Numerical modelling of wellbore behaviour in fractured rock masses

Wellbore instability problems have been encountered in many gas and oil fie lds. Fractures and a high differential stress (i.e., a large difference bet ween the principal effective stresses in the horizontal plane) are consider ed to be major causes of wellbore instabilities, including hole enlargement and high Volume caving. This paper describes the results of a series of nu merical analyses carried out using UDEC (Universal Distinct Element Code) t o obtain a better understanding of wellbore instability. The effects of two different fracture patterns (randomly polygonal fractures and aligned frac ture sets) and three different far-field stress conditions have been invest igated for various wellbore pressures, corresponding to different mud densi ties during drilling. It is known that the differential stress has an impor tant influence on wellbore instability. A comparison of the numerical resul ts with the analytical solution for homogeneous elastic behaviour has been carried out. The numerical analysis shows that the effect of a high differe ntial stress is exacerbated by the presence of adversely connected fracture s near the bore wall. The drilling mud density significantly affects the de formation of and the mud loss from a wellbore during drilling. It should be chosen carefully in the light of the fracture geometry and far-field stres s. (C) 1999 Elsevier Science B.V. All rights reserved.