THE INTERACTION BETWEEN THE WELLBORE AND PREEXISTING FRACTURES

Down-hole imaging techniques have revealed that natural fractures can be reactivated in the vicinity of a wellbore. Fracture reactivation, h owever, has never been analysed in detail. In particular, no one has q uantified the influence of in-situ state of stress, fracture friction angle, wellbore pressure, mud invasion in the fracture plane and fract ure location on the short and long term stability of the fracture. We have therefore little knowledge of the importance of the phenomenon an d of the conditions which lead to fracture reactivation.This work aims to quantify the influence of various controlling parameters on fractu re instability. For this purpose, we have extended a method which was previously used to analyse the interaction of tensile cracks with the wellbore. The fluid invasion into the fracture is taken into account b y assuming a length of invasion along which the fluid pressure is equa l to the wellbore pressure. This work shows that fracture reactivation due to near wellbore effects is likely to occur although the fracture movement may not be easily detected. However the occurrence of this m echanism is quite sensitive to the fracture location with respect to t he wellbore and to the fracture orientation. This work has also reveal ed the possibility of two length scales in fracture instability: the f irst length scale is related to the interaction of the wellbore with t he fracture and the second one is related to fracture instability with respect to the far-field state of stress.