A new time-based model makes projects more achievable, less complex, and less costly.
Carbon capture and sequestration (CCS) could be the foundation of a cleaner future, especially for industries where CO₂ generation can’t be avoided. Thanks to a breakthrough computational model developed by SCS Engineers, that future is much nearer. It changes how critical pressures are calculated by considering both the ‘where’ and the ‘when,’ the model’s precision yields an Area of Review (AoR) much smaller than that suggested by standard methods.
This equates to finding a missing puzzle piece. “The model’s accuracy brings onsite CO₂ storage into focus,” says Dr. Charles Hostetler, Senior Project Advisor and Subject Matter Expert with SCS Engineers. “CCS is now a real option that reduces dependence on expensive pipelines or long-distance transport, and eases public concern about safety or land use.”
A Leap Forward for Area of Review Modeling
To obtain a Class VI well permit, applicants must define the AoR, the subsurface area where injected CO2 and brine might leak from the caprock into underground sources of drinking water (USDW). The AoR boundary is defined by calculating ‘critical pressure,’ which is the pressure required to lift CO2 and brine from the injection zone up the length of an abandoned borehole or well into a USDW.
The standard USEPA method is based on reaching hydrostatic equilibrium, or the point at which brine will no longer migrate. It uses a potential energy comparison to establish the balance point between pressure differentials from two onsite structures: a borehole filled with CO2 brine and an abandoned oil and gas exploration well.
“But here’s the catch,” says Charles. “The standard method doesn’t consider how pressure fluctuates over time, or how long it takes for things to equalize.”
As a result, the standard method often underestimates the critical pressure. In turn, it drastically overestimates the extent of the AoR boundary, sometimes by as much as 50 to 100 square miles. This makes CCS projects much larger, more complicated, and more expensive than they need to be, often affecting a project’s viability.
“Our model looks at how brine moves over time and with changing pressure in the injection zone,” says Charles. “It produces a more accurate, right-sized AoR, which makes carbon sequestration more achievable.”
Why Timing Is Everything for Critical Pressure
In 2023, SCS Engineers began working with the USEPA to design a groundbreaking Computational Pressure Model (CP Model) for a deep well site on the West Coast. This model factored in real-world influences, including temperature, pressure, and salinity, to see how hydrostatic pressure and brine migration changed over time. Built with site-specific subsurface and salinity data, the CP Model divided the subsurface into detailed cells that represented the USDW, the CO₂-brine-filled borehole, the abandoned well, and the CO₂ injection zone. The result was a project-specific pressure timeline that included:
Two Big Takeaways from the CP Model
The model proved eye-opening, changing theories about risk in carbon storage. “The dynamics of carbon sequestration have to be considered to assess potential risks accurately,” says Charles. This is because:
1. Over-pressurization Doesn’t Last Forever The model showed that the period of overpressure in the injection zone was actually limited, and that’s a big deal. Why? This also limits the potential for upward brine migration. The CP Model revealed that once the injection stopped, pressure in the injection zone dropped quickly.
2. Hydrostatic Equilibrium Takes Its Time The rate of brine flow through an improperly plugged well is a complex function of time. The CP Model found that the system took a long time to reach hydrostatic equilibrium. The standard method assumed balance was reached right away, but the model showed it’s actually a slow process.
Mapping Endangerment Potential
After two years of collaboration, SCS Engineers and the USEPA fine-tuned the CP Model and took things a step further. They developed detailed site contours that accounted for pressure increases, salinity, and subsurface conditions.
“But instead of using just one data point,” says Charles, “we used thousands of data points to create a dynamic map that pinpointed where migration risks were higher or lower.”
Using the Endangerment Potential Map, SCS Engineers determined that the migration risk from any single CCS borehole was about 1 in 10 million, providing the USEPA with a strong safety margin. Even better, both the CP Model and the map can be updated with real-time monitoring data from the Pre-injection Testing and Operation phases to refine ongoing site management.
This new time-based model makes carbon sequestration projects more achievable, less complex, and significantly less costly, accelerating the path to a cleaner future. “This model doesn’t just change how we calculate critical pressure,” says Charles. “It carves an achievable path to cut carbon emissions, even for hard-to-abate sectors, and that’s a win for everyone.”
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