During CO2 injection operation start-up, strong transient flow occurs. It may lead to sudden changes in CO2 flowing pressure and temperature profiles, and possible phase transition. Moreover, wellbore flow behaviour affects the temperature at which CO2 enters the reservoir posing operational risks such as hydrates precipitation in the reservoir, ice formation around the wellbore, and thermal shocks of the casing. This indicates the importance of a reliable simulation of the coupled wellbore and reservoir flow to design the injection operations. The T2Well-ECO2M coupled wellbore-reservoir simulator allows the modelling of three-phase flow with the possible coexistence of liquid and gaseous CO2. T2Well-ECO2M offers a numerical and a semi-analytical approach to calculate heat exchange between the wellbore and surrounding formation. The full numerical approach is the most accurate, however, it requires building and solving spatial grids with many additional discretisation elements just for the modelling of heat conduction. The semi-analytical approach for wellbore heat exchange is based on the Ramey's method with or without time-convolution. The numerical and the semi-analytical option with time-convolution approach options were compared with reference to the simulation of dry CO2 injection at constant wellhead rate and enthalpy in a simple 1D radial reservoir containing CO2 and immobile brine. The conceptual model, considered in both approaches, assumes a reservoir at depth of 3067 m with a radial extension of 2000 m. The evolution of flowing pressure and temperature profiles as well as the radial distribution of reservoir thermodynamic conditions are compared. The computation time to solve the time-convolution semi-analytical case was approximately 2.5 % of the time necessary to solve the numerical case with, overall, a good approximation of the numerical solution.
The process of capturing CO2 from industrial sources and its injection into formations underground, Carbon Capture and Storage (CCS), could contribute significantly to the reduction of greenhouse gas emissions into the atmosphere. Possible candidates for CO2 storage are (i) deep saline aquifers (providing the most of estimated storage capacity); (ii) depleted oil and gas reservoirs; (iii) unmineable coal layers; (iv) salt domes (Grimston et al., 2001). Depleted gas reservoirs are well-favoured candidates for CCS due to their proven storage integrity, known subsurface conditions and properties, and the fact that most of the surface and subsurface equipment can be re-used for CO2 sequestration (Raza et al., 2018). However, due to particular sensitivity of the CO2 phase stability, depleted gas reservoirs with very low pressure and/or high permeability may cause an undesirable presence of two-phase conditions in the injection wellbore (Lindenberg, 2011). This happens during the transient flow, and is probably caused by a pressure difference between the incoming high-pressure CO2 and the initial pressure inside of the wellbore, resulting in sudden changes in CO2 pressure, density and temperature, and possible phase transitions. This is important since wellbore flow behaviour affects the temperature at which CO2 enters the reservoir.