Well-Integrity Risk-Assessment Strategy Applied to CO2 Sequestration Project
Authors: Carpenter Chris 
Published: February 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 22348, “Scrutinizing Well Integrity for Determining Long-Term Fate of a CO2 Sequestration Project: An Improved and Rigorous Risk-Assessment Strategy,” by Parimal A. Patil, SPE, Asyraf M. Hamimi, and M. Azuan B. Abu Bakar, Petronas, et al. The paper has not been peer reviewed. Copyright 2022 International Petroleum Technology Conference. Reproduced by permission.

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Depleted hydrocarbon reservoirs are considered inherently safe for carbon sequestration, but a high density of wells penetrating the carbon dioxide (CO2) storage reservoir could compromise containment performance in a carbon capture and sequestration (CCS) project. A risk-management methodology can be incorporated to evaluate primary and secondary barriers in existing plugged and abandoned (P&A) and development wells to ensure long-term viability of CO2 sequestration projects. The complete paper evaluates well-integrity and CO2 leakage risks along the wells in a depleted field that penetrates the CO2 storage reservoir.

Background
The identified CO2 storage site offshore Malaysia is a depleted hydrocarbon field discovered in the early 1980s. Subsequently, two appraisal wells were drilled to further assess the field’s development potential. The structure is a north/south anticline with an aerial extent of approximately 35 km2 and a vertical closure of 100 m on top of the reservoirs. Eighteen major and minor gas-bearing reservoirs exist in the field. The hydrocarbons from deeply buried reservoirs were produced over a period of approximately 15 to 25 years through deviated wellbores. In total, 24 wells are in the targeted field; of these, three are abandoned exploration and appraisal wells and 21 are development wells drilled from the platform. All exploration and appraisal wells are P&A, while 21 development wells are still accessible from the platform. High uncertainties are associated with the P&A wells because the well sites were restored per a regulatory requirement in which the casings were cut below mudline and a surface cement plug was placed with no intention of re-entering these wells. Development wells, on the other hand, were assessed and screened for reuse by conversion into CO2 injectors.

Understanding Well Integrity for CO2 Storage
Potential leakages that may occur through various mechanisms during geological storage of CO2 in the storage field include failed caprock and trap integrity and leakage along existing wellbores.
Parameters that could cause leakage of CO2 because of failed caprock include existing faults or fractures, reactivation of faults, development of new fractures during injection, and caprock failure caused by pressures exceeding fracture pressure during or after injection. The geological analysis of the depleted field for potential development as a future CO2 storage site must understand and mitigate associated risks by integrating information from various databases. However, the integrity of wells in the storage project must be ensured over very long time scales, in the thousands of years.