Today, most operators prefer to use cold production for heavy oil before beginning more expensive thermal-recovery processes. The high viscosity of heavy oil causes low mobility and low well productivity. However, in some cases well productivity is 10 to 20 times that predicted by Darcy＇s law because sand produced with the oil creates wormholes or because gas comes out of solution and results in "foamy oil." Global experience with these phenomena enables various generalizations to be made to provide optimal design for cold production from a given reservoir.