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Abstract:
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Carbon sequestration is a method of capturing and storing excess anthropogenic CO₂ in the subsurface . When CO₂ is injected , the temperature and pressure at depth turn it into a supercritical (SC ) fluid , where density is that of a liquid , but viscosity and compressibility resemble a gas . Ultimately the SC CO₂ is trapped at depth either by low permeability sealing layers , by reactions with minerals , or by dissolving into fluids . The injected CO₂ is buoyant and initially exists as a non -aqueous hydrophobic layer floating on top of the subsurface brine , up against the upper sealing formation , but over time it will dissolve into the brine and potentially react with minerals . The details of that initial dissolution reaction , however , are only poorly understood , and I address three basic questions for this research : What is the fundamental kinetics of SC CO₂ dissolution into water ? How fast does dissolved CO₂ diffuse away from the source point ? And what geochemical conditions influence the dissolution rate ? To answer these questions I employed a high pressure flow -through approach using a column packed with coarse quartz sand . The system was both pressure and temperature controlled to have either liquid or SC CO₂ present , and was typically run at 100 Bar , 0 .5 to 2 .5 mls /min , and 28 -60°C . After establishing the hydraulic parameters for the column using two conservative tracers (Br , As ) , injections (5 and 20 [mu]l ) were made either as aqueous solutions equilibrated to high pressure CO₂ , or as pure liquid or SC CO₂ into 0 .1 mmol NaOH . For all experiments the pH of the system was monitored , and [CO₂] over time was calculated from those data . For injections of brine with dissolved CO₂ , transport was conservative and was nearly identical to the conservative tracers . The CO₂ quickly mixes in the column and does not react with the quartz . The liquid and SC CO₂ injections , however , do not act conservatively , and have a very long tailing breakthrough curve that extends to tens of pore volumes . I hypothesize that the SC CO₂ is becoming trapped as a droplet or many droplets in the pore spaces , and the long breakthrough tail is related either to the rate of dissolution into the aqueous phase , the diffusion of dissolved CO₂ away from the phase boundary , or the reaction with the NaOH , limited to the narrow contact zones in the pore throats . Because of the speed at which acid -base reactions occur (nanosecond kinetics ) , I infer that the rate limiting step is either surface dissolution or diffusion . From plots of ln[CO₂] v . time I obtained values for k , the specific rate of the dissolution reaction R= -k[CO₂] . No trend for k was seen with respect to changes in temperature , but k did show a trend with respect to changing flow rate . k increased from an average value of 3 .05x10⁻³ at 0 .5 ml /min to an average value of 3 .38x10⁻³ at 1 .6 ml /min , and then held constant at the higher flow rates , up to 2 .5 ml /min . I interpret these data to show that at low flow rates , the reaction is diffusion limited ; the fluid nearest the contact zone becomes saturated with dissolved CO₂ . At higher flow rates , the fluid is moving fast enough that saturation cannot occur , and the kinetics of the dissolution reaction dominate . Simple geometric models indicate that the CO₂ /water interface is shaped like a spherical cap , indicating that the snapped -off CO₂ is forming a meniscus in the pore throat , limiting the surface area across which dissolution can occur . |