This paper demonstrates the effects of chemical potential mechanism on strength and deformation properties of, and wellbore stability in Pierre II shale through consolidated undralned triaxial and borehole collapse tests. Some of the shale samples for triaxial tests were treated with filtrates of two waterbased muds with different KC/concentrations at elevated pressure and temperature. The triaxial tests were performed undralned at a range of effective confining pressures. A series of borehole collapse tests was conducted on thick-walled cylinders of the same shale material. Mud filtrate was circulated along the borehole wall at a zero average overbalance pressure in some of the tests before the borehole pressure was reduced to induce borehole failure, while the cell pressure was maintained constant. Comparison of the triaxial test results on treated and untreated samples showed that the treated samples have higher modulus and peak strength but lower Poisson's ratio and Skempton's pore pressure parameters A and B than those untreated samples. The extent of the effects, attributable to chemical potential mechanism, was different for the two water-based muds. The thick-walled cylinder test results showed that the mechanism resulted in a signiticant increase in wellbore stability due to a combination of increase in effective mud sup-port and material strength.

Shales account for 75% of drilled sections and cause 90% of wellbore instability-related problems. They are generally chemically active and react with incompatible drilling fluids, resulting in timedependent change in mud support and possibly rock strength. Traditionally, physico-chemically induced instabilities were managed by using oil-based muds. However, due to environmental concerns, the industry has now re-focussed on the use of environmental friendly and low cost water-based muds (e.g. Reid et al. 1993). As such, a growing interest in understanding the interaction mechanisms between shale and water-basedrilling fluids, which is essential in designing new muds to manage wellbore instability problems in shale, has been witnessed in recent years (Mody & Hale 1993, van Oort et al. 1994, Tan et al. 1996, 1998 & 1999, Horsrnd et al. 1998).

This paper describes consolidated undrained triaxial and borehole collapse tests conducted on Pierre II shale incorporating two water-based muds. The purpose of the study is to determine the effects of chemical potential mechanism on deformation and strength properties of, and on wellbore stability in Pierre II shale. Some of the samples for the triaxial tests were treated with the mud tiltrates at an elevated temperature and pressure to induce the effects of chemical potential mechanism in the treated samples. The triaxial tests were then conducted at a range of effective consolidation pressures using the techniques developed specifically for testing low permeability shales (Wu et al. 1997). Borehole collapse tests were conducted on thick-walled cylinders of the same shale material. Prior to borehole pressure reduction, mud tiltrate was circulated at a zero overbalance pressure along the surface of the borehole wall in some of the tests. The effects of chemical potential mechanism could then be assessed by comparing between experimental results of reference samples (without treatment or mud tiltrate circulation) and the treated or mud tiltrate circulated sampies.

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