The Implication of Time-Dependent Filter Cake Buildup on Wellbore Stability and Breakdown Pressure
摘要
Existing poromechanical models fall short in wellbore stability analysis without considering combined effects of hydro-mechanical coupling, thermal diffusion, and transient nature of a filter cake buildup, especially for high-pressure high-temperature deep formations. To fill this gap, we consider these processes with their time effects and derive the coupled porothermoelastic analytical solutions of the time-dependent stress and pore pressure for the general geometry of an inclined wellbore. Terzaghi’s effective stresses are then combined with Drucker–Prager’s shear failure criterion and the rock’s tensile strength to calculate the corresponding collapse and fracturing mud weights. An example of an extended reach horizontal wellbore is presented to demonstrate the effects of a non-uniform filter cake buildup on the stresses in wellbore stability analysis while drilling. The example shows that hydro-mechanical coupling without filter cake plays an essential role in shrinking the mud-weight window with time. Wellbore cooling decreases the effective stresses as well as the collapse and fracturing mud weights. The buildup of a filter cake increases the effective tangential and radial stresses and tends to widen the mud-weight window with time. Also, a field case study is presented to show the applicability of the solution in calculating the abnormalities of excess breakdown pressure during well completion well stimulation. We apply the newly derived analytical solution to the field case and calculate breakdown pressures. Our results agree well with the values reported from the field case. Sensitivity analysis shows that the breakdown pressure decreases linearly with the decrease in wellbore temperature and the increase in rock’s volumetric thermal expansion coefficient. Breakdown pressure can be reduced by 8.7% by cooling the wellbore up to