  We investigate the nucleon self energy through the sixth chiral order in the
covariant $SU(2)$ chiral perturbation theory ($\chi$PT) in the single baryon
sector. The validity of the extended on-mass-shell (EOMS) renormalization
scheme is explicitly verified to two-loop order, manifested by the miraculous
cancellation of all nonlocal divergences and power-counting-breaking (PCB)
terms that are nonanalytic in pion mass. Using the $\sigma_{\pi N}$ term
determined from the latest lattice simulation to constrain some unknown
higher-order low energy constants (LECs), we predict the nucleon mass in the
chiral limit to be $856.6\pm 1.7$ MeV. It is found that the EOMS scheme
exhibits quite satisfactory convergence behavior through ${\cal O}(q^6)$ around
physical point. We also predict the pion mass dependence of the nucleon mass to
the accuracy of ${\cal O}(q^6)$, which is in satisfactory agreement with the
recent lattice results over a wide range of pion mass.
