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.