A good informative article by Stellae Energy’s Head of Subsurface David Pickering is reposted here.
O&G folks aren’t necessarily very familiar with SUPERCRITICAL FLUIDS, but they are an important concept for those working in geothermal / CCS and something with which they need to become comfortable. I have drawn a couple of “phase diagrams for geologists”, below. Lots of colour, not too many numbers…
SUPERCRITICAL FLUIDS (SCFs) can be defined as substances at a temperature and pressure above their critical point, where distinct liquid and gas phases do not exist (but below the pressures that turn them to solids). Instead, SCFs have some unusual properties that make them useful for the GEOTHERMAL AND CARBON SEQUESTRATION.
For example, SCFs have densities similar to that of a liquid, but viscocities / mobilities more like that of a gas. They have a very high enthalpy and, like a gas, can diffuse through a porous solid.
SUPERCRITICAL STEAM is already used in coal-fired and nuclear power stations, but not yet utilised in geothermal. The Icelandic Deep Drilling Project has attempted to access supercritical hydrothermal steam (>400degC, >250 atm, >4km depth), which has the potential to yield 5-10x as much energy per unit volume.
With little energy wasted transfering heat from steam that comes out of the ground to the steam that will spin a turbine, there are massive benefits in the early phases of the power generating cycle. The challenge obviously lies in that the tools & power plants aren’t quite technologically ready to handle the extreme PT conditions. The immense heat of supercritical water also mean that the acids that can cause immense problems in high temp / sub-critical systems are in theory no longer a problem.
Compared to water, CO2 has a more reachable critical point at just 31degC / 73 atm, making it ideal in CCS projects. SUPERCRITICAL CO2 is denser compared to gaseous CO2, allowing the storage of a greater volume in SEQUESTRATION. It is also buoyant compared to oil and water, so rises to fill the top of a geological trap.
Recently, ScCO2 is being researched as a WORKING FLUID IN GEOTHERMAL PROJECTS. GreenFire Energy in California have tested a monobore well with critical CO2 being injected in the centre of the well. The large difference in wellbore density between cold injected ScCO2 and hot produced ScCO2 results in a large buoyancy force that causes natural circulation in the geothermal reservoir (THERMOSYPHONING).
Although the mass heat capacity of ScCO2 is less than that of water, the fluid density to viscosity is 1.5x that of water. So although the rate of geothermal energy produced compared to water is considerably less, if we consider the buoyancy drive and better flow capacity of ScCO2, the reduced need for pumping and surface power consumption mean that power production of ScCO2 would be about equal to that of a water-based system.