Separate URLs for blog & portfolio

Hi Guys,

I'm wanting to move all blog posts to a url structure of http://www.website.com/blog/post-title which I have done by adding a custom permalink structure of /blog/%post-title%/ which has worked ok. The issue is that this change has had a knock on effect, making the companies portfolio nest under the blog. (ie. http://www.website.com/blog/portfolio/post-title). Im wanting to have 2 separate structures similar to below:

blog== http://www.website.com/blog/post-title
portfolio == http://www.website.com/portfolio/post-title.

Would really appreciate any insight you can give me.

Thanks in advance.

Dan