction analyses show clearly the differences of swelling clay content between shale samples. Thermal gravimetric analyses also validate that a simple analysis of the water loss below 100 øC gives a good insight about the composition and swelling behavior of shales. It also show that the water loss below 55 øC, is strongly dependent of the activity; and that this dependence quickly decreases as temperature increases, which suggest that the role of activity in the swelling behavior of shale only takes place at lower temperatures.


Oil and gas wells represent the fundamental infrastructure in hydrocarbon exploitation. Drilling the wellbore is the first and, usually, the most expensive step in oil and gas production activities. As a consequence, designing a stable and safe well has become a critical issue in petroleum engineering. Wellbore stability, mainly in shales, is one of the important problems encountered during drilling. More than one hundred million dollars are spent worldwide each year in remedial works. In the past, one solution has been to use oil-base muds, which can often eliminate the swelling problem associated with shales; however, environmental concerns increasingly restrict their use, and in many cases water-base muds are now required. The development of improved water-base mud for shale stability is the primary concern in the drilling industry. Wellbore stability has occupied the attention of many researchers this past decade and some discrepancies have been reported, not only in their chemomechanical approach, but also in the understanding of their hydration behavior, and their characterization.

Shale are sedimentary rocks that have distinct laminated layered characteristics and high clay content. The chemical processes, responsible for this formation must be understood in terms of two mechanisms: 1) neoformation. actual precipitation from solution; and, 2) transformation. whereby a new clay mineral inherits a significant part of its silicate skeleton from preexisting minerals, usually a phyllosilicate.

These processes include chemical weathering in soils, formation of authigenic minerals at the sediments depositional site, formation of diagenetic minerals after deposition, and clay minerals formed by hydrothermal alteration. Shales are, therefore, subjected to phenomena such as hydration, swelling, slu'inking, and strength reduction when exposed to water and ions. The mechanisms controlling these reactions are very complex and are not fully understood. These reactions result from the hydrophilic nature of the clay particles, which are somewhat, altered by both the chemical and mechanical environments. The chemical effects are due to the intermolecular forces between the clay particles and the ionic pore fluid inside the shale, as well as the composition of the drilling fluid. However, it has been recognized that the type and amount of clay groups and subgroups play an important role in distinguishing different hydration behaviors, as a result of their charge deficiency location (silic

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