Thick-Walled Cylinder Testing of Clays for the Study of Wellbore Instability

Abstract This paper describes two new automated, high pressure thick-walled cylinder (TWC) devices that have been designed and built in order to study wellbore instability in saturated hard clays. Both devices have the same internal diameter (Di = 2.5 cm) but different outer diameter (Do = 7.6 and 15.2 cm) and allow for independent control of the vertical stress and radial pressures acting on the inner and outer walls of the annular soil specimen, as well as pore water pressure. The apparatus design enables back-pressure saturated specimens to be re-consolidated under stress path control, and simulation of shearing due to de-pressuring of the model wellbore under a variety of drainage conditions. Test results for resedimented Boston blue clay (RBBC) show that there are minimal effects of specimen height on the model borehole response. A comparison of tests from the two TWC devices indicates that the outer diameter of the specimen has a significant effect on the cavity pressure-volume behavior and on the redistribution of pore pressures within the test specimen. The borehole response is influenced by drainage conditions, where drained tests attain lower minimum borehole pressure than undrained tests.