Chapter 10 - Wellbore Instability Due to “Block Loosening” in Fractured Rock Masses

Abstract

This chapter examines the effects of the presence of fractures on wellbore instability under various differential stress regimes using numerical modeling techniques. Numerical analyses have been carried out using the program Universal Distinct Element Code (UDEC) to investigate (1) the effects of the far-field differential stress on the mechanical behavior of a wellbore; (2) the effects of hydraulic pressure in, and mud loss from, the wellbore because of drilling; and (3) the behavior of wellbores with different fracture geometries. The chapter uses fracture geometry to examine the effects of differential stress on the stability of a wellbore in an originally isotropically fractured rock. To examine the response of the rock around a wellbore subject to different wellbore pressures, the hydraulic aperture, flow rate, and hydraulic pressure were monitored at points on two fractures in each of the two analyses. The results of the numerical analyses have confirmed that the presence of fractures and the existence of high-differential stresses are important factors in wellbore instability.