Average of number of divisors of shifted exponential sequence

Question

Let $a$ be a fixed positive integer greater than 1. We define the sequence $u_n=a^{n}-1$ for all positive integers $n$. Then are there any results in literature for asymptotic value of the sum $$\sum_{n \le x} \tau(u_n)?$$

Here, $\tau(n)$ denotes the number of divisors of $n$. Thanks in advance for any assistance.