A WC product may belong to none, one or more WC categories. Supposing you just want to get one WC category id.
global $post;
$terms = get_the_terms( $post->ID, 'product_cat' );
foreach ($terms as $term) {
$product_cat_id = $term->term_id;
break;
}
Please look into the meta.php file in the "templates/single-product/" folder of the WooCommerce plugin.
<?php echo $product->get_categories( ', ', '<span class="posted_in">' . _n( 'Category:', 'Categories:', sizeof( get_the_terms( $post->ID, 'product_cat' ) ), 'woocommerce' ) . ' ', '.</span>' ); ?>