Guest post by David Middleton
THE 240MWE FACILITY IS THE LARGEST POST-COMBUSTION CARBON CAPTURE PROJECT IN THE WORLD
WASHINGTON, D.C. — Secretary of Energy Rick Perry took part in a ribbon-cutting ceremony today to mark the opening of Petra Nova, the world’s largest post-combustion carbon capture project, which was completed on-schedule and on-budget. The large-scale demonstration project, located at the W.A. Parish power plant in Thompsons, Texas, is a joint venture between NRG Energy (NRG) and JX Nippon Oil & Gas Exploration Corporation (JX).
“I commend all those who contributed to this major achievement,” said Secretary Perry. “While the Petra Nova project will certainly benefit Texas, it also demonstrates that clean coal technologies can have a meaningful and positive impact on the Nation’s energy security and economic growth.”
Funded in part by the U.S. Department of Energy (DOE) and originally conceived as a 60-megawatt electric (MWe) capture project, the project sponsors expanded the design to capture emissions from 240 MWe of generation at the Houston-area power plant, quadrupling the size of the capture project without additional federal investment. During performance testing, the system demonstrated a carbon capture rate of more than 90 percent.
At its current level of operation, Petra Nova will capture more than 5,000 tons of carbon dioxide (CO2) per day, which will be used for enhanced oil recovery (EOR) at the West Ranch Oil Field. The project is expected to boost production at West Ranch from 500 barrels per day to approximately 15,000 barrels per day. It is estimated that the field holds 60 million barrels of oil recoverable from EOR operations.
The successful commencement of Petra Nova operations also represents an important step in advancing the commercialization of technologies that capture CO2 from the flue gas of existing power plants. Its success could become the model for future coal-fired power generation facilities. The addition of CO2 capture capability to the existing fleet of power plants could support CO2 pipeline infrastructure development and drive domestic EOR opportunities.
The Petra Nova carbon capture system was installed in the W.A. Parish generation station. This is the largest and cleanest fossil fuel generaton station in the United States:
W.A. Parish Electric Generation Station, Thompson, Texas
Owner/operator: Texas Genco Holdings Inc.
Texas Genco has invested heavily in upgrading its W.A. Parish coal- and gas-fired plant southwest of Houston. Although this nine-unit, 3,653-MW plant is the largest fossil-fueled plant in America, its NOx emissions have been reduced to microscopic levels. Based on those levels, W.A. Parish could rightly claim that it is among the cleanest coal plants in the U.S.
Texas Genco’s W.A. Parish Electric Generation Station (WAP) is the largest coal- and gas-fired power facility in the U.S. based on total net generating capacity. It and its owner, Texas Genco Holdings Inc., operate in the Electric Reliability Council of Texas (ERCOT), one of the largest electric power markets in the nation. Over the past few years, the majority-owned subsidiary of Houston-based CenterPoint Energy Inc. has met the challenge of adding emissions control equipment to these baseload units while maintaining the availability and reliability required by ERCOT’s competitive market.
In the process, Texas Genco has emerged as an industry leader at reducing emissions and demonstrating new NOx-control technologies. The company’s fleet of plants operates at one of the lowest NOx emission rates in the country, and WAP likely emits less NOx on a lb/MMBtu basis than any coal-fired plant of any size in the U.S. Cleanliness is costly; the company has spent more than $700 million on new emission controls since 1999.
With the commissioning of another round of emissions-control equipment this year, NOx emissions from Texas Genco’s Houston-area power plants—including WAP—will be 88% lower than 1998 levels. These actions play a major role in the Houston/Galveston Area Ozone State Implementation Plan and are helping to clean the air in the greater Houston area. To honor the accomplishment, the W.A. Parish plant was recently given the Facility Award by the Power Industry Division of the Instrumentation, Systems, and Automation Society (Research Triangle Park, N.C.) for installing equipment to reduce emissions and improve reliability while minimizing operational costs.
[…]
The W.A. Parrish Generation Station has a generating capacity of about 3,660 MW (2,740 MW of coal and 1,190 MW of natural gas capacity). Its total capacity is approximately the same as the ten largest solar PV plants in the U.S. combined (3,713 MW). From 2002-2009, W.A. Parrish operated at 85% of capacity. The war on coal gradually reduced its operations to 57% of capacity in 2016.
