Why does underground oil shale heating take months or years?

Underground oil shale heating is a slow process, typically taking months or years, primarily because rock is an excellent insulator.1

In the context of petroleum production, in situ processing requires heating the rock formation to trigger pyrolysis—the process of breaking down kerogen macromolecules into liquid and gaseous hydrocarbons. Because the surrounding geological formations do not conduct heat efficiently, the thermal energy takes a significant amount of time to penetrate the rock mass and reach the temperatures necessary to liberate these products.12

This slow, gradual heating occurs at lower temperatures—roughly 325–400 °C (620–750 °F)—compared to the rapid heating used in surface retorts, which operate at higher temperatures to achieve faster results.1