Suresh Kumar Govindarajan

CO2 sequestration, especially at field-scale, presents several challenges when compared to laboratory-scale experimentation due to the complexity of real-world geological conditions, as well as the differences in the spatial and temporal scales at which processes occur. Below are the answers to your questions based on the comparison between laboratory-scale (e.g., core-flooding experiments) and field-scale CO2 sequestration:

1. Consequences at Field-Scale Affecting CO2 Storage Efficiency and Safety

(a) Gravity-Override and Viscous-Fingering Impact

• Adverse Mobility Ratio and Density Contrast: At the field-scale, the density contrast between the injected super-critical CO2 (sc-CO2) and the formation brine, as well as the mobility ratio (the ratio of CO2 viscosity to brine viscosity), plays a significant role in the CO2 spreading pattern. If the CO2 has a lower viscosity than the brine, gravity override and viscous fingering can occur. This leads to CO2 spreading laterally instead of displacing formation brine vertically, which may reduce the efficiency of CO2 storage and increase the risk of leakage.Gravity Override: The CO2, being less dense than brine, will tend to rise and accumulate at the top of the formation. This reduces the amount of CO2 that can be stored in the deeper layers and risks capillary pressure-driven migration, potentially causing leakage into overlying formations. Viscous Fingering: This phenomenon occurs when the CO2 displaces brine unevenly due to viscosity differences. It can create channels or fingers of CO2 moving faster than the surrounding brine, thus limiting the displacement efficiency and potentially creating bypassed zones that could later release CO2.
• Field-scale consequences: These factors, particularly in heterogeneous reservoirs, impede the ability to predict and control CO2 movement accurately, resulting in lower efficiency and potentially compromising safety by increasing the risk of leakage.

(b) CO2 Accumulation and Fracture Formation in Cap-Rock

• Fracture Creation: At field-scale, large volumes of CO2 injected into the reservoir can lead to significant pressure buildup. If the pressure exceeds the fracture strength of the cap-rock, fractures can form, potentially allowing CO2 to leak into overlying formations or into the atmosphere.Safety risks: The risk of CO2 leakage due to fractures in the cap-rock is a critical concern for field-scale CO2 sequestration, as fractures can provide pathways for CO2 to migrate beyond the intended storage formation, undermining the storage integrity.
• Field-scale consequences: This issue is particularly dangerous in formations with low capillary strength and complex cap-rock integrity. Fracture formation, therefore, poses a significant threat to the safety of large-scale CO2 storage operations.

(c) Residual Brine Saturation and Trapping Capacity

• Residual Trapping Capacity: In CO2 sequestration, residual trapping refers to the CO2 that is immobile and trapped in small pores, which is critical for long-term storage security. High residual brine saturation can reduce the effectiveness of this trapping, as it limits the available pore space for CO2 to be trapped in the formation.
• Field-scale consequences: If residual brine saturation is high due to the inability of CO2 to displace brine effectively (e.g., caused by poor sweep efficiency or viscous fingering), this may reduce the overall trapping capacity of the storage site and compromise the long-term containment of CO2.

2. Laboratory-Scale Experiments and Their Effectiveness in Evaluating CO2 Injection Behavior

At the laboratory scale, experiments like core-flooding using WAG (Water-Alternating-Gas), CO2-SWAG, CO2-foam injection, or direct viscosification of CO2 can provide valuable insights. Here's how these methods address different parameters:

• (a) Pressure Drop During CO2 Injection: Core-flooding experiments provide a controlled environment to measure pressure drop during CO2 injection. Methods like WAG or CO2-foam injection can simulate more realistic injection conditions, affecting pressure drop. The formation of foam or the alternation of water and gas phases may help reduce the pressure drop by enhancing CO2 mobility and improving sweep efficiency.
• (b) Residual Brine Saturation: Core-flooding experiments can directly measure residual brine saturation after CO2 injection by monitoring how much brine remains trapped in the pores after CO2 has been injected. Techniques such as CO2-foam injection can also reduce residual brine saturation by improving CO2 displacement efficiency.
• (c) Breakthrough Time: Breakthrough time refers to the time it takes for CO2 to reach the production well after injection begins. Laboratory experiments like core-flooding allow precise measurement of breakthrough times for different injection strategies (e.g., WAG, CO2-SWAG). These methods provide insights into the effectiveness of each strategy in delaying or preventing premature CO2 breakthrough.
• (d) Brine Production @ 1 PV Injection: By injecting one pore volume (PV) of CO2, laboratory-scale experiments can measure how much brine is produced and how CO2 interacts with the formation brine. This helps in understanding the brine displacement behavior and potential for brine leakage or storage capacity reduction during CO2 injection.

3. Field-Scale vs. Laboratory-Scale: Heterogeneity and Forces in CO2 Sequestration

• Complexity of Heterogeneity: Laboratory-scale experiments typically focus on smaller, more uniform core samples, where heterogeneity is simplified or controlled. However, field-scale heterogeneity is often much more complex and is characterized by layered formations, fractures, and variations in permeability and porosity. At field scale, heterogeneity introduces significant variability in CO2 distribution and flow dynamics.
• Capillary, Viscous, and Gravity Forces: The interplay between capillary, viscous, and gravity forces is indeed different between laboratory and field scales. Laboratory experiments usually assume homogenous or simplified porous media to isolate specific mechanisms, while field-scale processes are influenced by more complex geological features.
• Capillarity in Laboratory-Scale: While laboratory experiments can capture the capillary pressure effects from small-scale heterogeneities, it can be challenging to replicate the large-scale capillary forces that occur in the field, especially when dealing with layered reservoirs and large differences in permeability. At field-scale, capillary forces are strongly influenced by the heterogeneity of the reservoir (e.g., variations in rock properties, fractures, and interfaces between different geological layers), which affects the distribution of CO2 and brine. The field-scale capillary pressure is more dynamic due to the larger range of pore throat sizes and more complex distribution of fluids.
• Taylor-Aris Dispersion: In the field, advection and dispersion (including Taylor-Aris dispersion) are more pronounced due to the large-scale heterogeneity, leading to non-ideal, highly diffusive transport behavior. At the laboratory scale, Taylor-Aris dispersion is typically less significant, or at least more easily controlled.

Conclusion:

• Field-scale CO2 sequestration is far more complex than laboratory-scale testing due to the large spatial heterogeneity, pressure buildup, capillary forces, and real-world geological constraints that cannot be fully replicated in a lab setting.
• While laboratory-scale experiments like core-flooding can offer useful insights into local-scale behavior and mechanisms like pressure drop, residual brine saturation, and breakthrough times, they have limitations in capturing the full complexity of field-scale processes, especially in terms of heterogeneity and long-term effects.
• The interplay between capillary, viscous, and gravity forces is indeed different at the two scales, with field-scale heterogeneity leading to differential flow patterns that laboratory-scale experiments cannot fully simulate. Therefore, laboratory-scale tests provide valuable information, but they must be complemented with field-scale studies and real-world monitoring for effective CO2 sequestration management.