The Petra Nova carbon capture system will enable the plant to capture about 90% of the CO2 from 240 MW of its coal capacity. It is expected to capture about 1.6 million tons of CO2 per year. The cost of the carbon capture system was approximately $1 billion, with the taxpayers picking up 19% of the tab. Normally, I would call this a pointless waste of money. It won’t have any effect on atmospheric CO2 or the weather. However, this carbon capture system actually serves a useful purpose:
The Captured CO2 will employ Enhanced Oil Recovery to enhance production at the West Ranch oil field, which is operated by Hilcorp Energy Company. It is expected that oil production will be boosted from around 300 barrels per day today to up to 15,000 barrels per day while also sequestering CO2 underground. This field is currently estimated to hold approximately 60 million barrels of oil recoverable from EOR operations
How Carbon Capture Works
The West Ranch oil field has produced about 390 million barrels of oil since 1938. CO2 injection will boost the production from 300 to as much as 15,000 barrels of oil per day. The EOR could lead to the recovery of 60 million barrels of oil that would otherwise be “left in the ground.” Irony is such a beautiful thing!
And the really cool thing about this project: It makes money!
FiscalNotes
NRG’s Petra Nova Plant Captures Carbon, Boosts Bottom Line
An interview with David Greeson, Vice President of Development, NRG Energy Inc.
by Brian Wellborn
NRG Energy Inc. (NRG) and JX Nippon Oil & Gas Exploration jointly operate the Petra Nova Carbon Capture project, the world’s largest retrofit post-combustion carbon capture system, at the W.A. Parish Generating Station southwest of Houston.
Fiscal Notes recently spoke with NRG Vice President of Development David Greeson to discuss the Petra Nova project and learn what makes its capture system unique, environmentally sound and profitable.
Fiscal Notes: What are Petra Nova’s broad environmental goals?
David Greeson: The goal of the Petra Nova project is to capture more than 90 percent of the carbon dioxide (CO2) in the exhaust flue gas from an existing coal-fired unit at the W.A. Parish power plant. We want to prove it’s feasible to build a carbon capture system on schedule and on budget. Demonstrating the system working at full commercial scale will provide a path forward to address CO2 emissions from existing coal-fired plants, both in the U.S. and around the world.
In addition, we’re looking to create a commercial structure that couples power generation with oil recovery for potential long-term viability — not only to pay for the carbon capture and storage system but also to provide an economic return for investors.
[…]
Fiscal Notes: How economically viable is Petra Nova’s carbon capture process?
Greeson: As long as oil is priced at around $50 per barrel or above, sales of the oil from the West Ranch field will pay for the Petra Nova project.
[…]
The price of CO2 for EOR projects is generally pegged to the price of oil. At >$50/bbl, the sale of the CO2 to Hilcorp will pay for the carbon capture system. Projects like this do not need subsidies.
This will enable the coal-fired plants to operate at a higher capacity and prevent 60 million barrels of oil from becoming “stranded assets.” I just love irony!
Addendum
4/19/2017
DOE estimates that CO2 EOR could recover about 85 billion barrels of oil from existing U.S. oil fields:
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CO2 sequestration is a misguided follow-on to the technologically misleading concept of ‘carbon footprint’. The planet atmosphere came dangerously close to the low level which would have resulted in extinction of all land plants and animals at the end of the last glaciation. It is still impoverished for CO2. Following are CO2 levels at various times and conditions:
Carbon dioxide levels, ppmv
40,000 Exhaled breath
20,000 No symptoms in healthy young people below this level
8,000 OSHA limit for 8 hr exposure
5,000 OSHA limit for continuous exposure
5,000 Approximate level 500 million years ago
1,500 Artificial increase in some greenhouses to enhance plant growth
1,000 Approximate level 100 million years ago
1,000 Common target maximum for ventilation design for buildings
405 Current atmospheric level
275 Atmospheric level before industrial revolution
190 Atmospheric level at end of last glaciation
150 All land plants and animals become extinct below this level.
This is emphasized graphically as a bar chart in Figure 7 of the analysis at http://globalclimatedrivers2.blogspot.com
I think I can see why this caught your attention. It has a certain cool factor that pumping CO2 into the ground don’t have.
Energy. The less expensive the better. Sometimes workers ask where their raise went. The power bill people. Did you not take basic math in school. More likely you voted for someone who dazzled you with magical thinking.
