從浦項事件學習：解決地熱能源造成的地震問題
By Josie
Garthwaite
2017年11月的某個下午，一場規模5.5的地震襲擊了南韓浦項，造成數十人受傷，1700多位居民必須遷往緊急住宅。最近發表的研究指出罪魁禍首是一項地熱能源發展計畫。
2017年南韓浦項發生的地震和地熱發電廠有關。來源：Yonhap/EPA-EFE/Shutterstock
「我們有十足把握。」史丹佛大學的地球物理學家William
Ellsworth表示。「在科學研究上我們通常不會這麼肯定，但此例是鐵證如山。」包括Ellsworth以及首爾國立大學的李岡根在內的科學團隊，5月24日在《科學》(Science)發表了評論，描述浦項的失敗經驗帶來的教訓。
浦項地震為何如此受人矚目？因為它是目前為主跟加強型地熱系統(enhanced
geothermal system)有直接關聯的最大地震。加強型地熱系統一般會在地下開通新的管道，使地球內部的熱能可以到達地表並用來發電。包含美國在內，現在有越來越多國家開始推動低碳能源，這項技術的出現可以持續提供穩定的電力，補足發展條件嚴苛的風力和太陽能。有些研究估計加強型地熱系統可以達到美國目前發電裝置容量的10%。因此了解浦項的地熱系統出了什麼差錯，可以讓其他地區發展這項極具潛力的能源時更加安全。
數十年來運用地熱資源來發電的地方，通常地下深處的熱能和水氣會通過原本就能讓水流通的岩層冒出來。而在浦項和其他加強型地熱系統的計畫當中，會在不透水的岩層當中注入流體來撐出裂隙，製造用來提取地球內部熱能的管道。唯有透過這種方式，這些地方的熱能才能用來發電。
Ellsworth說：「我們半個世紀以前就已經知道，把流體用高壓注入地球可能會造成地震。」Ellsworth是史丹佛大學誘發與觸發地震研究中心的主任之一，也是該校地球能源與環境科學院的教授。
接下來的段落中Ellsworth解釋浦項計畫出了什麼差錯。他們的分析結果不只可以幫助降低下一代地熱發電廠的致災風險，對於依據類似技術來壓裂岩石的其他計畫也有助益。此外，他也會討論為什麼有這些風險，他還是認為加強型地熱在再生能源中可以擔起重任。
加強型地熱系統如何運作？
WILLIAM ELLSWORTH：加強型地熱系統的目的，是在高溫而且透水性相當差的岩石中，製造可以讓水流動的裂隙網路。有了裂隙網路之後再鑿兩口井就能進行熱能交換：把冷水注入其中一口井，經過地底加溫之後，再從另一口井把熱水抽上來。
開鑿地熱井並在井壁襯上鋼管的施工過程和技術，就和建造油井時用的一樣。但在井底會留下一段開放的岩石，接著施工人員會以高壓把水灌進來，撐開已經存在的裂隙，或是創造出新的。
這些微小的裂隙有時候會產生微小的地震，問題在於地震變得太大的時候。
是什麼原因造成南韓浦項發生大地震？
ELLSWORTH：他們注入高壓流體之後，其中一口井按照計畫產生了裂隙網路。但注入另一口井的水，卻讓一條通過這口井，但之前並未發現的斷層開始活動。
沿著斷層帶傳來的水壓減輕了原本作用在斷層上的力，使斷層變得容易滑動。雖然工作人員有停止注入流體或是降低壓力，但微小的地震仍然持續了幾個禮拜，而且隨時間經過地震還變得越來越大。
人們應該要從這些跡象警覺到不需要相當巨大的外力就能引發大地震。這點便是特別危險的地方：注入流體造成的壓力成為了地震發生的最後一根稻草。
地熱或其他類型的能源計畫中，因為注入流體產生的地震帶來的威脅，目前有什麼方法可以監視並降到最低嗎？
ELLSWORTH：一般而言，世界各地的政府不希望鑽井和注水造成的地震影響人民。實務上來說，政府和鑽井業者傾向把重點放在防止人們可以感覺到的小地震，而非避免可能造成嚴重損害，但機率低上許多的大地震。
因為如此，許多計畫採用「交通號誌」系統來管理。地震很小的時候是綠燈，工程可以照常進行。如果地震開始變大就調整操作方式。當地震太大就得整個停下來至少一段時間。此時就是紅燈狀態。
許多地熱和油氣鑽探計畫依據的假設認為，只要注入井裡的流體不超過某個體積，引發的地震就不會超過某個強度。在某些地方這個假設也許是對的，但浦項的經驗告訴我們並非全然如此。
什麼是更好的方法？
ELLSWORTH：我們必須隨時考慮意外或誘發地震發生的可能性。重要的是，看待這件事的觀點要從「危害」(hazard)提升成「風險」(risk)。危害是指傷害或威脅的可能來源；風險則是傷害或威脅造成損害的機率。以這樣的思路來說，跟浦項一樣大的地震不管是襲擊人口密集的都市，或是杳無人煙的沙漠，都是一樣的的危害，但在都市造成的風險卻高出許多。
雖然造成嚴重損害的事件發生機率可能很小，但我們需要體認它們的存在，並在進行決策時列入考量。或許得到的決定是這項計畫根本就不該執行。
舉個例子：如果計畫開始前便知道可能引發規模5.0的地震，就能估計地震造成的損害與傷亡。如果我們得出不同規模的地震發生機率，政府當局就能決定是否接受這些風險，以及計畫應該在什麼樣的規範下執行。
計畫開始進行之後，執行單位與政府之間仍要持續交流。如果一個斷層開始活動，地震造成損害的機率跟著提高，政府當局和專案管理或許得站出來說「到此為止」。
綜合從浦項得到的研究結果，您覺得加強型地熱發電的發展腳步應該要慢下來嗎？
ELLSWORTH：自然的地熱系統是潔淨能源很重要的來源之一，但這些系統的數量不多而且耗盡的速度很快。如果我們可以找到安全的方法來建立加強型地熱系統發電廠，在電力供給和暖房系統上就多了一個低碳選擇，對人類來說具有相當大的利益。
屬於史丹佛大學的共同作者還有博士後研究員Cornelius Langenbruch。其他共同作者來自於瑞士的蘇黎世聯邦理工學院、紐西蘭的威靈頓維多利亞大學、美國的科羅拉多大學波德分校、中國地震局、韓國的首爾國立大學、全南大學和忠南大學。
研究經費來自於韓國能源科技研究所。
Lessons from Pohang: Solving
geothermal energy's earthquake problem
On a November afternoon in 2017, a
magnitude 5.5 earthquake shook Pohang, South Korea, injuring dozens and forcing
