Effect of Stratum Properties on Wellbore Stability.

The influence of stratum properties on wellbore stability was studied by numerical simulation. The results showed that the safe mud density window (SMDW) for both permeable and impermeable strata becomes narrower with increase in the ground stress non-uniformity coefficient. The probability of wellbore wall instability increases where the strata are permeable. If the ground stress non-uniformity coefficient is constant in this case, then there is a linear increase in the pressure at which stability loss and wellbore-wall collapse occur and a linear increase in stratum fracture pressure (SFP). Here, fracture pressure increases more rapidly than collapse pressure. The SMDW narrows with rise in pore pressure and widens with an increase in stratum tensile strength and the cohesive force between molecules. The SMDW is wider for permeable strata than for impermeable strata in this case. The width of the SMDW also increases with an increase in the internal friction angle, but it becomes narrower with an increase in the effective stress. The SMDW is not affected by increases in the Poisson's ratio or porosity if there is no permeation but becomes smaller if there is permeation. In the absence of a supporting force, wellbore stability decreases drastically and the formation near the wellbore also becomes unstable. [ABSTRACT FROM AUTHOR]