Fluid particles paths in a centered annulus with rotation of the inner pipe.
New drilling problems emerge with the development of extended reach wells offshore. The long drilled sections, often with high deviation or even horizontal deviation, lead to poor hole cleaning. The cuttings tend to settle along the walls creating additional friction which could be critical for the drilling process and could reduce the amount of removed cuttings.
These situations are particularly observed during the sliding phases. For these wells, cleaning phases have to be performed frequently to avoid problems such as stuck pipes or pack-off during the tripping periods. Furthermore, during the cementing phases, the fluids displacement operations have to be perfectly controlled to ensure good security of the well and optimal cement placement.
To solve all these hydraulic problems, it is necessary to have first a good knowledge of the flow between the drill string and the hole walls in the annulus, depending on the drilling parameters. It will then be possible to define the critical drilling conditions to each problem which will give fruitful information for the drilling program preparation. For instance, better knowledge of the mechanisms controlling cuttings removal will help to optimize the hole cleaning.
The classical approach consists in using both driller knowledge, which provides basic rules, and dedicated software, based on experience correlation. However, this is not sufficient for new complex wells, as some parameters, essential when deviation occurs, are missing. The mud real rheology, the rotation and eccentricity of the drill pipe, often neglected in these approaches, has a real influence on the flow behavior.
3D modeling approach
Calculated velocity profile and shear rate contours in an eccentrated annulus for a Power Law fluid.
For a better understanding of the phenomena, a 3D numerical approach is most suitable. It is the only approach which provides a detailed flow pattern in the entire 3D domain for each solved variable such as pressure, velocity, viscosity, stresses. Once it has been validated, it is a powerful tool for parametric studies. Useful information for the hole-cleaning problem is available, such as the weak velocity areas that are critical for the cuttings removal, the shape of the velocity profiles, the areas of high viscosity and so on. It is still an approach mainly used in research projects but it is very innovative for solving drilling problems.
IFP has developed this 3D numerical approach in a project concerning extended reach wells, to study the mud flow in the annulus as a function of the drilling parameters. Both rotation and eccentricity of the drill pipe and rheology of the mud have been taken into consideration. The commercial fluid mechanics software, Estet, has been modified to take into account the non-Newtonian behavior of the drilling mud. A Herschel-Buckley law has been chosen because it is well suited to represent the drilling mud, especially with a yield stress. Validations have been made using both experimental measurements and numerical results from other software.
After this validation phase, an analysis of several drilling configurations has been made. The influence of the drilling parameters has been identified, including these being the positive influence of the rotation and the importance of the eccentricity. All these results will be presented at the next SPE Annual Confer-ence in Dallas this October.
To go further in the modeling of hole cleaning, it is necessary to take into account the cuttings in the annular flow, and analyze the way they will be carried in the mud depending on the drilling parameters, especially the well inclination. A lot of theoretical work has already been done to understand the mechanisms of cutting removal. Most of this work considered only a single particle, and without all of the important drilling parameters. The 3D numerical approach is then of great interest as it can take into account the total amount of cuttings, and can be used to study the conditions for their settling on the wall, solving the transient equations.
IFP has decided to use the same 3D numerical approach as described previously for modeling of hole-cleaning. A Lagrangian two-phase flow method is used to track the solid particles in the mud flow. This approach is new because both non-Newtonian and two-phase behaviors are assumed. In that aim, the work will continue in a JIP in order to provide new criteria defining critical drilling conditions for hole cleaning, based on these 3D results, laboratory experiments, and well data analysis.
Estet software version 3.4, EDF-Simulog.
Herschel, W., Buckley, R., ASTM proceedings, Part II.
The author would like to thank the ARTEP association for giving permission to publish these results.