Flushing formation with CO2-enriched water

[Kyle Horner]

Hi Brian, Craig

 

I am using GWB 8.012 to simulate the flushing of a formation with variably CO2-saturated groundwater. I am taking a multiple-step approach, described below, and run into hiccups during a few of the steps. I would appreciate your insights into my workflow in GWB, and any tips how to resolve the issues I seem to be having. I have attached a few working files for your reference

 

Workflow:

 

1. 
    
   
2. Calculate maximum CO2 solubility in regional groundwater outside of GWB (e.g. Solubility model of Duan et al 2003, 2006).
   
3. Modify Thermo.dat CO2 ion size to -0.5 to invoke Helgeson's polynomial fit to calculate activity of CO2(aq).
   
4. Open React w/ modified thermo.dat. Enter regional groundwater concentration into basis. Swap CO2(aq) for H+ in basis and specify CO2(aq) concentration. Balance on bicarbonate. Run React, suppressing all mineral precipitation during this step to simulate rapid equilibration of groundwater with CO2.
   
5. Pick-up CO2-equilibrated groundwater (the flushing solution) to reactants (Pickup->Reactants->Fluid).
   
6. Check Flush Configuration option in React.
   
7. Define mineral assemblage for 1m3 of formation in Initial System tab. Increase mass of H2O to reflect volume of water in porespace of 1m3 of formation. Add additional basis species at nominal concentrations (0.01mg/L) where required so that all ions in flushing solution are in basis.
   
8. Flush system using varying quantities of reactants ("Reactant Times" > >1)
   

The two issues I have arise in Steps 4 and 6.

 

First, following the pick-up of the CO2-enriched groundwater in Step 4, the reactants tab appears to un-swap H+ and CO2 (that latter of which is absent from reactants). It also assigns an incorrect value to HCO3-. Is GWB incorrectly picking up the solution, or am I misinterpreting the way GWB handles this step? So far, before flushing, I have been adding CO2(aq) to the reactants tab and entering the values for H+, CO2(aq), and HCO3- calculated in Step 3.

 

Second, as the formation mineralogy consists of numerous silicate phases, I often run out of basis species to swap for minerals in Step 6. As a work-around I have been omitting quartz from the formation defined in the basis tab to free up Si to swap for another, more reactive phase. Is there another way to define a complex mineral assemblage in GWB flush models that will allow a greater number of mineral phases to be included? Or is GWB limited to the number of minerals that can be defined by swapping out basis species?

 

Thanks for your help with this. Please let me know if I can explain anything further.

 

Cheers,

Kyle

CO2_Equilibration.rea

Formation_Flushing.rea

[Brian Farrell]

Hi Kyle,

 

Nice to hear from you again after the Melbourne course.

 

After you run the first model, take a look at the text file and compare the "Aqueous species" and "Original basis" sections. The former section lists the concentration and activity of all the actual species in the system (H+, CO2(aq), HCO3-, NaHCO3, CaHCO3, CO3--, etc.) while the latter lists the thermodynamic components (H+, HCO3-, etc.). When React picks up the your calculation results, it uses the components, not the species concentrations. It looks like you're taking values from the actual species, but you don't need to do this.

 

As for the number of buffer minerals allowed, that is a limitation arising from thermodynamics, not the GWB. In a true equilibrium state, you can't have a fluid in equilibrium with "extra" phases. What you can do, however, is to add extra minerals to the Reactants pane. Section 26.5 of the Geochemical and Biogeochemical Reaction Modeling text (or Section 4.2 in the GWB Reaction Modeling Guide) contains an example that might be useful to you. In the example, a fluid at equilibrium with Kaolinite, Muscovite, and Quartz (but not feldspar) is exposed to a reactant mineral Albite, which dissolves into the fluid at a rate calculated from a kinetic rate law.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC

[Kyle Horner]

Hi Brian,

 

Thanks for the prompt reply. Your comments on the pickup feature have cleared up the issue I was having. The kinetic examples were also useful as our aim is to eventually include kinetics in the model runs.

