Experimental investigation on the stability of gas hydrates under near-wellbore conditions during CO2 injection for geologic carbon storage

Abstract

Carbon capture and storage (CCS) is one of the key technologies in reducing the CO2 concentration in the atmosphere. The injection of CO2 into depleted reservoirs, saline aquifers and salt caverns to permanently store it in geologic formations is called geologic carbon storage (GCS).In this paper, the focus is set on the possible hydrate formation due to cooling while injecting CO2 in depleted gas reservoirs (DGR) for GCS, in which the reservoir pressure is so low that the pressure difference between injected fluid and reservoir at sand face can result in operational challenges. The temperature reduction due to abrupt pressure reduction near the wellbore, caused by the so-called Joule-Thompson (J-T) effect, plays a crucial role in the design of CO2 injection. Detection of hydrate in the early stage (equilibrium point) as well as the metastable zone lead to avoid interruption of the CO2 injection process due to plugging effects in the near wellbore area.Therefore, this study focused on the formation and equilibrium conditions as well as estimation of the metastable zone of CO2 hydrates formed in brine of different concentrations and at different CO2 phases. An experimental setup was build up and presented. Moreover, the main component of gas reservoirs, CH4, was taken into account and hydrates from mixed gas spheres were investigated. Also N2 and O2 were added to CO2 in small amounts because these gases can be introduced from the surface CO2 sequestration process as impurities. The studies were carried out in a temperature range of (266–284) K and in a pressure range of about (2.5–9.0) MPa adapted from the project field specifications. A good agreement of the equilibrium data with the predicted date from HYDOFF program by Sloan in all cases studied was found. The novelty of the present study is the investigation of the gas hydrate formation points (at cooling rates of 0.45 K/min) to estimate the metastable zone of gas hydrate. For all tests, the metastable range was quantified. Possible subcooling before gas hydrate formation was found to be between (1.4–9.0) K. This is an important outcome of the study as this implies that CO2 injection without gas hydrate forming seems to be still possible although operating conditions are already submerged into the GHSZ.Additionally, it was observed that the salinity of the water phase has significantly higher influence on the gas hydrate formation and dissociation than the components of gas mixture. Nevertheless, gas impurities (N2, O2) act as retardant in contrast to CH4 which doesn't affect the CO2 hydrate stability zone significantly.