ABSTRACT
Nonlinearity has been observed in stress-strain behavior of most sedimentary soft rocks, including shales. In addition, due to the low permeability of shales, excess pore pressures will be induced in the material in response to the volume change of the rock matrix during drilling. To obtain a precise prediction of deformations around wellbores in shales, the nonlinearity and effect of induced pore pressure are required to be incorporated to the constitutive models. This paper develops a simple nonlinear model encompassing the most important characteristics of shale stress-strain behavior. The model is investigated using two FLAC-based models, one is a time-dependent model that incorporates coupled flow-mechanical interaction and another a steady state time-independent analysis that only accounts for mechanically induced pore pressures. Predictions from the two analyses are compared with the results of borehole collapse tests on thick-walled hollow cylinders of a synthetic shale. The investigation shows that there are large differences between predictions obtained from the two numerical models. The nonlinear coupled numerical model is shown to be in good agreement with the results of the laboratory tests.
