Abstract
The "Smear Effect" during Casing Drilling can help to increase the fracture gradient, mitigate lost circulation problem and reduce Non Productive Time (NPT) in drilling operations specifically narrow pore-fracture pressure sedimentary basins and deep offshore applications. This is due to plastering of cuttings to the wellbore wall which may enhance wellbore hoop stress by wedging created fractures or increase fracture propagation pressure. Although some field applications have increased wellbore integrity, uncertainties remain regarding the mechanisms by which this occurs and how to operationally capture the maximum attainable wellbore pressure. More importantly, the "Smear Effect" has never been quantified scientifically. The uncertainties include the optimum ratio of the casing to the open hole size, casing rotation, contact between the casing and the mud cake, type of drilling fluid, casing eccentricity and the fluid's hydraulic complexities. This paper provides new insights on "Smear Effect" by presenting numerical and analytical models to to clarify and optimize relevant parameters in casing drilling applications. Advanced finite-element methods were used to model the near wellbore area under considerations of various pipe/open hole ratios and mud-cake contacts in each simulation. The models consider geomechanical properties in both the mud cake and formation. Actual operational data from one of the fields in South Texas was used for simulations. Analytical models were also used to calculate the contact force for each set of casing sizes and rotations. The force then used as the new boundary conditions in finite-element models and mud cake failure was evaluated.
The results indicate that casing rotations up to 100 rpm cannot result in mud cake failure; however, this cannot be confirmed firmly due to uncertainties regarding mud cake properties. In addition, increasing the casing ratio will reduce the risk of mud-cake failure when increasing casing rotations. Hydraulic calculations show a sharp increase in the bottom-hole pressure when the casing-annulus size ratio exceeds 0.8 in concentric situations.
