Suresh Kumar Govindarajan

Your question covers several complex aspects of geological CO2 storage and leakage risk assessment. Here are the answers to your questions:

1. Assessment of sealing error accuracy in the absence of exploration and production data

Assessing the accuracy of sealing error in the absence of exploration and production data can be challenging, but it is not impossible. In this case, alternative methods can be used to estimate the sealing capacity, such as:

• Mathematical and physical simulations: These simulations can use general data about geological formations, such as soil type, porosity, permeability, and other geological parameters, to estimate sealing capacity.
• Models based on similar geological formations: In the absence of specific data, data from similar areas or soils that have already been explored and show similar geological structures can be used.
• Preliminary estimates based on geophysical methods: Even without production data, geophysical data (such as seismic data) can provide information about geological conditions and potential risk areas.

It is possible to estimate sealing capacity and leakage risk without direct exploration and production data, but these estimates will be less accurate and carry greater uncertainty.

2. Sufficiency of (a) seismic interpretation, (b) quantification of clay in fault zones, and (c) likelihood of continuity of sand from well to fault for assessing uncertainty related to fault column heights

• (a) Seismic interpretation: Seismic interpretation can be very useful for understanding structural characteristics of the soil, such as faults, fractures, and faults. It can help determine potential CO2 migration pathways in deep aquifers, but on its own, it may have limited precision in assessing fault column heights or their stability.
• (b) Quantification of clay in fault zones: Quantifying clay content is important because clay has low permeability, which can help seal CO2 and prevent its leakage. Accurate measurement of clay content can improve assessments of where and how CO2 can be trapped, but it won't always resolve all uncertainties related to fault column heights.
• (c) Likelihood of sand continuity from well to fault: The continuity of sandy layers between the well and the fault can be a critical factor in determining CO2 migration pathways and leakage risks. Sandy layers have higher permeability than other soil types, so it is important to determine whether such layers are continuous and would allow CO2 to leak rapidly to the surface.

While seismic interpretation, clay quantification, and assessments of sand layer continuity can provide valuable information, they may not be sufficient on their own for a full uncertainty assessment, especially concerning fault column heights. These estimates should be supplemented with other data and models, such as hydrogeological simulations and fault mechanisms.

3. Impact of burial depth and clay content in fault zones on CO2 leakage pathways

• (A) Impact of burial depth: The depth at which CO2 is stored has a significant impact on its leakage. The greater the depth, the greater the stability, and the less likely CO2 is to migrate through the soil and escape to the surface. Furthermore, deeper formations often have more layers that can act as seals, reducing the risk of leakage.
• (B) Impact of clay content in fault zones: Clay is a natural sealant because it has low permeability. When clay layers are present in fault zones, they can act as barriers that prevent CO2 from leaking to the surface. However, in the case of faults or other geological processes, large clay layers may be disrupted, which reduces their sealing properties and may lead to CO2 leakage.

Both burial depth and clay content play a key role in assessing CO2 leakage pathways. The deeper the layers and the more clay present, the lower the risk of CO2 leakage. On the other hand, various geological processes, such as faults, can weaken these properties and increase the risk.

Conclusion:

Assessing the risk of CO2 leakage requires an integrated approach that combines geophysical, hydrogeological, and geological data. All three questions you posed are important for assessing and preventing CO2 leakage from deep geological formations.