The transient behaviour of CO2 flow with phase transition in injection wells during geological storage – Application to a case study

This article presents a study on the transient behaviour of CO2 in wellbores during injection of CO2 for geological storage or enhanced recovery of oil or gas. To deal with the transient flow equations, we developed a novel modelling scheme that fully includes the storage/inertial terms, and tested the scheme by simulating observations from a field trial of CO2 enhanced coal bed methane recovery. Because the field trial involved short periods of CO2 injection, transient thermal and wellbore storage effects played a significant role in the wellbore׳s hydraulics. It was shown that our model׳s simulation presented a satisfactory agreement with the field trial data. A further sensitivity analysis demonstrated the importance of representing wellbore transients. The results and observations from this work include the following. (1) A set of field measurements for low temperature (below −20 °C) carbon dioxide injection is presented, which can serve as an experimental benchmark for verification and validation of theoretical and numerical models. (2) A modelling scheme for transient flows is developed, which shows that for the transient flow concerned the storage /inertia terms in the flow equations must be included and that large errors can result if they are ignored. (3) Unlike a long-term steady injection problem, for short-term injection or at an early stage of injection the thermal exchange between the injecting carbon dioxide and its surroundings is mainly dependent on the thermal scenario in the immediate vicinity of the wellbore, and the geothermal gradient of far-field is less important in this situation. (4) When the injected carbon dioxide is in a two-phase state, the Joule–Thomson coefficient may no longer be the best approach to characterising the dependence of temperature on pressure. Instead, the phase transition and the heat transfer processes may play an equal or a more important role in determining the pressure and the temperature profiles of the fluid within the wellbore.