Boosting geothermal yields

 By Robin Wylie

A new study published in the journal Geothermal Energy used a numerical simulation to study the effect of CO2 sequestration into an aquifer in Louisiana, USA. It found that the introduction of supercritical CO2 into the aquifer, as well as reducing atmospheric levels, might also be able to increase the heat yield produced by the aquifer. This finding further advocates the dual use of CO2 sequestration together with geothermal production. The Gulf States of the USA have huge quantities of geothermal energy that are still untapped, and accordingly this study could pave the way for the wide-scale development of geothermal power generation in this region…

(Cover photo: A lake at Indian Creek, Louisiana by Finchlake2000)

Vast amounts of geothermal energy are lying beneath the Gulf States of the USA. And according to a new study, CO2 gas could be the key to harnessing it.

The Northern Gulf of Mexico (GOM) sedimentary basin in the Southern United States contains millions of cubic kilometer of subterranean geothermal aquifers. According to one estimate, these aquifers contain a gigantic 46 zeta-joules of energy (that’s 46 with twenty-onezeroes after it) – enough to meet the current power needs of the entire world for approximately 120 years.

But these geothermal riches remain frustratingly out of reach. The reason for this is financial: the relatively low temperatures of the Northern GOM aquifers makes exploiting them expensive (multiple boreholes need to be drilled to reap commercial quantities of geothermal energy).

As a result of this costliness, the geothermal projects that have been launched in the Northern GOM region to date have mostly targeted only the hottest (i.e. most profitable) aquifers, while leaving the cooler majority — which contain the bulk of the region’s geothermal energy — untapped.

And according to new research, the key to freeing the Gulf’s trapped heat energy could come from an unlikely source — carbon dioxide. A study in the journal Geothermal Energy has found that injecting CO2 gas into aquifers in the Northern GOM could significantly increase their heat yield, thereby making them more financially viable.

In the study, Tatyana Plaksina and Christopher White of Texas A&M University used numerical modelling to simulate the effects of injecting CO2 into a virtual geothermal aquifer, similar to the real-life “Camerina A” aquifer in southern Louisiana.

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The model predicted that by injecting small amounts of CO2 gas into the hot base of the aquifer — less than one gram (0.03 oz) per second — the amount of geothermal heat recovered from the water would be increased by around 0.1%, a small but significant rise.

What’s more, Plaksina and White believe that with higher CO2 injection rates, the output of geothermal plants could be increased even more.

Their finding suggests that CO2 injection could boost the profitability of geothermal plants in the Northern GOM, which in turn could induce energy companies to tap the vast store of clean energy sitting beneath the region.

What’s more, if the injected CO2 could be sequestered within the aquifer after being used for geothermal heat extraction, then geothermal plants could potentially become net removers of atmospheric carbon, rather than being ‘merely’ carbon neutral as they are currently.

As well as soaking up atmospheric carbon, performing CO2 sequestration during geothermal energy extraction would also bring financial benefits. This increased income would make hybrid geothermal extraction/CO2 sequestration projects even more commercially attractive, potentially promoting their development across the Gulf region and elsewhere.

There are currently no such hybrid power plants anywhere on Earth. However this new study is a promising theoretical step towards their development.

“I think application of CO2 for geothermal extraction has great potential both as a working fluid in geothermal energy production and a displacement fluid,” says Plaksina.

about the author
Robin Wylie
Freelance earth/space science journalist. Currently finishing off a PhD in volcanology at University College London.