Geochemical assessment of inorganic fluids from carbon capture, utilization and sequestration project in Saudi Arabia

Abstract

Saudi Aramco has designed and implemented a geochemical monitoring and surveillance program for the first and largest Carbon Capture, Utilization and Sequestration project in the Middle East. The objective of this program is to quantify geochemical changes in reservoir fluids during water-flooding and CO2 injection to evaluate geochemical reactions, mixing processes during water alternating-gas (WAG) cycles, arrival times of injected fluids, sealing attributes for CO2 sequestration, and scaling tendencies. From 2012 to 2021, 280 time-lapse water samples from four CO2 producer wells, one gas-oil-separation plant, and nine shallow aquifer wells were analyzed and interpreted on the temporal variation of reactive elements (Ca, Mg, HCO3, SO4) and conservative parameters (Na, Cl). Except for one producer well, water-flooding did not alter the composition of produced water in observation wells. Almost three years after starting WAG, the arrival of injection fluid was detected in March 2018. Mixing of formation water with injection water resulted to be the dominant physical process. The abundance of heterogenous dolomite layers and elevated Ca/Mg and Ca/HCO3 ratios of produced water from one of the wells imply the partial interaction with specific reservoir horizons. Analogue geochemical trends point toward a hydraulically connected system for three of the producer wells, with sealing features for the fourth. The observed results are essential to calibrate CO2 reactive transport models for the prediction of a long-term sequestration of CO2. The lack of any communication between the reservoir formation and overlying groundwater zones assures in-situ confinement of CO2 as important conclusion of this study.