This report summarises recent advances made in the calculation of the NNLO QCD corrections to the width difference between $B_q$ and $\bar{B}_q$ mesons. The focus is on the perturbative high-energy part of the calculation; more specifically, the matching coefficients between the $\Delta B = 1$ effective operators of the weak interaction and the $\Delta B = 2$ transition operator are calculated as a deep expansion in $m_c/m_b$. Some technical challenges in the calculation as well as the phenomenological implications of the newest theoretical predictions for $\Delta \Gamma$ and $a_\text{fs}$ are discussed.