Fake fog for outdoor field portrait?

My frame is roughly going to be 20' wide in a field and has a hill behind it. Is there a way to make enough fog that would hang low to the ground and cover a space let's say 20' x 20'?  And be there long enough that I can get a 20-30 minute session in?

Outdoors?  that might require a number of professional fog machines to cover that much space, especially if there is any wind. 

Don't they have those dry ice blocks or better yet, I wonder if a smoke grenade will do what you want?

The problem with smoke grenade is that it will be very thick and won't look like low hanging fog

I think your plans  to create fog outdoors for a photo shoot is going to fail unless your plan involves closeup portrait type work on a near totally still air day. For this, the spray can "Atmosphere" can be sprayed in front of, behind, but not at the subject just before each shot is taken. It will appear more like smoke though, and the air currents will dissipate it very quickly. 

Having created fake fog using dry ice and low speed fans to move the created fog onto a stage works, and we started making this fog just a few minutes before the curtain was opened.  We did this for several stage shows and it worked well. The process works quite well in a confined space with no air circulation other than the low speed fans blowing the fog from the dry ice and water onto the stage. It takes a few minutes for the fog to build up on the stage with the curtains closed, and then slides out and off the stage when the curtain is opened. Dry ice fog is carbon dioxide/air moisture mix and is heavier than air. It cools the moisture in the air to make the fog, and it disappears quickly as it dilutes and warms in the air.  This "heavier than air" dry ice fog built a nice layer across the stage and worked quite well within this controlled environment, but it did not last long when the curtain was opened.

Heated oil type fog machines don't make fog that settles to the floor/ground very well like real fog or dry ice fog does. It tends to mix with the surrounding air like smoke and will quickly mix with all of the air in the space. Keeping it lying low like real fog is going to be a challenge too, even when indoors with no air movement. I have done a few product type "still life" shoots in my studio using one of these fog machines and also with the Canned "Atmosphere" spray. It is also flammable, so cannot be used around open flames like candles. I prefer this Atmosphere spray for this type shot because it can be sprayed only when and where needed, but again it behaves more like smoke and not like fog. 

Charley