This appears to be a win-win situation, having nothing to do with being “green” or with climate. The CO2 in compressed form is simply acting as an industrial component, aiding in the production of oil. I had my doubts about it, but everything appears to be above board.
Financing Mega Scale Energy Projects
Further details on the challenges and details in this project.
A Case Study of the Petra Nova Carbon Capture Project
A very good way to do business!
December 4, 2008 …
The idea behind the new commercial from the Alliance for Climate Protection is that clean coal is like the emperor’s clothes: It doesn’t exist. Gore — the winner of the 2007 Nobel Peace Prize for his work on raising awareness about climate change — tells NPR’s Robert Siegel that clean coal technology is not “anywhere close to being a reality.”
http://www.npr.org/templates/story/story.php?storyId=97825453
From Shell …
Carbon capture: “the technology we cannot afford to ignore”
It’s significant and exciting. Think of the issues that currently shape our approach to the future of energy. The future of renewables is hotly debated at the moment, as is energy efficiency and how we approach nuclear power. The electrification of transport and heating are also big issues in how we think of the future of energy.
CCS is another one. Yet there is a sense that it is still to meet its potential. That needs to change. Think of all the effective international actions, policies and investment that have been rightfully put behind renewables in recent years. If the same level of effort was put into CCS, it could make a real difference.
http://www.shell.com/inside-energy/carbon-capture-the-technology-we-cannot-afford-to-ignore.html
Al Gore is a moron and generally wrong every time he flaps his gums.
Carbon capture and sequestration (CCS) only makes sense, if you intend to do something useful with the CO2, like piping it to old oil fields for enhanced recovery projects.
This is not new. Since 2000 the Dakota Gasification Company in North Dakota has been collecting CO2 from their syngas plant and piping it to the Weyburn oil field in Saskatchewan for Enhanced Oil Recovery.
The word “clean” should not appear in any of this. It is abuse of the language.
re: the use by the AUTHOR of this post the following verbiage (in more than ONE spot I might mention) “The Petra Nova carbon capture system was … ”
Shoot, I give up.
We have bought “lock, stock and barrel” into the terminology of the greens and the left …
That’s what it is. It is a carbon capture and sequestration system (CCS). It captures the CO2, post combustion. The CO2 will then be piped to an oil field to be injected for enhanced oil recovery (EOR).
The World Coal Association calls it “carbon capture”…
https://www.worldcoal.org/reducing-co2-emissions/carbon-capture-use-storage
Because that’s what its called.
Due to the pressure in the reservoir the CO2 is at supercritical conditions and becomes a solvent, that is it dissolves the oil and makes it less viscous and easy to flow. Some heavy oils are not that soluble and the asphaltenes are left behind. Supercritical extraction with CO2 is used to extract organic oils from plants and seeds.
The thing I really object to, as the expression “Clean Coal” is applied by industry to such projects, is that CO2 is being implicated as otherwise “dirty” unless captured. Rubbish.
Good point. This particular coal-fired plant was already very clean without the CCS, which does nothing to enhance its cleanliness.
As noted above, O2 from atmospheric gasses is problematic. But is it necessary to recover the CO2 separate from the other flue gasses to have the advantages of the effects of the CO2 on the viscosity of the oil? Presumably there would be little O2 in the flue gas.
“…its NOx emissions have been reduced to microscopic levels. Based on those levels, W.A. Parish could rightly claim that it is among the cleanest coal plants in the U.S.”
At long last. “Clean” here means clean. Here in Australia we still have “clean” in common usage when referring to minimal CO2 emissions. Just plain dumb, the more so when, in the same breath, “carbon” is used invariably to refer to carbon dioxide.
At long last, too, a CCS concept that aims to put the sequestered CO2 to good use. Unless and until that is demonstrably viable, CCS will remain an excruciatingly dumb idea.
David, my thanks to you for your capable education of many folks here about CCS, oil field operations, CO2 solvent injection and geology. Great job.
At the beginning of my career with CVX in Alberta I spent several years working on injection systems for an acid natural gas tertiary drive system in vertical reefs. H2S was about 13% in the injection gas, pressure was 6000 psi.
Three points to round things out for the inquisitive:
1. Separated CO2 at surface is not at atmospheric pressure. It is slightly less than wellhead pressure, say 900 psi for a 1000 psi well. Thus hard won high pressure is not wasted and the recycle compressors are much smaller.
