Humans have become very good at producing carbon, so good that we now have too much of it in the atmosphere. Fortunately, there’s a solution. Put it back where it came from, under ground. That’s not as simple as it sounds.
Carbon sequestration requires a receptacle, and existing reservoirs are just the thing. But which ones?
That question was the subject of a recent presentation by Joe Mello at CarbonExpo 2022. He is a reservoir engineer and vice president at Netherland, Sewell, and Associates, a petroleum consulting company.
“If somebody asks what makes a good sequestration reservoir, the really easy answer is, if it was a good oil and gas reservoir, it will be a good CCS reservoir,” he said. CCS is carbon capture and sequestration.
But, again, it’s not quite that simple. The geologic setting is important, yes, but so are the type of reservoir, the geomechanics, the depth of the reservoir, and myriad other factors.
Two main types of reservoirs are suitable for carbon sequestration: depleted oil and gas reservoirs, as Mello mentioned, and saline aquifers. Mello also mentioned active oil and gas reservoirs using CO2 for enhanced oil recovery but said that process is well documented, so he wasn’t going to talk about it, adding that much of the information applies to that use as well.
The US Department of Energy has estimated the total storage capacity of both kinds of reservoirs Mello talked about. Depleted oil and gas reservoirs in the US have a CO2 storage capacity of between 182 billion and 232 billion tonnes. For saline aquifers, that number is between 2.3 trillion and 21.6 trillion tonnes. “So, that’s a huge difference, and the bottom line is there’s just a lot more storage capacity out there in saline aquifers compared to oil and gas reservoirs,” Mello said. “Saline aquifers are just much more prevalent than depleted oil and gas fields. As anyone who’s drilled exploration wells looking for oil can probably tell you, there’s a lot of water reservoirs out there.”
Mello was quick to add, however, that depleted oil and gas reservoirs shouldn’t be discounted as sites for carbon sequestration just because fewer are available. Both types of reservoirs have their benefits.
Depleted oil and gas reservoirs generally have lower pore pressures, Mello said, making them a little easier to inject into initially. “And they may be less prone to geomechanical issues,” he said.
The factors determining pore space in the reservoir are a driving consideration, Mello said. “Things like porosity, thickness, and areal extent—those all define the size of the tank you’re working with. … All of those characteristics ultimately lead to more carbon that you can store in a given reservoir. And that’s attractive.”
Permeability is also important because it determines a reservoir’s injectivity. “Higher permeability may let you meet your target injection rate with fewer injection wells,” Mello said.
The most important factor, however, is the reservoir’s top seal, Mello said. “You can’t have a hydrocarbon accumulation without some sort of working top seal, right? Because oil and gas are more buoyant than water, and they would eventually migrate to the surface. Same thing is true for carbon dioxide.”
While an oil and gas reservoir will have a top seal, the wells that have poked through it constitute a double-edged sword for carbon sequestration. On the one hand, the work done with those reservoirs will have created a plethora of data that will be important in the consideration of carbon-storage projects. “By virtue of drilling the production well and producing hydrocarbons in the past, you’ve got more data in the form of logs or probably better seismic shot over those areas,” Mello said. “Also, just the production totals, the historical production volumes—using techniques like material balance, you can actually use those production volumes to estimate fairly accurately the ultimate storage potential of a given reservoir.”
On the other hand, poking holes in the top seal can create avenues for injected carbon dioxide to escape. “Every penetration into a top confining layer is a potential anthropogenic leak point, a manmade pathway for CO2 to escape your storage reservoir,” Mello said. “That’s less desirable. You do not want many penetrations into your caprock.”
Mello pointed out that the risk can be mitigated by vetting any wellbores in the path of the proposed CO2 plume to verify their integrity, “but if you’re dealing with hundreds and hundreds or thousands of penetrations, it increases the risk that something will slip by and CO2 can migrate out of zone.”
The swath of data that comes with depleted oil and gas reservoirs can boost the chances of commerciality, Mello said, giving a nod to the Society of Petroleum Engineer’s Storage Resource Management System (SRMS). The SRMS echoes SPE’s Petroleum Reserves Management System (PRMS), with storage capacity replacing reserves. “It’s very similar in logic and methods and language, and so anyone familiar with the PRMS can pretty easily pick up the SRMS and get right to work,” he said.
Just like in the PRMS, for a reservoir to fit in the SRMS, the storage capacity has to be commercial. “You can be commercial by turning a profit with a storage enterprise through storage fees or some sort of government incentives, but one thing I really like about the SRMS is that they sort of also allow the hurdle of commerciality to be met by coupling your CO2-injection project with a revenue- and CO2-generating project.”
Mello gave an example of an ethanol plant that is emitting CO2. “You don’t have to turn a profit storing the CO2 itself. You can turn a profit selling your ethanol and storing the CO2, sort of ring-fenced together,” he said. “And that’s really important because it recognizes that, going forward, companies may have incentives … that would encourage them or incentivize them to sequester their CO2 even if they’re not necessarily turning a profit at the act of sequestration itself.
“So, as projects move forward and it becomes more and more of these projects out there in the universe for investors to choose amongst, using the SRMS is sort of the unified language to benchmark project vs. project vs. project,” Mello said. “I think it’s going to grow in importance and commonality.”
Another major consideration when selecting a reservoir for CO2 injection is depth. At certain temperatures and pressures, CO2 becomes a supercritical fluid at a much denser state than it is under atmospheric conditions. This shift to supercritical fluid occurs at pressures approximately 3,000 ft under ground, Mello said.
“After it makes that supercritical shift, it becomes much denser and then … as you get deeper, it’s sort of a state of diminishing returns, where going deeper and deeper and deeper does not give you huge benefits,” Mello said. “So, it’s really a balance when you’re selecting a reservoir.”
Deep injections can bring geomechanical considerations, as well. Any injection into the subsurface increases pore pressure, which can reactivate faults. This induced seismicity can become more prominent when the faults are larger or are connected to the reservoir’s crystalline basement, Mello said. “When you see large pressure increases against faults that are connected to the basement that are large, you can see material induced seismicity.”
He pointed out that successful CO2-injection projects are dependent on injecting for a long time and keeping the CO2 in the ground, so candidate reservoirs need to be less vulnerable to major instances of induced seismicity. “Even if small seismic events don’t threaten the seal integrity of your caprock, they can still have an impact on the project in the form of public perception,” he said. “Understandably, a lot of people are not going to be super thrilled with earthquakes in their neighborhood.”
Mello recommends geomechanical due diligence when selecting a sequestration location. “That includes things like identifying preexisting faults and mapping them, identifying fracture networks in reservoir and caprock, identifying past seismic events throughout all the recorded history to understand the magnitude of potential seismic events and understand the stress conditions of the reservoir that you’re injecting into.”
All these considerations are part of the larger picture when selecting a site for CO2 sequestration, Mello said. “Site location, CO2 source, local incentives … all those things matter, and they have to be considered together,” he said. “As you’re considering a project or evaluating a project or screening a project, it’s important to keep the big picture in mind. And make sure that you’re really addressing all the concerns that make a good project and not focusing on just good CO2 sources, just good site locations, or just good geologic settings.”