more than 1,700 of the city’s residents into emergency housing. Research now
shows that development of a geothermal energy project shoulders the blame.
“There is no doubt,” said Stanford geophysicist
William Ellsworth. “Usually we don’t say that in science, but in this case, the
evidence is overwhelming.” Ellsworth is among a group of scientists, including
Kang-Kun Lee of Seoul National University, who published a perspective piece
May 24 in Science outlining lessons
from Pohang’s failure.
The Pohang earthquake stands out as by far the
largest ever linked directly to development of what’s known as an enhanced
geothermal system, which typically involves forcing open new underground
pathways for Earth’s heat to reach the surface and generate power. And it comes
at a time when the technology could provide a stable, ever-present complement
to more finicky wind and solar power as a growing number of nations and U.S.
states push to develop low-carbon energy sources. By some estimates, it could
amount to as much as 10 percent of current U.S. electric capacity.
Understanding what went wrong in Pohang could allow other regions to more
safely develop this promising energy source.
Conventional geothermal resources have been
generating power for decades in places where heat and water from deep
underground can burble up through naturally permeable rock. In Pohang, as in
other enhanced geothermal projects, injections cracked open impermeable rocks
to create conduits for heat from the Earth that would otherwise remain inaccessible
for making electricity.
“We have understood for half a century that this
process of pumping up the Earth with high pressure can cause earthquakes,” said
Ellsworth, who co-directs the Stanford Center for Induced and Triggered
Seismicity and is a professor in the School of Earth, Energy &
Environmental Sciences (Stanford Earth).
Here, Ellsworth explains what failed in Pohang and
how their analysis could help lower risks for not only the next generation of
geothermal plants, but also fracking projects that rely on similar technology.
He also discusses why, despite these risks, he still believes enhanced
geothermal can play a role in providing renewable energy.
How does enhanced
geothermal technology work?
WILLIAM ELLSWORTH: The goal of an enhanced geothermal
system is to create a network of fractures in hot rock that is otherwise too
impermeable for water to flow through. If you can create that network of
fractures, then you can use two wells to create a heat exchanger. You pump cold
water down one, the Earth warms it up, and you extract hot water at the other
end.