2. Acid gas recovery systems using amines (DRS or MDEA or other) are reliable but expensive and have been used to recover CO2 and H2S in fields worldwide for decades.
3. The NRG geologists would never use CO2 injection if they were worried about the integrity of the overburden rock. So leaving CO2 in the ground is not a problem. More likely is NRG would try to sell it to someone else in the future.
Amine solvent absorption is a well known process for hydrogen sulfide removal from natural gas. Using it to remove carbonic anhydride is a natural. The one hurdle is dealing with water as this would make the process more difficult in regeneration of the amine. I’m certain an adsorber wheel type Lungstrum heat exchanger could be employed only with a four pass configuration.
Chemical engineering beats alternative energy any day.
A ijnteresting article, but I would hope that Carbon capture does not end up with politicians saying that to Save the Planet we must bury all such CO2 from any future fossell fuel power stations. This seems to be the thinking by the likes of PM Malcolm Turnbull regarding any new coal fired power stations here in Australia. We have had cases in nature where CO2 has come to the surface and killed people. .
Michael.
Ordinarily, DOE money in a clean or renewable energy project is “the kiss of death”. If this one does, indeed, turn out to be reliable and commercially viable (and I hope that it does), it will be the exception to the rule. The energy landscape is littered with the rusting remains of failed DOE energy projects.
Oil boom!
Good pun, Brian.
My first oil field summer job was for Amoco at Gregoire Lake near Fort McMurray. The field was oil sand at depth, too deep to mine. So the experimental solution was a fire flood. We pumped air down the injector wells and lit a fire. The heat and steam created by the flame front mobilized the oil so it moved to producer wells. It all worked okay until the slight slope to the reservoir caused all the completions to start sliding downhill. That and the cost of Iconel completions eventually ended the experiment.
But it was a fun summer for an engineering student.
Environmentalists used to advocate for endangered species in the amazon rain forest, pollution in china, and the like. Then, they discovered the BIG ONE. This was the mother of all causes. The solution to this problem required government intervention in the free market on an enormous scale, to the point of even completely shutting down the industrial economy. Consequently, they soon became a united army aggressively calling for the end of fossil fuel use.
Then, one day someone decided to invent carbon capture technology. Suddenly you could trap that CO2 for a much smaller cost than building renewables. But wait a minute! That doesn’t require big government!
That wasn’t supposed to happen! The fallacy!
They never thought of that before they put all their eggs into one basket!
(For the record, climate change is a hoax therefore ANY mitigation is a waste of money, but that being said it’s kind of fun to watch the big green machine squirm at the possibility of carbon capture technology becoming more viable than renewables)
Careful on the cost assumption that CCS is less capital cost than renewables. CCS typically adds 30 to 50% to the cost of a new large power plant.
Fair enough, it’s still experimental so we don’t know yet. But I’ve looked at windmills from an entrepreneur’s perspective and I think it costs about 10 times as much as coal, on the ground. Payback period is in the range of 35-45 years vs. 7-10 for coal. Therefore, it really doesn’t take much to beat renewables.
So… CO2 from coal releases oil from which CO2 subsequently is released.
If the recovered oil were left in the ground and the CO2 from the coal not captured, what would be the net difference for CO2 reaching the atmosphere?
Isn’t this CO2 exchange rather than capture, or am I missing something and perpetual motion has been invented?
This is how capitalism defeats regulatory malfeasance.
This gives you something to fuel your plane with when you go globe trotting to save the planet or golfing on somebody else’s dime.
Another nail in the coffin lid of “peak oil” . The phase of carbon dioxide in this process is supercritical. This makes it an incredible solvent, not only lowering viscosity but also penetrating pore space and liberating tight oil. Supercritical carbon dioxide is used to remove caffeine from whole coffee beans. Because carbon dioxide is an acid anhydride, it preferentially extracts this alkaloid (organic alkaline compound).
Current hard rock unconventional Petro can greatly benefit from this technology.
David,
Thanks for the Great information and follow up with replies as always. I am all for use of CO 2 to improve well production but not for sequestration without positive results. Noting that two Oxygen Molecules are sequester for every one Carbon, I suggest we call it what it is: Oxygen Sequestration which might not sound to good to many of the enviro’s and the uninformed.
