Geochemical modeling of CO2 occurrence state in gas field produced water reinjection reservoir

Gas field produced water reinjection is similar to CO₂ geological storage in terms of site selection, well construction, injection and environmental monitoring. The co-storage of gas field produced water and CO₂ could maximize the use of stratum space, injection wells, environmental monitoring technologies, and promote efficient disposal of gas field produced water and large-scale development of CO₂ geological storage. Under the background of synergistic reduction of pollution and carbon emissions, the coordination of gas field produced water reinjection and CO₂ geological storage could provide a potential way to promote the synergistic reduction of pollution and carbon emissions and expand the efficiency path of CO₂ geological storage.

 The CO₂ injected into the reservoir would be presented in dissolution-mineralization phase and free phase, which changes with time. The evolution of the CO₂ occurrence state in the gas field produced water reinjection formation will directly affect the storage efficiency and long-term safety of the reservoir.  Based on the interaction mechanism of CO₂-gas field produced water-reservoir rock, the effects of CO₂ pressure, produced water salinity, reservoir rock type and formation temperature on the dissolution-mineralization phase CO₂ and gas phase CO₂ were systematically investigated by using PHREEQC program. PHREEQC is currently the most commonly used fluid rock geochemical reaction simulation software, which can simulate processes such as ion exchange, oxidation-reduction, mineral dissolution and precipitation. It has been widely used in the simulation of long-term complex hydrogeochemical reaction processes in fields such as CO₂+O₂ in-situ leaching of uranium, CO₂ geological storage, shale hydraulic fracturing, and groundwater remediation.

Combined with the changes of mineral composition of the reaction process and the dissolution-mineralization phase CO₂ proportion, the main controlling factors affecting the occurrence state of CO₂ in the gas field produced water reinjection formation were analyzed. The results show that (1) feldspar and chlorite are the main minerals to promote CO₂ mineralization reaction, while illite and calcite are the main carbon fixation minerals in the process of CO₂ geological storage. (2) CO₂ pressure is the main controlling factor affecting the CO₂ occurrence state in the gas field produced water reinjection formation, followed by the reservoir rock type and the gas field produced water salinity, and the formation temperature has the least influence on the change of CO₂ occurrence state. Under the simulated conditions, the change of CO₂ pressure leads to the change of proportion of dissolution-mineralization phase CO₂ in the sandstone and limestone system range from 47% to 72%, the variation of the proportion of dissolution-mineralization phase CO₂ caused by the difference of rock type range from 10% to 45%, the change of the proportion caused by the produced water salinity and formation temperature fall 2% ~ 30 % and 1 % ~ 15 %, respectively. The study shows that in the practice of gas field produced water reinjection combining with CO₂ geological storage, the corresponding injection volume and pressure should be adjusted in time according to the salinity of reinjection water, reservoir physical properties and temperature, so as to reduce the increase of leakage risk caused by gas phase CO₂ accumulation in the early stage of storage.