Will intergalactic travel be impossible in roughly 15 billion years?
Yes, over time the accelerating expansion of the universe (assuming the $\Lambda$CDM model is right) will separate gravitationally bound clusters of galaxies from each other with exponentially growing distances. However, this will not happen in 15 billion years.

This excellent paper analyses the expansion in detail. What takes about 17 billion years is for the distances to increase by 2.71 times. They predict that typical clusters will grow isolated in about 120 billion years, with the Local Group becoming isolated in 175 billion years from the other nearby groups.  

In this "era of isolation" the only reachable galaxies are the ones in one's local cluster. However, they will tend to merge over time into one big elliptical galaxy (like the Milky Way and Andromeda are likely to do in a few billion years). Extragalactic astronomy also becomes really boring since the remote clusters are redshifted so much they cannot be observed.

If one just travels outwards from Earth in some direction with a high velocity one will eventually find oneself in a last cluster beyond which there is nothing reachable.