When petroleum reserviors contain a low-gravity ( less than 20℃ API), high-viscosity oil and have a high porosity, secondary recovery methods are not effective for displacement of oil. For such reserviors, thermal processes have received the most attention. The injection of steam reduces the oil viscosity which causes an increase in the oil mobility. Depending on the way in which the heat is generated in the reserviors, the thermal processes can be devided into three categories: 1. In-situ combustion; 2. Steam injection; 3. Wet combustion.