Operators drilling a geothermal well line it with a
steel tube using the same process and technology used to construct an oil well.
A section of bare rock is left open at the bottom of the well. They pump water
into the well at high pressure, forcing open existing fractures or creating new
ones.
Sometimes these tiny fractures make tiny little
earthquakes. The problem is when the earthquakes get too big.
What led to the
big earthquake in Pohang, South Korea?
ELLSWORTH: When they began injecting fluids at high
pressure, one well produced a network of fractures as planned. But water
injected in the other well began to activate a previously unknown fault that
crossed right through the well.
Pressure migrating into the fault zone reduced the
forces that would normally make it difficult for the fault to move. Small
earthquakes lingered for weeks after the operators turned the pumps off or
backed off the pressure. And the earthquakes kept getting bigger as time went
by.
That should have been recognized as a sign that it
wouldn’t take a very big kick to trigger a strong earthquake. This was a
particularly dangerous place. Pressure from the fluid injections ended up
providing the kick.
What are the
current methods for monitoring and minimizing the threat of earthquakes related
to fluid injection for geothermal or other types of energy projects?
ELLSWORTH: Civil authorities worldwide generally
don’t want drilling and injection to cause earthquakes big enough to disturb
people. In practice, authorities and drillers tend to focus more on preventing
small earthquakes that can be felt rather than on avoiding the much less likely
event of an earthquake strong enough to do serious harm.
With this in mind, many projects are managed by using
a so-called traffic light system. As long as the earthquakes are small, then
you have a green light and you go ahead. If earthquakes begin to get larger,
then you adjust operations. And if they get too big then you stop, at least
temporarily. That’s the red light.
Many geothermal, oil and gas projects have also been
guided by a hypothesis that as long as you don’t put more than a certain volume
of fluid into a well, you won’t get earthquakes beyond a certain size. There
may be some truth to that in some places, but the experience in Pohang tells us
it’s not the whole story.
What would a better
approach look like?
ELLSWORTH: The potential for a runaway or triggered
earthquake always has to be considered. And it’s important to consider it
through the lens of evolving risk rather than hazard. Hazard is a potential
source of harm or danger. Risk is the possibility of loss caused by harm or
danger. Think of it this way: An earthquake as large as Pohang poses the same
hazard whether it strikes in a densely populated city or an uninhabited desert.
But the risk is very much higher in the city.
The probability of a serious event may be small, but
it needs to be acknowledged and factored into decisions. Maybe you would decide
that this is not such a good idea at all.
For example, if there’s a possibility of a magnitude
5.0 earthquake before the project starts, then you can estimate the damages and
injuries that might be expected. If we can assign a probability to earthquakes
of different magnitudes, then civil authorities can decide whether or not they
want to accept the risk and under what terms.
As the project proceeds, those conversations need to
continue. If a fault ends up being activated and the chance of a damaging
earthquake increases, civil authorities and project managers might say, “we’re
done.”
From everything
you’ve learned about what happened at Pohang, do you think enhanced geothermal
development should slow down?
ELLSWORTH: Natural geothermal systems are an
important source of clean energy. But they are rare and pretty much tapped out.
If we can figure out how to safely develop power plants based on enhanced
geothermal systems technology, it’s going to have huge benefits for all of us
as a low-carbon option for electricity and space heating.
Additional Stanford co-authors include postdoctoral
research fellow Cornelius Langenbruch. Other co-authors are affiliated with
ETH, Zurich in Switzerland, Victoria University of Wellington in New Zealand,
University of Colorado, Boulder, the China Earthquake Administration, and Seoul
National University, Chonnam National University and Chungnam National
University in the Republic of Korea.
The work was supported by the Korea Institute of
Energy Technology.
原始論文：Kang-Kun Lee, William L. Ellsworth, Domenico
Giardini, John Townend, Shemin Ge, Toshihiko Shimamoto, In-Wook Yeo, Tae-Seob
Kang, Junkee Rhie, Dong-Hoon Sheen, Chandong Chang, Jeong-Ung Woo, Cornelius
Langenbruch. Managing injection-induced seismic risks. Science,
2019; 364 (6442): 730 DOI: 10.1126/science.aax1878
引用自：Stanford's School of Earth, Energy &
Environmental Sciences. "Lessons from Pohang: Solving geothermal energy's
earthquake problem:“
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