Geysering

I said this blog was going to be aimed at research non-related to my PhD, and already I digress.

Carbon dioxide (CO2)-charged cold geysering isn't exactly what my PhD is about (if only I knew *exactly* what my PhD is about...), but it's related in the sense of fluid dynamical processes that occur beneath the ground and lead to eruptions.  I'm more interested in eruptions of the volcanic sense, but the actual process might be similar.  Or it might not be.  Regardless, it's interesting, and it's important for looking at what might happen during carbon capture and storage (CCS) efforts.

So what is a cold geyser?  It's an erupting aquifer system driven by CO2 bubbles rather than steam.  Crystal Geyser near Green River, Utah, is a good example that has been studied extensively by geologists at the Universtiy of Cambridge.  CO2-laden water pools in a confined aquifer.  A borehole drilled through a confining layer, often in an oil field, into the CO2-laden aquifer provides a path for the pressurised water and CO2 to reach the surface.  The water column generally provides enough pressure to keep the CO2 in solution, but a decrease in pressure causes the CO2 bubbles to expand or "boil," displacing the water and starting the eruption.

In a world that's trying to figure out how to deal with excessive amounts of anthropogenic CO2, one of the sexiest plans is to inject it deep into geological formations where it will hopefully stay put for a very long time.  But what will actually happen if there's a route to the surface?

Crystal Geyser may provide some answers.  Field studies document the periodic eruptions of this unintentionally-created geyser that was born when a prospective oil well was drilled into a fault zone above a natural CO2 reservoir.  While it's simple enough to measure eruption duration, it's a bit more complicated to measure gas emission mass.  And even more complicated to measure liquid emission mass.  However, studies by researchers in Cambridge, the US Department of Energy and Lawrence Livermore National Laboratory provide data which could describe a relationship between eruption duration and time until the next eruption as well as the total mass of CO2 emitted over a given period.

One study looks at 76 days worth of eruptions from the summer of 2005.  Sensors at the base of the geyser measure temperature and fluid movement, and the start time and duration of 140 eruptions yield a bimodal distribution:  two thirds of the eruptions were short (7-32min) and one third were long (98-113min), with a strong correlation between eruption duration and time until next eruption.  Indeed, long and short eruptions seemed to alternate over the course of the study.

The detailed physics of the cyclic eruption mechanism are not fully understood, but here's an explanation of the process as summarised in Lu et al 2005:  The water level rises in the well as it recharges from the groundwater.  When overflow starts, the few bubbles rising with the liquid begin to decrease the hydrostatic pressure at the top of the well and any slight density change progresses down the well.  The vapour flash point--the level at which bubbles start to form--drops deeper into the well and more gas is released.  Bubbles coalesce to form slug flow which leads to larger eruptions, and almost all of the fluid in the well contributes to releasing CO2 gas.  At this stage, the fresh supply of fluid that is recharging the well cannot provide the gas required to maintain the slug flow because the fluid pressure is too high for gas to exsolve.  So the water level in the well falls quickly, but the hydrostatic pressure is rebuilt as the incoming fluid rises.  The flash point curve (describing where boiling will occur) moves upward as well.  The eruption repeats when the water level reaches the wellhead and bubbles begin to decrease hydrostatic pressure again.

But we still don't know exactly how much fluid comes out during each eruption.  Nor the details of the physics behind the recharging process.  We've got records of cycles in pressure and temperature deep within the geyser, and geochemical data that show the amount of gas released.  Could we combine all of this to provide a detailed physical mechanism of what's going on beneath the ground?

The ideal way to gain some better understanding would be to build a simple laboratory model in which to recreate some simple gas exsolution processes.  Showing exactly what mechanism results in what eruption timescale, based on experiments using a bubbly fluid that is decompressed to allow bubbles to grow and drive liquid out of the chamber, could yield interesting results and would allow measurements of volume erupted.  The key component would be to recharge themodel chamber with gaseous fluid...and to explain how this recharge occurs in a natural system.

Although, this may not be what the carbon capture and storage advocates want to hear...

Gouveia, F. and Friedmann, S.J., Timing and prediction of CO2 eruptions from Crystal Geyser, UT., Lawrence Livermore National Laboratory Report (2006).

Lu, X. et al., Measurements in a low temperature CO2-driven geysering well, viewed in relation to natural geysers.  Geothermics 34:389-410 (2005).