Abstract

This paper describes an integrated methodology to characterize shales from an engineering point of view. Shale samples from offshore Brazil were used throughout the study. The proposed methodology includes laboratory tests designed to characterize the rock individual constituents, i.e., solid particles and pore fluid, and to describe the rock microstructure. Complementary laboratory tests on the same materials indicated low potential of reactivity when in contact with saline solutions in spite of the presence of highly expansive clay minerals. The paper suggests that other elements besides simply the characteristics of the clay minerals should be considered when evaluating the potential of reactivity of shales.

Introduction

The drilling of shaly sequences of inclined wellbores often needs constant interruptions to solve problems such as tight hole, excessive solids in the drilling fluid and bit balling. The consequences are: increase in drilling time and costs. Ten years old estimates indicate that these drilling problems, on average, are responsible for 20 to 30% of the drilling costs and, amongst these, 80 to 90% occur when drilling through shales1,2. Current drilling scenarios of highly inclined wells and large water depths have kept the industry searching for solutions to minimize the impact of such problems.

Cases of wellbore instabilities when drilling through shales have been reported for over 60 years3. The first attempts to face instabilities in shales were based in empirical approach in which the signs of instabilities were reported generating systems of classifications oriented towards identifying the eventual drilling problems4,5. Since then, it was established that the most likely cause of instabilities in shales were related to the clay minerals present in the shales and its expansive characteristics when exposed to the drilling fluids. Attempts to follow a rational approach to analyze problems of wellbore instabilities date from the 60s when experimental studies were first carried out6. Next, several studies were carried out to define more appropriate modes to drill wells through shales considering not only the drilling fluid density but also its chemical composition7–15. Studies on physico-chemical phenomena (ion transport and hydration of clay minerals) are quite recent and practical application of the results is still limited16.

Most of the shale test results reported prior to 1990 were carried out on samples that had lost part of its natural, down hole fluid content due to exposition or inappropriate storage prior to testing. Several studies indicated that swelling effects or reactions between shale and fluids observed during experiments could be interpreted as caused by the loss in saturation of the samples and not only by the presence of expansive clay minerals17–19. Actually, fully preserved shale samples did not show any indication of interaction even when immersed in deionized water20. After these new findings, it became clear that some effort would have to be placed in understanding which elements of shales would have to be described in order to evaluate the potential destruction of its structure when in contact with fluids.

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