Abstract: It is possible that pore pressure of formation around bore-hole is changed whenever during the overbalanced drilling or the under-balanced drilling. The change of pore pressure affects stress distributions around borehole and wellface stability. Filter cake quality will affect formation pore pressure around borehole. The effects of filter cake quality on radius stress, hoop stress,
axial stress and tangential stress have been studied quantitatively
in this paper. The result show that, with filter cake quality in-creasing, the support of drilling fluid acting on borehole wall in-creases, the ability that formation around borehole resists tan-gential failure increases and wellface stability improves. On the other hand, the stresses between rock grains increase the ability that formation around borehole resists tensile failure increases and wellface stability improves. The effect of filter cake quality
on wellface stability of low permeability formation is much sharply.

Key words: filter-cake quality, stress, wellface stability, flow-deformation coupling, rock deformation