Gas/Liquid spreading in area

I want to play in 3D space with gasesand liquids, but I need somehow create system for spreading these in closed area. Something similar like in “Oxygen Not Included” game (also made in Unity).
Any practical ideas how to create this system in 3d space?

I it’s bit late but you can search about Cellular Automata. It has lot of usage in physical calculations like fire spreading, gas spreading, sand and water simulations. There is a article about it Cellular Automata for Physical Modelling, it’s bit old but still useful.