Casing while Drilling (CwD) is an efficient method by which to increase the fracture gradient in narrow pore-fracture pressure sedimentary basins and deep offshore applications. It offers hydraulic improvements and the ability to plaster cuttings to the wellbore wall, which can enhance the wellbore's hoop stress by wedging the created fractures. Although successful field applications of increasing wellbore integrity have been reported, uncertainties remain regarding the mechanisms and how to operationally capture the maximum attainable wellbore pressure. These uncertainties include the hydraulic complexities of fluids, role of Particle Size Distribution (PSD) and how it relates to the plastering effect, type of drilling fluids, borehole shape, role of lost circulation materials (LCM), and casing eccentricity.
This paper presents numerical, analytical and experimental methods to study the contributing factors in CwD applications. Laboratory experiments were conducted to evaluate the Particle Size Distribution (PSD) and filtration rate of the mud mixed with cuttings from a recently drilled well. Several tests were conducted using Permeable Plug Testing (PPT) equipment to evaluate the role of different LCMs, to fill the PSD gap and to capture the strengthening effect.
In addition, advance finite-element methods were used to model the near wellbore area and hoop stress changes with consideration of the formation's poro-elastic properties. Furthermore, the frictional pressure lost during the CwD operation was evaluated using Computational Fluid Dynamics. Analytical models were used to investigate different boundary conditions when applying finite-element analysis.
The numerical simulations and laboratory experiments in this work were based on a recently drilled well in South Louisiana, where severely depleted sections were drilled successfully. Previous drilling records in this area report multiple problems with lost circulation, tight holes and other wellbore stability issues. Results from the numerical models and laboratory experiments agree well with field observations. The analysis presented in this paper indicates that an optimum PSD can significantly mitigate lost circulation and minimize the need to add LCM.