Carbon Mystery

[mcfay]

Hi Tom,

 

I am injecting a known mass of CO2 in 2 wells into a 6 m thick 200mx100m aquifer using X2t to calculate mass balance between rock and water. Pretty simple 75% qtz, 15% K-spar, 10% calcite rock composition, looking at dissolution/precipitation reactions with calcite. I have run models for 1 to 0.001 m3/m2/yr discharge rates and 22 g/l and 104 g/l salinity. The high 104 g/l salinity models indicate low CO2 solubility, near zero calcite precipitation and some calcite dissolution. But I am having trouble accounting for the mass of CO2 inject in the fluid or rock using 'elemental composition of rock and fluid' outputs. Some CO2 is leaving the model domain but I still cannot find the CO2, even after accounting for outflow. I am running the models for 10,000 years plus, and am wondering if it is an iteration error? I thought I had figured it out by assuming that the lack of CO2 in rock or fluid compared to what I had injected (corrected for outflow), was accounted for as a gas phase and thus should have a high CO2 fugacity, but the high salinity model at 1 m3/m2/yr has the lowest CO2 fugacity, and this model has the most CO2 unaccounted for...i.e. the mass balance gets worse with higher salinity and groundwater flow rate. My epsilon is 1e-7 which is low so again I think iteration error. I also tried summing the individual dissolved CO2 species to check if the 'total C in fluid' values were similar and they were close but not exact. Overall the 'total carbon in fluid + rock' output says carbon is leaving the system, but I cannot figure out where it is going. Any suggestions?

[Tom Meuzelaar]

Hi Tom,

 

I am injecting a known mass of CO2 in 2 wells into a 6 m thick 200mx100m aquifer using X2t to calculate mass balance between rock and water. Pretty simple 75% qtz, 15% K-spar, 10% calcite rock composition, looking at dissolution/precipitation reactions with calcite. I have run models for 1 to 0.001 m3/m2/yr discharge rates and 22 g/l and 104 g/l salinity. The high 104 g/l salinity models indicate low CO2 solubility, near zero calcite precipitation and some calcite dissolution. But I am having trouble accounting for the mass of CO2 inject in the fluid or rock using 'elemental composition of rock and fluid' outputs. Some CO2 is leaving the model domain but I still cannot find the CO2, even after accounting for outflow. I am running the models for 10,000 years plus, and am wondering if it is an iteration error? I thought I had figured it out by assuming that the lack of CO2 in rock or fluid compared to what I had injected (corrected for outflow), was accounted for as a gas phase and thus should have a high CO2 fugacity, but the high salinity model at 1 m3/m2/yr has the lowest CO2 fugacity, and this model has the most CO2 unaccounted for...i.e. the mass balance gets worse with higher salinity and groundwater flow rate. My epsilon is 1e-7 which is low so again I think iteration error. I also tried summing the individual dissolved CO2 species to check if the 'total C in fluid' values were similar and they were close but not exact. Overall the 'total carbon in fluid + rock' output says carbon is leaving the system, but I cannot figure out where it is going. Any suggestions?

 

Hi Mathew:

 

Can you provide a script so I can take a look at this? Be sure to include the database if different from the default databases provided with GWB. If you want to keep this private, email them to me at gwb@rockware.com.

 

Regards,

 

Tom

[mcfay]

Hi Tom

 

I sent you the script to your email. But this may be a similar issue to the post from Dec 9 by Tom about gas units in React. If there is a volume of gas not dissolving in the brine then the model may be losing gas CO2 gas mass at each timestep. I guess Tough React is the modeling software of choice to fully resolve these issues.

 

Thanks

 

Mathew