Saudi Aramco is implementing its first carbon-dioxide enhanced oil recovery (CO2-EOR) carbon capture, utilization and storage (CCUS) demonstration project in a small depleted part of a carbonate reservoir, as part of its environmental stewardship and its efforts to mitigate CO2 emissions. It is worth mentioning that Saudi Aramco does not require EOR oil for many decades to come. This project is being implemented primarily to demonstrate the feasibility of sequestering CO2 through EOR in the Kingdom and using it as grounds to test new monitoring and surveillance (M&S) technologies. The injected CO2 is expected to react with formation fluid (oil and water) and rock minerals for geological storage to occur. Reservoir simulation studies suggest about 40-50% of the injected CO2 will be sequestered permanently in the reservoir.

To understand the geological trapping process and also to identify and quantify any inadvertent "out-of-zone" CO2 leakage, a comprehensive geochemical monitoring (geochem) program has been developed and is being deployed. The objectives of the geochem program are to monitor the project before, during and after CO2 injection to assess the efficacy of the sequestration process. In addition, the geochem data will provide the pertinent information for calibrating the CO2 reactive transport model to be used to quantify and predict the amount of CO2 that will be sequestered over a long period. The geochem plan includes monitoring the changes in basic ionic compositions of reservoir and aquifer water, pH, dissolved organic carbons (DOCs), volatile organic carbons (VOCs), BTEX, etc., and changes in isotopes of carbon, oxygen, hydrogen. Furthermore, the plan also includes monitoring the changes in soil gas (CO2, CH4, O2, Ar, Rn, and He) concentration.

Baseline and time-lapse data are being collected. These provide the basic input to our reactive transport model. The results from the analyses of the reservoir fluid indicate very little value may be derived from monitoring time-lapse isotopic changes. Isotope clumping is considered as a better approach to understand and quantify the amount CO2 being sequestered. This paper reviews the design and implementation of the geochem plan for the project, operational issues, the results obtained to date and the use of "clumped-isotope" geochemistry. Plans for further implementation of the geochem program and the lessons learned will be shared. The geochem plan is deemed robust and will help with understanding, tracking, monitoring and predicting the geo-sequestration process.

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