Uncertainty and Risk Due to Known Knowns, Known Unknowns and Unknown Unknowns in Saline Aquifer CO2 Sequestration
Authors: Tewari Raj Deo , Pratama Edo , Chidambaram Prasanna , Refani Mohammad Oggi , Sedaralit M Faizal , Eydinov Dmitry , Tiwari Pankaj Kumar , Das Debasis Priyadarshan 
Published: February 2024

Abstract
Sustainable economic development and environment protection is pivotal for life on earth. Geological storage of increasing emission of CO2 in saline aquifers has huge potential. But saline aquifers are poorly understood due to paucity of data. Known knowns, known unknowns and unknown unknowns are strikingly different in saline aquifers compared to hydrocarbon fields. These uncertainties introduce inaccuracy in studies and thus uncertainty and risk management become inseparable part of CO2 storage development planning. Fit-for-purpose approach incorporating the uncertainties must be taken with correct understanding of the data requirements based on geological complexity, physical and chemical processes being modelled. Business decisions are made under uncertainty because uncertainty may be mitigated to some extent but cannot be eliminated completely. Ignoring uncertainty is not a reality rather it has to be uncovered, recognized, understood, assessed, and addressed in an unbiased way.
Multiple scenarios of static and dynamic models generated by incorporating the seismic, geological, well logs, pressure, temperature, and core data in order to address the known knowns, known unknowns and unknown unknowns. 3-way coupled numerical simulation including geomechanics and geochemistry conducted to derive the number of CO2 injectors and water producers, injection and water production rates, well spacing and completion types, produced water disposal, and storage volumes etc. Effect of pressure management for plume movement, geomechanics and geochemistry are critical for seal integrity and storage volume. Ranges of solutions estimated to avoid the deterministic high and low cases that is often inefficient as high-case of ‘hole-in-one’ may suggest unplausible optimistic scenario whereas the pessimistic low-case of ‘train-wreck’ may be economically unattractive. Measurement, monitoring, and verification plans designed for during and post closure of CO2 injection to ensure mindful operation and integrity of subsurface, surface, and marine environment with remedial actions if it happens unexpectedly.
Multiple scenarios generated incorporating uncertainties. Structural, solubility trapping, residual and mineralization of CO2 understood. Importance of saline aquifer size, water producers to manage the reservoir pressure and plume movement contemplated. Uncertainty variation is different between hydrocarbon fields and saline aquifers. It decreases with time in former whereas same increases in saline aquifer. Monitoring, measurements, and validation (MMV) plan for containment, conformance, and maintenance of injectivity and operational sustainability is firmed up. The amount of CO2 that may get recycled along with water can potentially widen the containment risk and would challenge the viability of project economics as handling of produced water requires large offshore treatment facility. Hence aquifer which get replenished may not be ideal choice due to minimal pressure changes, but the isolated big aquifers promise umpteen potential. Dispersion modelling studies suggests unlikely leakage of CO2 at seabed in qualified storage site and not affecting the marine environment.