To cast field type use these steps:

1. Set meta_key within a custom query

$query->set('meta_key','custom_field');

or

$args = array(
    'meta_key' => 'custom_field',
);

2. Set meta_type to “numeric”

$query->set('meta_type','numeric');

or

$args = array(
    'meta_type' => 'numeric',
);

3. This will allow you to add ordering for numeric values

$query->set( 'orderby', "custom_field" );

or

$args = array(
    'orderby' => 'custom_field',
);