The presence of microfractures is now recognized as a common cause of severe wellbore instability, particularly in ultra-low permeability rocks such as shales and some limestones.

In unstable microfractured formations, it is important to avoid pressurizing the fractures with drilling mud. This paper examines the rock mechanics aspects of wellbore instability in microfractured formations, and identifies drilling fluids and drilling practices that reduce the problem. Improved stability can be achieved by the use of certain drilling fluid additives that seal fractures. Limited success is seen with sized solids, asphalts, gilsonites or conventional lost circulation materials but a recently developed additive for use in oil- or water-based muds appears to prevent much of the instability normally observed.

The mechanism of action of this additive is discussed and a recent field case history presented.

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