1. n. [Geology, Drilling Fluids]

The rate of increase in temperature per unit depth in the Earth. Although the geothermal gradient varies from place to place, it averages 25–30 degC/km [15 degF/1,000 ft]. Temperature gradients sometimes increase dramatically around volcanic areas. It is particularly important for drilling fluids engineers to know the geothermal gradient in an area when they are designing a deep well. The downhole temperature can be calculated by adding the surface temperature to the product of the depth and the geothermal gradient.

Synonyms: temperature gradient,  thermal gradient

See: high-pressure, high-temperature filtration test,  mud engineer