Jul 2024

Heat Content & the Capacity for Heat

Heat Content & the Capacity for Heat

Heat Content & the Capacity for Heat

It’s because the heat capacity of water is higher than the heat capacity of the human body, we are getting cold when staying longer in the swimming pool.

And due to the fast transfer of energy (high thermal conductivity), the snack from the buffet seems so appealing.

Yet, whether we ultimately freeze in water depends on the absolute water temperature.

To get this water lukewarm, energy must be supplied. The warmer, the more energy. Rocks, however, need just a quarter of the energy that water would need, when heated up to the same temperature.

No wonder that flow and storage capacities of the rock and the ability of a porous medium to transmit fluids through it is – next to the temperature present – crucial for heat harvest.

But also, no wonder, geothermal assets are targeting ‘temperature’ and not heat capacity. And on top of this, Mother Earth has supplied already sufficient energy to heat up both rock and fluids. That’s good 😊

The heat capacity of a rock saturated with one or more fluids is best being estimated on a volumetric basis. Somerton states the following equation for this:

ρbФp = (1-Ф) ρsCps + Ф(SnρnCpn)

ρbФp … volumetric heat capacity of fluid-saturated rock
Cps… heat capacity of rock solids
Cpn… heat capacity of fluids
Sn… fractional saturation of fluids
ρn…density of fluids
ρs…density of rock solids
n… summa from 1 to n

Having a vapour phase present has a negligible effect. Not so for liquid phase.

Reference:
Somerton, W.H. (1992) Thermal Properties and Temperature-related Behaviour of Rock/Fluid Systems. Elsevier.

  • Heat Content and Capacity for Heat: Slide 1. Rocks hold energy in form of heat.
  • Heat Content and Capacity for Heat: Slide 1. The more heat is added to a rock, the more energy it holds. This energy is called heat content and often given in cal/g. The rocks' capacity of holding this energy decreases with increased temperature. The capacity for heat decreases.
  • Heat Content and Capacity for Heat: Slide 3. Heat content diagram
  • Heat Content and Capacity for Heat: Slide 4. The warmer the rock, the less energy it can hold. The first derivate tracks these changes. This is the petrophysical property heat capacity, which is obviously a function of temperature.
  • Heat Content and Capacity for Heat: Slide 5. Heat capacity diagram
  • Heat Content and Capacity for Heat: Slide 6. Heat capacity is one of 3 major geothermal rock properties. Specific heat capacity is relative to water. Pure water has about 4 times the heat capacity of dry rock. The heat content of a compound of a rock is equal to the sum of the heat content of its individual parts. The vapour phase will not increase total heat capacity - but the liquid phase does.

About the Author

Claudia Steiner-Luckabauer, Principal Petrophysicist and Geothermal Expert at the HOT Energy Group

  • Dr Claudia Steiner-Luckabauer is the Principal Petrophysicist and geothermal expert at the HOT Energy Group. She has been working in a leading role on numerous integrated reservoir characterisation and field development projects, focussing on integrated petrophysical evaluation and formation evaluation.
  • As HOT’s subject matter expert, Claudia has extensive experience assessing mature oil fields, heavy oil fields, gas condensate fields, gas fields, underground gas storage, clastic and carbonate settings, low-salinity environments, unconsolidated reservoirs, and fractured reservoirs (chalk, carbonate, granite basement, sandstone). She is also an experienced instructor of academic and industry workshops on various topics in geosciences and has authored numerous papers.

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