Numerical modelling of CO2 migration in heterogeneous sediments and leakage scenario for STEMM-CCS field experiments

The dynamics and plume development of injected CO2 dispersion and dissolution through sediments into water column, at the STEMM-CCS field experiment conducted in Goldeneye, are simulated and predicted by a newly developed two-phase flow model based on Navier-Stokes-Darcy equations. In the experiment, CO2 gas was released into shallow marine sediment 3.0 m below the seafloor at 120 m water depth in the North Sea. The pre-experimental survey data of porosity, grain size distributions, and brine concentration are used to reconstruct the model sediments. The gas CO2 is then injected into the sediments at a rate of 5.7 kg/day to 143 kg/day. The model is validated by diagnostic simulations to compare with field observation data of CO2 eruption time, changes in pH in sediments, and the gas leakage rates. Then the dynamics of the CO2 plume development in the sediments are investigated by model simulations, including the leakage pathways, the fluids interactions among CO2/brine/sediments, and CO2 dissolution, in order to comprehend the mechanisms of CO2 leakage through sediments. It is shown from model simulations that the CO2 plume develops horizontally in the sediments at a rate of 0.375 m/day, CO2 dissolution in the sediments is at an overall average rate of 0.03 g/sec with some peaks of 0.45 g/sec, 0.15 g/sec, and 0.3 g/sec, respectively, following the increase in injection rates, when some fresh brine provided. These, therefore, lead to a ratio of 0.90~0.93 of CO2 leakage rate to injection rate.