A few comments from an engineer that has spent his entire career reducing emissions from coal fired power plants.
The location of this plant is ideal for CO2 sequestration. An oil field located nearby that needs a pressurized gas to extract the oil. There are a few other sites that are currently operating.
However even at this site you are not reducing CO2 but transferring it. It is being removed from the combustion gas as a gas and being compressed into the ground as a gas rather than in the atmosphere. It will be in the ground until something happens that will release it into the atmosphere. Kicking the can down the road.
The federal government has spent 100s of millions of dollars on CO2 sequestration. They have mapped out caverns in the ground that could hold the CO2. Pumping huge quantities of a gas into caverns in the earth’s core, what could go wrong with that? Well, the CO2 will find any small crack and work its way out to the atmosphere. If there a shift or fissure in the rock surrounding the cavern it would release huge amounts of CO2. CO2 is heavier than air and the gas would hug the ground filling all voids like ditches, creeks and basements. The breathable air would be displaced at those locations resulting in death of any living creature in that area.
Sequestering CO2 from coal fired power plants is technically feasible. However it takes huge amounts of power to accomplish. This makes a power plant less efficient, resulting in more fuel being burned, resulting in more CO2 emissions.
The large quantity of power consumed to remove CO2, the large capital costs to build the plant and the expensive operating costs to operate the plant makes a new coal fired plant not economically viable. Add the liability of injecting huge amounts of CO2 in caverns in the earth and coal fired plants are dead.
Delaying the release of CO2 into the atmosphere gives the biosphere more time to do it’s own sequestration thing.
There are no “caverns in the earth’s core.” Nor does CO2 sequestration entail pumping CO2 into “caverns” or anywhere near “the earth’s core.”
It involves drilling or modifying existing wells in depleted oil and gas field and pumping it into pressure-depleted porous reservoirs. The process would not exceed the fracture gradient of the rocks. The injected CO2 would be trapped by the same geologic seals as the now-depleted oil & gas reservoirs were. The injection wells would then be plugged and abandoned. So there would be zero chance of this happening:
Over time, traces amounts of CO2 may leak to the surface; however this would pose no threat to anyone.
Enhanced recovery of oil through CO2 injection has been going on since 1972. There has never been a case where a catastrophic CO2 blowout has occurred.
https://www.netl.doe.gov/file%20library/research/oil-gas/CO2_EOR_Primer.pdf
http://petrowiki.org/CO2_sequestration
https://youtu.be/OtfuYlhDjw4
Ritter, K., Shires, T.M., and Lev-On, M. 2014. Methane Emissions From Natural Gas Systems: A Comparative Assessment for Select Industry Segments. Presented at the SPE International Conference on Health, Safety, and Environment, Long Beach, California, USA, 17-19 March. SPE-168379-MS. http://dx.doi.org/10.2118/168379-MS.
CO2 is heavier than air, especially when cooled by expansion from underground compressed storage. When it leaks to the surface, it forms an oxygen-displacing bubble that suffocates animals and insects. Catastrophic blowout is not necessary to kill. Significant CO2 leaks have happened multiple times in the USA, and multiple times with Denbury Resources alone. I am not aware of any human fatalities so far due to EOR CO2, but there have been wildlife kills. http://www.ernstversusencana.ca/denbury-fined-662500-for-mississippi-blowout-of-co2-injected-in-high-pressure-enhanced-oil-recovery-so-much-carbon-dioxide-came-out-that-it-settled-in-hollows-suffocating-deer-and-other-animals. Natural CO2 blowouts and industrial accidents have killed many people. This detailed actuarial study predicts about 200 deaths per year by 2050 if large-scale CO2 sequestration is attempted (http://minh.haduong.com/files/HaDuong.ea-2011-ActuarialRiskAssessmentCCS2050.pdf).
CO2 is naturally corrosive when in contact with moisture as it forms carbonic acid which is a problem for steel pipes. Of course, the tertiary EOR process is CO2 combined with water flooding. The new injection wells are likely to be stainless steel for this reason, but all the old wells at an EOR field represent possible leak points unless they are properly located and abandoned with cement plugs. It is often more the rule than the exception that old wells are not properly documented. Turning an oil field into a sequestration site is going to be a massive job. Not doing it right is what resulted in Denbury’s leaks and fines.
