Find the code where you register your portfolio post type and add/change this arguments:
'has_archive' => 'work', //under work you will see a list of portfolio items
'rewrite' => array(
'slug' =>'work_item',
'with_front' => false,
),
then manually go to Settings > Permalinks and refresh your permalink structure
after that your items will be listed under /work
as an archive
and work_item/name_of_post
as singles.