Custom Post Type conflicting with page?

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.