| Summary: | Cuttings transport modeling was analyzed with a major focus on three dimensional (3D) Computational Fluid Dynamics (CFD) approaches including rheometry and to a lesser extent on one-dimensional modeling and dimensional analysis. As a first step, the relevant parameter space was analyzed and field values typical for the Norwegian Continental Shelf were established. Dimensional Analysis was applied to further understand the parameter space and to establish a process description based on a polynomial. For the fluid phase (i.e., the drilling fluid or a drilling fluid model system), the classical General Newtonian Fluid (GNF) concept was investigated by means of rheometry and the example of polymeric solutions (Polyanionic Cellulose (PAC) dissolved in distilled water) typically used in experimental cuttings transport studies. It is shown that the GNF assumption only holds if the fluid is at steady-state with respect to its microstructure and that such a steady-state may be hard to achieve in experimental works because of the long rheological timescales of the fluid. Concerning the solid phase (i.e., the cuttings), the performance of the typical modeling concept utilized in cuttings transport research, namely the Kinetic Theory of Granular Flow (KTGF) in combination with a frictional viscosity model accounting for dense granular regions, was evaluated by means of CFD simulations of the cliff collapse problem. Several fluids (air, water, two PAC solutions) and spatial scales (cliff height and particle diameter), among other parameters such as the cliff’s aspect ratio and initial solid volume fraction were investigated. While the typical sloped deposits were obtained in most cases shortly after the collapse these were found to be unstable: The top layer of the sediment bed continues flowing after the collapse which eventually leads to an entirely flat deposit. This is attributed to the utilized modeling approach which is not capable of handling the top sediment bed layer successfully. As an alternative, a modeling approach prominent in the field of environmental sediment transport modeling was tested. The dense region is dynamically excluded from the computational domain, and the Exner equation is used to describe the evolution of the sediment bed. Problems such as proper closures for the bed load transport models as well as contact problems were encountered, disqualifying this approach for use of cuttings transport simulations within the scope of this project. The relevance and magnitude of turbulence and dunes in wellbore flows were estimated and several pipe and annular single-phase RANS simulations were compared with DNS data (generated in the AdWell project) for Newtonian and shear-thinning fluids. While wellbore flows are laminar to transitional (mostly depending on the fluids’ viscosity), none of the turbulence models investigated appears to be universally applicable. However, this part is still in progress and only preliminary conclusions are presented. A subproblem of cuttings transport, a particle subjected to a cross-flow of a mildly viscoelastic, shear-thinning fluid, was investigated by means of CFD. The particle is treated in a Lagrangian manner and the particle-induced shear is accounted for in the computation of the fluids viscosity as seen by the particle. Several cases were investigated and the model was validated with results from the literature. Discrepancies are found close to the lower channel wall were the particles in the experiments are advected much farther than in the simulations. Finally, drill pipe rotation in combination with orbital drill pipe motion was investigated. Specifically, the effect of forward, i.e., synchronous, and backward, i.e., asynchronous, whirl (SW and AW, respectively) on cuttings transport was evaluated and compared with classical concentric and eccentric arrangements. AW and, more dramatically, SW improve cuttings transport, albeit depending on other system parameters such as the fluid’s rheological parameters and the drill pipe’s rotational rate. However, for the parameter space investigated, best transport of cuttings was obtained in a positively eccentric drill pipe system because the main flow occurs at the top of the bed and consequently high shear stresses acting on the bed. This thesis highlights current shortcomings and potential for improvement of CFD cuttings transport simulation. Further work is required on all individual topics to achieve better quantitative results and to integrate subscale models into a model on the annular scale. The findings presented here will hopefully contribute to a more comprehensive understanding of cuttings transport and support further development of CFD models applicable to the annular scale and more coarse, real-time models applicable to the entire wellbore. Digital full text not available
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