Numerical Simulation Study On Migration And Fate Of CO₂ For Carbon Dioxide Sequestration In Saline Aquifers

Slowing down and controlling global warming trend by reducing CO₂ emissions have become an important issue in current world. With huge CO₂ sequestration potential, proper depth, wide distribution, CO₂ sequestration in saline aquifer has become one of the most promising carbon emission reduction measures. Although researchers have carried out a lots of studies, a few of them explored the effect of model parameters（such as temperature, pressure, CH4 content, CO₂ injection/feeding rate, local topography error and short-term mineral reactions） and structure on short-term CO₂ migration and fate, as well as the impact of rate laws and regional groundwater flow on long-term CO₂ sequestration. Resolving these key questions will directly affects the uncertainty and safety of CO₂ sequestration in saline aquifer.To explore whether the factors above will significantly affect migration and fate of CO₂ and safety of CO₂ sequestration, In this study we selects Sleipner in Norway as research area, and takes the sandstone layer（“Layer 9”） on top of Utsira reservoir as the target layer. We simulate and analyze the effect of model parameters and structure on short-term plume migration with three-dimensional multi-phase flow numerical model; using one-dimensional radial multi-component multi-phase reactive transport model, we systematically analyzes the impact of rate laws and regional groundwater flow on long-term CO₂ migration and fate through employing different rate laws and groundwater flow velocities. The main work and conclusions are listed below:（1） Three-dimensional multi-component multi-phase flow numerical model for Layer 9 was built with GEM. By introducing CH4 impurity, the model was calibrated with the seismic data in 1999, 2001, 2002, 2004, 2006, 2008. The study shows that under the condition of horizontal permeability anisotropy, a better calibrated model is obtained by adjusting temperature and CH4 content. Compared with previous research, the calibrated model here can better explain the elongated, northward extension of the observed plume, and the calibrated temperature and CH4 content are 33.5 oC and 2.4%, respectively.（2） Based on the calibrated 3D multi-component multi-phase flow numerical model for Layer 9, many numerical experiments were designed through changing model parameter values and model setting. By comparing the results of different numerical experiments, the effect of model parameters and structure（including temperature, pressure, CH4 content, CO₂ injection/feeding rate, local topography error and short-term mineral reactions） on short-term CO₂ migration and fate was analyzed. The results indicate that among four factors of temperature, pressure, CH4 content and CO₂ injection/feeding rate, short-term plume migration is more significantly affected by temperature and CH4 content. If rising temperature or CH4 content, plume migration can be promoted by reducing gas phase density, but the migration direction of the plume is restricted by the topography. Even if great uncertainties exist in temperature, pressure, CH4 content, CO₂ injection/feeding rate, the percentage of short-term structure trapping and dissolution trapping has small error limit, and simulation values are about 93±2% and 7±2%. Due to the potential local topography error, it is difficult to obtain the result that fully agrees with actual plume distribution, therefore the model should be calibrated by comparing the plume trend. Dissolution of calcite favors the topographic high in Layer 9.（3） 1D radial multi-component multi-phase reactive transport model for Layer 9 was built with TOUGHREACT/ECO₂ N. Many numerical experiments were set by changing precipitation and dissolution rate laws of feldspar and other silicate minerals. Through comparing the results of different simulation experiments, general effects of rate laws on long-term CO₂ sequestration were studied. The results show that changing the dissolution rate laws of the main soluble silicate minerals can influence the silicate reactions and mineral trapping by impacting the sensitivity of the relevant coupled reaction’s rate to the acidification of brine. The steeper the slope of rate- ?Gr（Gibbs free energy of reaction） relationships, the more sensitive the coupled reaction rate and the mineral trapping are to the acidification of brine. The predicted fraction of CO₂ mineral trapping when using the linear rate law for feldspar dissolution is twice as much as when using the non-linear rate law. Rate law uncertainties of mineral precipitation have little effect on mineral trapping evolution for Utsira-type formation because the dissolution of primary aluminosilicate minerals are much slower and are the rate-limiters in this study.（4） Based on the built 1D radial multi-component multi-phase reactive transport model for Layer 9, a new numerical experiment was set regardless of velocity of groundwater flow. By the comparison among simulation results of different experiments, general effects of regional groundwater flow on long-term CO₂ sequestration were explored. The study shows that neglecting the influence of regional groundwater flow will significantly underestimate the fraction of mineral trapping. This is because fresh brine from upstream continuously erodes CO₂ at the tail of the CO₂ plume, generating a larger area of the acidic brine for mineral trapping. If regional groundwater is ignored, the percentage of mineral sequestration for Layer 9 would be reduced to 3.8% from 22% after 10,000 years. Groundwater flow can constantly erode CO₂ plume, thus making gas phase rapidly disappear. In the case that groundwater flow is not considered, gas phase still occurs at year 10,000. While, if groundwater flow is taken into account, gas phase will completely disappear at year 7,000.