Concepts and Misconceptions of Mud Selection Criteria: How to Minimize Borehole Stability Problems?
Authors: Santos H. , da Fontoura S.A.B. 
Published: December 2007

Abstract
Problems when drilling through shales are usually attributed to inadequate weight and/or composition of the drilling fluid. Drilling fluid is most often selected based on experience (after trial and error attempts), based on screen tests with available samples, and finally based on some concepts developed in the past. This paper discusses some of the tests used to select the drilling fluids, their limitations and usefulness. Also, it questions some of the existing concepts, such as activity, semi-permeable membrane, chemical potential due to difference in activity, shale swelling mechanisms, and the capillary effects in the laboratory. Specifically designed laboratory tests conducted with downhole preserved shale cores show that these concepts cannot explain shale-fluid interaction. Most of these misconceptions were originated from inadequate laboratory practices. A description of new interaction mechanisms between shale and water and oil-based solutions and a discussion on how to actually reduce stability problems are also presented in this paper.

Introduction
In any exploration project, mud selection plays an important role both technically and economically speaking. Problems deriving from an inadequate selection of either the weight or composition of the drilling fluid are various, but not restricted to: walls instability with hole enlargement or loss circulation; stuck pipe; increased torque and drag; excessive loss of time reaming and trying to drill ahead; formation damage with reduction in reservoir performance; and, in a limit situation, part of the well, or even the entire well is lost. Several additives have been specifically developed to take care of each problem. The drilling fluid cost is, then. directly proportional to the amount and types of additives added to the mud. Although responsible of a significant percentage of the well cost, an adequate drilling fluid, sometimes expensive, can save much more if some of the related problems described before can be avoided. Proper selection of the drilling fluid is, therefore, decisive to minimize these problems and reduce development costs.
Even though a significant amount of research has been conducted in the past to improve drilling fluid performance, persistent problems in the field has led to the conclusion that much more needs yet to be done. Despite the fact that it is the most drilled formation and present the majority of the problems during drilling, shale has not received the primary focus and, therefore, not many cores are available for testing. Research has attempted to understand shale-fluid interaction mechanisms using outcrop samples or old shale cores. Most results, if not all of them, have been biased by mishandling procedures and led to wrong conclusions.
The purpose of this paper is to show that some of the concepts derived in the past and used today to select the best composition of the drilling fluid do not apply to downhole conditions. This paper initially describes the most commonly used criteria in the selection of the best mud, their limitations, and how they have led to the misconceptions derived. Specifically designed tests were conducted in well preserved downhole shale samples to check some of the most important concepts applied in drilling fluid selection. Results show that shales do not behave as a semi-permeable membrane, even when an oil-based fluid is used. Shale swelling mechanisms, shale activity, capillary effects, and use of chemical potential as a driving force were evaluated in laboratory tests. A new understanding of shale-fluid interaction is proposed. The impact of these new concepts on the operations and suggestions to effectively minimize borehole stability problems are also discussed.

Mud Selection Criteria
Laboratory tests and field experience are usually used to select the drilling fluid.
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