Definition - What does Thermal Gradient mean?
Thermal Gradient is the rate of temperature change as the depth of an oil well or a gas well increases. It is a physical quantity denoted by temperature per unit length. The commonly used units to represent Thermal Gradient are Degree Fahrenheit per 100 feet or Degree Celsius per meter. The SI unit for Thermal or Temperature Gradient is Kelvin per Meter (K/m).
Petropedia explains Thermal Gradient
As the depth of drilling increases during drilling process of a well, the temperature as well as the Thermal Gradient of the well also increases. It is also known as geothermal gradient. Average Temperature Gradient ranges from 25 to 30 degree Celsius per km or 15 degree Fahrenheit per 1000 feet. Thermal Gradient can be expressed as dt/dL.
Thermal Gradient is a vector quantity and is given by the formula:
dT = dt/dL
dT = (dt/dx, dt/dy, dt/dz)
L = x, y and z the three coordinates of location of interest under the ground.