Effect of Initial Pore Pressure Distribution on Deformations Around Wellbores in Shales

ABSTRACT: Due to the low permeability of shales, excess pore pressure will be induced in the material in response to the volume change of the rock matrix during drilling. The excess pore pressure response is time-dependent, due to fluid flow associated with non-uniform pore pressure distribution in the material. Previous research has shown the considerable effect of induced pore pressure on stability of wellbores drilled in shales and a significant difference between time-independent and time-dependent analyses. The excess pore pressure is largely dependent on the initial pore pressure and its distribution in the formation. To obtain a precise pre-diction of deformations around wellbores in shales, a realistic initial pore pressure distribution is required to be incorporated into the analysis. This paper investigates the influence of initial pore pressure distribution on borehole deformation of a thick-walled hollow cylinder of a synthetic shale. The investigation is conducted using two FLAC-based models, one is a time-dependent model that incorporates coupled flow-mechanical interaction with assumed uniform pore pressure distribution and another similar model with a more realistic non-uniform pore pressure distribution. The numerical predictions obtained are assessed against experimental observations in which a non-uniform initial pore pressure distribution is present. The investigation shows that there are large differences between predictions obtained from uniform and non-uniform pore pressure distributions. The numerical model with non-uniform pore pressure distribution is shown to be in better agreement with the results of the laboratory tests than those from uniform distribution.