Most wellbore stability problems occur while drilling shale intervals. Although the drilling industry invests large amounts of time and money each year to deal with the problem, the interaction between drilling fluids and shale is complex and not well understood. Generally speaking, fewer wellbore stability problems occur when drilling with invert emulsion drilling fluids (IEF) than with water-based drilling fluids (WBF). In this paper, the theory governing the interaction between shales and invert emulsion drilling fluids is briefly reviewed.

A new testing device developed by the University of Oklahoma was used to study changes in rock strength of shale cores exposed to invert emulsions. Two shale samples, one from a deepwater environment and another from an onshore environment, were exposed to invert emulsions having varying chemical activity (water phase salinity) levels. The stresses required to fail the samples were directly measured under in-situ conditions. The results showed that under some conditions the invert emulsion strengthened the samples and under other conditions the shales were weakened. These results are interpreted using current osmotic pressure and membrane efficiency theory of invert emulsions.

Using a set of in-situ drilling and wellbore stress conditions, traditional elastic wellbore stability modeling is used to predict changes in pore pressure at the wellbore wall as a function of changes in invert emulsion chemical activity. Porochemoelastic modeling is also performed to predict changes in shale pore pressure as a function of time. The modeling results are compared to the shale strength data obtained in the laboratory.

Lastly rock strength measurements of shales exposed to IEF are compared to those exposed to WBF having the same chemical activity (water phase salinity) levels. The differences in performance are explained in terms of osmotic pressure and membrane efficiency theory.

With the laboratory and computer modeling results, the drilling engineer can better appreciate the effect of invert emulsion interaction with shales, which can enable better planning of future wells, especially those having narrow safe drilling windows.

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