Heavy oil is recovered by introducing heat into the reservoir through thermally controlled processes. Steam flooding and in situ combustion or air injection are the most frequently-used thermal recovery methods. Steam flooding is used extensively in the heavy oil reservoirs in California. Experiments with cold production and sand injection and horizontal well production of heavy oils have been conducted mainly in Canada and Venezuela, which have extensive heavy oil reservoirs. Steam flooding is conducted by injecting steam into reservoirs that are relatively shallow, permeable, and thick, and contain moderately viscous oil. The dominant mechanism in thermal recovery by steam is the reduction in the viscosity of the oil, allowing flow to the wellbore. Problems with reservoir heterogeneity and steam distribution are being overcome. Steam flooding production in the U. S. averages nearly 500,000 BOPD.

In situ combustion introduces heat in the reservoir by a process of injection air and downhole ignition to burn portions of the oil to displace additional oil. The combustion front is sustained and propagated through continuous injection of air into the reservoir. Premature breakthrough of the combustion front contributes to operational problems. Both steam flooding and in situ combustion have high surface facility costs and require special safety measures.

Low-permeability heavy oil resources offer huge potential for adding to U.S. reserves. Diatomite formations in California alone contain 12-80 billion barrels of original-oil-in-place that could be tapped with a successful thermal recovery—primarily steam injection—scheme. One NETL project lends support to the technical case for thermal recovery from diatomite formations. This illustration, which shows computed tomography-derived images of water being taken up by an oil-saturated diatomite core (with times below it shown in minutes), is featured in the fact sheet entitled Heavy and Thermal Oil Recovery Production Mechanisms, Project No. DE-FC26-00BC15311.