It is well known that the series $\sum_{p\in \mathbb{P}} \frac{1}{p}$ diverges where $\mathbb{P}$ denotes the set of primes. Brun proved that $\sum_{p\in \mathbb{P_2}} \frac{1}{p}$ converges where $ \mathbb{P_2}$ denotes the set of twin primes. Now for an even integer $k$ let $\mathbb{P}_k = \{ p, q \in \mathbb{P}: |p-q| \leq k\}$. By Zhang's result we know that $\mathbb{P}_k$ is infinite for $k$ larger than some threshold $N$. My question is what is the smallest value of $k$ for which $\sum_{p\in \mathbb{P}_k} \frac{1}{p} = \infty$? Or does no such finite value exist? Moreover, what are the asymptotics for these sums as $k \to \infty$?