Call Grunt task from subdirectory

I'm struggling with how to make this work. I have a casper test in which I need to run a Grunttask at the end of. The problem is the directory structure which I've added below.

package.json

Gruntfile.js

casperjs/

--- web/

------ tests/

--------- test.js

And in test.js I'm calling grunt.task.run(['mytestname']); When I run the casper test I get "uncaughtError: ReferenceError: Can't find variable: grunt".

Any ideas? Thanks!