A study on the thermal-hydrodynamical-coupled CO2 flow process in the Ordos CCS-geological-formation

Non-isothermal flow in wellbores and geological formations is an important process associated with geologic storage of CO2. Technically, simulation of it is a highly challenging task as it often encounters phase transition problems for the fluids involved, which may greatly influence the profiles of pressure, temperature, and flowrate along the wellbore and consequently changes the injectivity of the relevant gas injection. In this paper we present a coupled non-isothermal wellbore/reservoir model for simulating the relevant flow behaviors, in which the non-isothermal wellbore flow model CO2Well and the reservoir simulator TOUGH2/ECO2N are combined to form a novel coupled flow model. The model developed is compared and verified by the existing wellbore-reservoir simulator T2Well with two CO2 injection examples. These examples demonstrate that the present method has considerable computational advantages in simulating wellbore flows when phase transition occurs. We then apply this model to the Ordos CCS demonstration project in China, and investigate the sophisticated, thermal-hydrodynamically-coupled flow processes observed there owning to injection of the condensed, cold CO2 that was directly captured from a nearby coal liquefaction plant. The results obtained by the simulation, along with the relevant field observations, provide in-depth insights into the CO2 flow behaviors in both the wellbore and the geological formation. For example, the simulation revealed that, in the wellbore, the heat exchange between the injected CO2 and the surrounding rock plays a critical role for the temperature distribution. It was also observed that phase transition occurred in the transferred period between the injection time and the shut-in time. In addition, both the simulation and field observation showed that most CO2 injected entered the topmost layer due to the high transmissivity of the rock. The simulation also showed that the temperature-change zone travelled slower than the CO2 plume. For example, the CO2 plume extended to 430 m away from the injection site after 31 months of CO2 injection, while the temperature-change zone extended merely about 90 m away, much smaller than that of the CO2 plume. The simulation suggests that the injectivity at this site is about 0.83 kg/(MPa.s). The simulation also suggests that the injection temperature should be relatively high (e.g., above - 3 degrees C if the injection rate is more than 9.45 kg/s) to avoid formation of CO2 hydrate in the subsurface.