chapter  4
24 Pages

Numerical scheme for calculation of shear failure gradient of wellbore and its applications

Xinpu Shen, Mao Bai & Russell Smith

The mud weight window (MWW) is the range of mud density values that provides safe support to the wellbore during the drilling process at a given depth. If the mud weight value selected is within the MWW range, the wellbore is stable and no plastic deformation will occur on the wellbore surface. Furthermore, when a safe mud weight is selected within the MWW, no mud loss will occur. The MWW is defined by two bounds. Its lower bound is the shear failure gradient (SFG), which is the minimum mud weight required to avoid plastic failure of the wellbore; its upper bound is the fracture gradient (FG), which is the maximum mud weight value that will not induce fracture openings. Because natural fractures usually exist within various kinds of formations and wellbores are usually vertical, in practice, the value of the minimum horizontal stress is used as the value of FG.