Also glossed over in this article is the fact that CO2 capture saps energy from the power plant, significantly reducing its thermal efficiency. So it must burn more fossil fuel and generate more polluting emissions to generate the same amount of electricity. Sequestering carbon is actually a plan to increase the consumption rate of fossil fuels and reverses the trend of increasing the efficiency of power plants. My research indicates 65% carbon capture reduces thermal efficiency (increases heat rate) about 15%. This plant is supposed to do 90% capture, so that impact is likely the 25-30% reduction that some commenters have suggested.
A full-blown national campaign to sequester CO2 underground can only be entertained by those with no sense of the geographic scales involved, no concept of the increased polluting emissions, and no sense of the substantial immediate mortality risks compared to the imperceptible and distant climate benefit that is still only based only on falsified climate models.
Blowouts are rare; but they do happen. A $650k fine is trivial. Nor is it pertinent to CO2 sequestered in a properly abandoned oil field.
Predictions of x number of deaths are as worthless as the EPA’s claim that the CPP will prevent 90,000 asthma attacks per year.
In this specific case, the CCS will only affect 10% of the generation station’s coal capacity and it will lead to the production of 60 million barrels of oil, which would otherwise be stranded.
You are correct that the scale of sequestration above and beyond EOR is huge and that the mythical threat of CAGW does not justify the cost.
Super-critical CO2 at 1000 PSI / 74 bar has less than 1/4th the CO2 per unit of volume as as crude oil, so it requires more than 4 times the volume to sequester the same amount of carbon back into the ground as is vacated in producing the oil containing it. So we know before we begin that this is a dead end for the volumes needed for climate change purposes. The alternative plan has become injecting CO2 in to deep saline aquifers. For either of these options, stimulating earthquakes is going to be an issue. Not because pressures exceed the fracture gradient of rocks, but for the same reasons as reinjected fracking disposal water is linked to them — because water is a lubricant, and because it is chemically active in such reactions as serpentinization at depth, that change rock density.
Roger that. There is no climate-related justification for CCS.
But, if a little bit of CCS can enable coal-fired plants to remain in service longer, operate at a higher capacity and help recover some of the 85 billion bbl of oil recoverable through CO2 EOR… It’s a good thing.
Regarding injection wells, the rocks are already saturated with brine before wastewater is injected. The injection wells do increase the pore pressure and this can trigger induced seismicity. The fix for this is to carefully site injection wells where they are sufficiently distant from faults and to manage the pore pressures more carefully.
..You claim to be an engineer, yet, you do not know the definition of the Earth’s core ??
Did someone kidnap and replace seaice1? This version is managing to make sense.
http://breakingenergy.com/2017/04/18/bipartisan-support-grows-carbon-capture/
What’s the quote about bad ideas made even worse through bi-partisan politics?
The last time 99 Senators agreed on something, we got the TSA.
David, your information is true for the sites that use the CO2 to force out oil. However as I stated earlier those sites are unique and there are not a lot of them located near coal fired plants. Department of Energy initiated a program to identify and map caverns throughout the United States to accept injection of CO2 for storage. They called them deep geological formations. They have assigned risk with each area. I personally worked on these projects. These caverns would be used to store removed high pressure CO2 that would be pumped into them.
I worked on a potential coal fired plant that used their proximity to one of the caverns as a potential for the plant to install CO2 sequestration at a future date. Unfortunately the plant was never built, too expensive. For additional reading go to:
https://www.netl.doe.gov/research/coal/carbon-storage/atlasv
The word “cavern” doesn’t appear anywhere in the document because CCS has nothing to do with caverns. DOE put together an atlas of geological formations suitable to be CO2 sequestration reservoirs:
https://www.netl.doe.gov/research/coal/carbon-storage/atlasv
These are formations which are porous, permeable and sealed by impermeable overburden.
Provided care was taken to not exceed the fracture gradient when operating the injection wells, there is absolutely no chance of the sort of catastrophic blowout you described.
Here’s the link to the full atlas…
https://www.netl.doe.gov/File%20Library/Research/Coal/carbon-storage/atlasv/ATLAS-V-2015.pdf
?w=680
I would imagine that even if cracks did form, and even if these cracks did make it all the way to the surface, all that would happen would be a very slow seepage of CO2 into the atmosphere. Slow enough that even sensitive instruments would have trouble detecting it.
And that would be a worst case scenario.
That’s a SyFy channel scenario.