Leakage Rate Modeling in Depleted Gas Fields Safeguards Well Integrity, CO2 Storage
Authors: Carpenter Chris 
Published: February 2022

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 205537, “Safeguarding CO2 Storage by Restoring Well Integrity Using Leakage Rate Modeling Along the Wellbore in Depleted Gas Fields Offshore Sarawak,” by Parimal Patil, SPE, Prasanna Chidambaram, SPE, and M. Syafeeq B. Ebining Amir, Petronas, et al. The paper has not been peer reviewed.
Ensuring the long-term integrity of existing plugged and abandoned (P&A) and active wells that penetrated a studied CO2 storage reservoir was key to reduction of leakage risks along the well path for long-term containment sustainability. With a goal of developing depleted gas ﬁelds as carbon dioxide (CO2) storage sites in oﬀshore Sarawak, determining the complexity involved in restoring the integrity of these P&A wells, as well as the development wells, is critical. Leakage-rate modeling (LRM) was performed to identify and evaluate the associated risks for designing the remedial action plan.

CO2 Storage Challenges in Central Luconia
Storage sites in this study include three major depleted gas reservoirs in the Central Luconia gas ﬁeld oﬀshore Sarawak. Hydrocarbons from deeply buried reservoirs were produced through drilling of vertical and deviated wells. Thirty-eight wells were drilled in these three depleted gas reservoirs, of which 11 are exploration wells that had been P&A; 27 wells include active producers, water injectors, and idle wells.
By estimating the performance of well-barrier elements in the legacy P&A wells and existing development wells, an assessment of well integrity analysis was performed. The casing, as well as the cement, may degrade with time depending on the downhole temperature, pressure, stress conditions, and formation ﬂuids. Degradation of wellbore cement in the presence of carbonized acid ﬂuids poses a risk of creating leakage pathways. Understanding the local stress conditions acting on the cement/casing/formation sheath is important to increase knowledge regarding leakage-pathway creation, together with geochemical and geomechanics processes.
CO2 Storage Well Integrity Risk Assessment
Once the locations of the existing P&A wells were identiﬁed, site-inspection data helped deﬁne
the preparation required to assess possible gas seepage or the existence of leakage pathways along these wellbores. The measured leakage amount of gas/CO2 could then be used to backfeed the simulation model to identify the leakage system along the wellbore. The method uses desktop modeling studies together with site surveys using a remotely operated vehicle (ROV) or side scan sonar. The modeling helps simulate various leakage scenarios, while actual ﬁeld measurements for any bubbling at the seabed provide the critical inputs to reﬁne the modeling work to deﬁne the risk associated with each well.
To gauge the integrity of storage sites for each well, a comprehensive understanding of critical factors and dominant criteria had to be considered as part of the data-evaluation process. Tables 1 and 2 of the complete paper define the risk-assessment elements for the existing P&A exploration and appraisal wells and development wells, respectively, used for the risk-assessment exercise. The risk-assessment process includes identification of risk, risk analysis, and risk evaluation. All wells that penetrated the CO2 storage reservoir were benchmarked as a baseline for a corrosive storage environment. Such data are useful for forecasting the possible leakage rates at the CO2 sequestration site.