The curious case of Caroline

Caroline is a curious beast. It is not just that she just keeps on giving and that she turns a handy profit for her owners. The curiosity is in what she gives.

Since she started back in the late 1960's Caroline has given more than twice what could have reasonably been expected. And still there seems to be little let up in what she has to offer. Which makes her a tidy little earner for her owners.

Which is all very strange, because what Caroline gives is CO2.

Caroline is a commercial production borehole in South Australia near Mount Gambier. But she is unlike most other production boreholes which typically flow oil or natural gas. Rather, Caroline flows pretty close to pure CO2 to service our need to fizz our drinks, among other things.

Given our problem with CO2 emissions from energy production - Australia emits round 200 million tonnes of CO2 each year to supply our electricity needs - you might ask, why on earth are we paying good money to pump even more CO2 out of the ground in boreholes like Caroline?

The answer is simple. Making pure CO2 is a pretty expensive business.

The CO2 derived from combustion of coal in an electrical power generator, like Hazelwood in Victoria's Latrobe Valley, makes up just a fraction of a flue gas mix largely dominated by nitrogen.

It takes a lot of energy to separate that CO2 from the other flue gases, as is required for commercial grade CO2 or for carbon capture of storage (CCS). In fact, the energy penalty is around 30% for conventional CO2 flue gas separation processes. In other words, to clean a flue gas stream into a near pure CO2, requires making about 30% more CO2.

That's an expensive overhead.

So that is why when you can find more or less pure reservoirs of CO2 simply by drilling into them, as the Caroline borehole has, you can turn a handy profit.

But Caroline has other stories to tell.

The accumulation of CO2 in subsurface reservoirs, such as that tapped by the Caroline borehole, tells us that the earth's crust has significant capacity to hold fluids over long geological timescales. Of course, we exploit this very fact by flowing around 7 billion tonnes of oil and natural gas from sub-surface reservoirs each year to serve our energy needs.

But just as we can, and have, exploited the capacity for the earth's crust to store such fluids, Caroline provides us with the insight that we can return CO2 back underground from where it came, and have some confidence it will stay there. And that is the essence of the storage side of CCS.

But Caroline also tells us that the crust is not always a perfect reservoir. Already Caroline seems to have provided more than twice its anticipated yield of CO2. So as we draw off CO2 from the Caroline reservoir, the reservoir must be naturally recharging.

To understand this we need to think of the crust in terms of a transient fluid reservoir, in which fluids are constantly passing though the tiny holes that lie between the grains of sand and other components that make up the solid rock mass.

This framework is precisely what allows us to understand groundwater aquifers. As the tiny water filled holes - or pore space - in these aquifers are recharged by surface water infiltration high in the landscape they are drained by discharge down the aquifer flow path to lower parts of the landscape. Understanding that the pore-space is just temporary transit point for the water on route through the aquifer leads to the recognition of a dynamic system. When we pump ground water from boreholes at a rate exceeding recharge, we necessarily deplete the groundwater store. Managing groundwater aquifers for long-term use requires balancing discharge and recharge.

Similar principles hold for carbon-bearing fluids such as oil, natural gas and CO2, though the natural source of these fluids comes from the rock mass itself, through the "maturation" of buried organic matter or oxidation of deep inorganic carbon buried 100's of kilometers down in the earth's mantle.

In the case of Caroline, the CO2 derives from processes deep in the earth's mantle allied to the volcanic province in south-eastern South Australia and western Victoria. On its way to the surface, this deeply sourced CO2 temporarily accumulates in the reservoir several kilometers deep tapped by the Caroline borehole. But because the reservoir must be naturally recharging and draining, the CO2 it contains is destined to make its way to the surface, to recharge the atmosphere as part of the natural carbon cycle.

The notion that the earth's crust is naturally fluxed by carbon-bearing fluids is dramatically illustrated in parts of the world such as Timor Leste, where natural gas (mainly methane) leaks to the surface in such large quantities that it sustains numerous natural flames [see footnote 1].

In our quest to provide the numerous benefits that derive from cheap and accessible energy, we have dramatically impacted on the rates of natural carbon cycling through our earth's crust. By depleting one reservoir of carbon we have necessarily enriched another - the Earth's atmosphere.

Noting we didn't set out with the desire to enrich the atmosphere in CO2, we now understand an unintentional consequence of this activity is a significant change in our climate system, with a very high probability of particularly dire consequences for Australia. The curious case of Caroline shines a light on the challenges and opportunities of one strategy for how we might manage the problem, namely CCS. Caroline shows us that CO2 can readily be returned to the crust, but doing so comes at a significant expense and with some risk. Caroline shows us that to manage the problem we need to better understand the crust's natural fluid cycling.

Perhaps more pertinently, the curious case of Caroline suggests we might benefit from rethinking our relationship with our earth's crust. Rather than simply treating it as a source of resources to be extracted or a repository for our waste, there is potentially greater value in treating it as a system to managed for the variety of services it can provide. For example, as we now understand is true of our groundwater systems, there is benefit in attributing value to the pore space, beyond the value of the fluids that can be extracted in the short-run.

The idea of "crustal services" has obvious analogies with the way ecologists frame ecological services and is a theme that I hope to return to in a future post.

1. Timor is essentially a highly fractured natural gas reservoir. In fact its geology shares much in common with that of the Timor Sea gas fields such as Greater Sunrise and Bayu-Undan. The only significant difference is that geologically Timor has been literally smashed up in the process of tectonic "collision" between the leading edge of the Australian continent and the Indonesian volcanic island chain. That "collision" commenced about three million years ago, and on Timor has lead to the fracturing of the reservoir seals that would otherwise trap the gas in deep underground reservoirs.

Mike Sandiford, Director, Melbourne Energy Institute, University of Melbourne

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