Reactive transport of CO2 in a brine cavity Dirk and Meredith EGEE 520 Project Presentation

Introduction CO2 emissions are increasing as a result of the combustion of fossil fuels Sequestering CO2 into deep saline aquifers is a viable method to reduce these emissions Geographic abundance Close proximity to industrial emission sources Favorable chemical compositions of saline waters Presence of Ca2+, Mg2+, Fe2+, Sr2+ Sequestration mechanisms Hydrodynamic trapping: CO2 exists as both immiscible and dissolved phases due to low flow velocity Mineral trapping: Conversion of gaseous CO2 into solid carbonates -The premise of this project is the fact that CO2 emissions are increasing and have been increasing as a result of the combustion of fossil fuels -Sequestering these CO2 emissions into deep saline aquifers has been suggested as a viable method to reduce the emissions -primarily because these saline aquifers are in geographic abundance and they are in close proximity to the industrial emission sources -saline aquifers also have formation waters with favorable chemical compositions of metal cations within -these include calcium, magnesium, iron and strontium – all of which can be converted into stable carbonates through the following sequestration mechanisms -the first mechanism is called hydrodynamic trapping – after injection of CO2 into the aquifer, it can exist as an immiscible gas plume or dissolved phase -the low flow velocity of the aquifer can cause the CO2 to stay trapped this way for millions of years -building on the idea of hydrodynamic trapping is mineral trapping – this is basically the conversion of gaseous CO2 into solid carbonates through the reaction with metal cations just like the ones I listed above 11/15/2018

Formulation Modeled system as 2-D system with sides of 1 meter Process of modeling in steps Diffusion into a box Adding Navier-Stokes and advection diffusion with functions to describe the change in fluid density due to infusion of CO2. Adding a second advection diffusion equation for brine and the advection conduction equation for heat transfer Generate sinks with reaction and add the appropriate terms to advection diffusion and conduction equations -The model we used for simulation is a 2-Dimensional system with sides of 1 meter -instead of beginning with the complex system, we decided to model our system with steps that increased in complexity -First we intention is to model the saline aquifer as a container of simple geometry such as a cylinder or a cube. The geometry of a real aquifer will affect the fluid behavior, however a simple case can illustrate a great deal about the basics of fluid behavior in a given system. Supercritical carbon dioxide is not completely miscible in aqueous solutions and is less dense than the saline solutions. Thus, the injection will be modeled by setting the top boundary of the system at a constant carbon dioxide concentration equal to that of the maximum solubility. It is also known that the density of saline/CO2 solutions vary according to temperature, pressure, and CO2 concentration.[6, 11, 13-15, 17] The solutions become denser with higher CO2 content.[19] Finally, reactions can occur in the solution and in so doing, release heat.[17] The goal, time allowing, is to incorporate all of these affects into one model to gain insight into the behavior of this system over time. The modeling will occur in a series of steps. First, the diffusion of carbon dioxide will be modeled using constant density and the diffusion equations.[19-21] Next, the effects of carbon dioxide concentration and pressure on fluid density will be modeled and applied to the fluid mechanics equations. This will add convection to diffusion. Finally, a sink term will be added to the mass balance equations, a heat generation term will be added to the energy equations proportional to the sink term, and the temperature effects on density will be added.[22] This should model reaction within the system. These steps should yield a model that is relatively accurate and attainable within the bounds of this class 11/15/2018

Governing Equations 11/15/2018

Solution 11/15/2018

Verification 11/15/2018

Parametric Study Temp Profile D=0.001 tc=500 Temp Profile -In our parametric study we varied the diffusion coefficient and thermal conductivity coeffcients -this is a movie of the original solution’s CO2 profile with Diff Coeff=5E-5 and tc=100000 -in this movie we can see CO2 beginning to diffuse and mix or react with the brine solution -The CO2 rich solution is beginning to sink towards the bottom of the box and convect -This is a movie showing what happens when we increase the DiffCoeff to 0.001 -Right away we can see that there is a much thicker band of diffusion and only one main downflow of CO2 compared to the 4 or so in the original solution -this is a movie showing what happens to the temperature in the original solution -even though there isn’t much change in the temp, we can see that it is decreasing in the middle, which makes sense since this is an endothermic reaction -this is a movie that shows what happens when we decrease the tc to 5000 and keep the DiffCoeff at 0.001 -we can see more of a temperature gradient and the beginning of conduction occurring -by decreasing the tc even more to 500, we can see even more instability and conduction occurring as the -we can also see this reverse or upward flow of the system Temp Profile D=0.001 tc=500 Temp Profile D=0.001 tc=5000 CO2 Profile D=0.001 tc=100000 Original solution – Temp Profile D=0.00005 tc=100000 Original Solution – CO2 Profile D=0.00005 tc = 100000 11/15/2018

Conclusion CO2 emissions are going to remain an issue for years to come Research has shown that saline aquifers are viable for sequestering CO2 The Navier-Stokes advective-diffusion and advective-conduction equations are known to model real behavior and have been verified with COMSOL 3.2 in this study COMSOL 3.2 produces results that are reasonable Mass diffusion rate and thermal conductivity parameters play a major role in fluid behavior Given more resources, we are confident that an accurate model for this system could be attained -to conclude, CO2 emissions are going to remain an issue for years to come and research has shown that saline aquifers are viable for sequestering CO2 and consequently reducing emissions -from common knowledge, the Navier-Stokes advective-diffusion and advective-conduction equations are known to model real behavior of this system and have been verified using COMSOL -and we think that COMSOL has produced reasonable results -from our work with this system and COMSOL, we found that the mass diffusion rate and tc parameters were the major factors affecting the outcome of the system -we consequently had to alter these parameters to get a solution in a reasonable about of time -given more resources and time, we are confident that a more accurate model for this system can be attained using COMSOL 11/15/2018