Reaming while drilling can be a more economical drilling technique than conventional drilling. Downhole failures can occur for a variety of reasons, and failure as a result of excess dynamic loading on the bottom hole assembly (BHA) components may be the most mysterious. One of those mysteries is the static and dynamic distribution of axial load between the bit and the reamer. This paper presents an approach using the explicit finite element method to model the static and dynamic interactions between the bit, the reamer and the formation of the BHA.
The results for axial displacement from the explicit finite element method are compared to theoretical results for a selfweighted column. Also, the axial vibrations are compared to the theoretical natural frequencies. Once the comparison showed the explicit FE method gives the same theoretical results, the model is expanded to better reflect actual downhole conditions. The model includes a bit and a reamer from which the interaction can be studied with an emphasis on axial loading. A weight dependent velocity boundary condition is used to model formation penetration of the bit and reamer. This models the relationship between the axial load on the reamer and the bit. This so-called drill-ahead model can be used to better understand the dynamics of the BHA.
Results from this study show the amplitude of the axial oscillating force on the reamer was greater than that of the force on the bit. Drilling with a more aggressive reamer will decrease the side forces on these components for the case studied. In addition, for a change of load at the top of the model, the reamer can take more of the transient load than the bit.