Experimental Study of Wellbore Instability in Clays

This paper presents the results of an extensive program of laboratory model wellbore tests that have been performed to study wellbore instability in saturated clays. The tests were conducted on resedimented Boston blue clay (RBBC) anisotropically consolidated to vertical effective stresses up to 10?MPa by using two custom-built thick-walled cylinder (TWC) devices with outer diameters Do = 7.6 and 15.2?cm. The experimental program investigated the effects of specimen geometry, mode of loading, strain rate, consolidation stress level, and overconsolidation ratio (OCR) on deformations of the model wellbore measured during undrained shearing. Results indicate that for normally consolidated clays most of the change in cavity pressure occurs at volumetric strains less than 5% after which the borehole becomes unstable. Increases in outer diameter and strain rate led to a reduction in the minimum borehole pressure. Stress-strain properties were interpreted by using an analysis procedure originally developed for undrained plane strain expansion of hollow cylinders. The backfigured undrained strength ratios from these analyses for normally consolidated specimens range from su/σvc′ = 0.19–0.22. Overconsolidation greatly improves the stability of the borehole, and interpreted undrained strength ratios from the TWC tests are consistent with well-known power law functions previously developed for elemental shear tests.