wp_query: How to retrieve current term slug in custom taxonomy?

Using the WP Types Plugin, I setup a custom post type (slug: portfolio), which has a custom taxonomy (slug: portfolio-category).

The custom taxonomy has three items (slugs: group1, group2, group3) to organize portfolio posts.

On each single portfolio page I am displaying 3 related posts from the same taxonomy term, filtered by slug:

global $post;

$term_list = wp_get_post_terms($post->ID, 'portfolio-category', array("fields" => "names"));

      $wpex_port_query = new WP_Query(
          array(
              'post_type' => 'portfolio',
              'showposts' => '3',
              'orderby' => 'rand',
              'post__not_in' => array($post->ID),
              'no_found_rows' => true,
              'tax_query' => array(
                  array(
                     'taxonomy' => 'portfolio-category',
                     'field'    => 'slug',
                     'terms'    => $term_list,
                    )
                )
            )
        );

This basically works, but I am wondering if there is a better way than creating a global variable $post? What is the recommended way to handle this?