 

If I modify my approach to add the 'extra' phases through the reactant tab, how will that work with the flush configuration? Won't using 'reactant times >1’ create a titrating effect, slowly adding the 'extra' phase to the system rather than having it present initially? I am happy for that to happen with the flushing fluid, but not for the formation minerals.

 

Ultimately, we are trying to define a unit volume with specified mineralogy (based on XRD/XRF data) that we can flush. The constraints on the basis are understandable as (for example) Feldspars, quartz, and kaolinite cannot be in thermodynamic equilibrium with one another. However, they can coexist in a formation before equilibrium is reached (as is the case in our study area). I would like to define an initial system where the formation water and formation mineralogy are not at steady-state, but I do not see how to in GWB. Given the thermodynamic constraints, is it possible to define our (non-equilibrium) unit volume through the basis tab?

 

Perhaps a Flush model isn't the way to go. I have had a look at X1t, but I think the restrictions on basis swapping to define an initial unit volume are still present.

 

Cheers,

Kyle

[Brian Farrell]

Hi Kyle,

 

In the flush configuration, a reactant fluid (solvent water plus solutes) displaces the existing fluid from the system. Kinetic minerals added from the Reactants pane are not gradually titrated into the system (as if they were a simple reactant) or removed from the system (as in a flow-through model). Rather, their entire mass coexists with the fluid from the start, and they are allowed to dissolve into (or precipitate from) the fluid at a rate determined by the kinetic rate law you specify.

 

The number you specify for "reactants times" will be multiplied by the mass you specify for a kinetic mineral, so you might divide the mass you specify by the "reactants times" to get the actual mass that you want.

 

As an example, say you have a kg of solvent water with a few buffer minerals at equilibrium, plus an additional mineral which is not in equilibrium with that water-rock assemblage. Set up the equilibrium system in the Basis pane, set a flush configuration, add 1 kg water in the reactants pane, and specify a number for reactants times - let's say 10. If you want 100 g of some non-equilibrium mineral in your system, then add a kinetic mineral to the Reactants pane and specify a mass of 10 g. 100 g of the non-equilibrium mineral will be exposed to the equilibrium system.

 

Hope this helps,

Brian

[Kyle Horner]

Hi Brian,

 

I have added the kinetic reactants into the mix, and that seems to have sorted out the last of the issues I was having. Thanks again for the help.

 

Cheers,

Kyle

[John Kaszuba]

I'm working on a problem using an approach that is similar to the one described by Kyle. In my flush model, however, I wish to fix aqueous CO2 activity to simulate buffering by a separate supercritical CO2 phase. I'm running into a brick wall with this. My latest and greatest attempt was to revise the database by adding a fictive supercritical CO2 phase as a mineral, but this approach yields multiple mass imbalance errors for minerals, aqueous species, and gasses. I'd be curious to hear what folks think. I've attached two input files that I've developed, one to equilibrate a groundwater with CO2 and a second to flush this water through a rock.

 

Thank you

 

John

Kaszuba input-equilibrate with CO2.rea

Kaszuba input-flush model.rea

[Brian Farrell]

Hi John,

 

Have you tried using a fixed activity buffer? If the system is configured such that the CO2(aq) activity is at its desired level, you can go to the Reactants pane and click add - Fixed - Aqueous... - CO2(aq). For more information, see section 3.5, Buffered paths, in the GWB Reaction Modeling Guide. Section 14.2, Fixed activity and fugacity paths, in the Geochemical and Biogeochemical Reaction Modeling text contains a more detailed explanation.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions

[John Kaszuba]

Brian

 

In my system, the rock + pore water was pre-equilibrated in a previous calculation and picked up for use in the basis pane of my current calculation; this rock is flushed with brine that was pre-saturated with CO2 and picked up for use in the react pane of my current calculation. I had looked at using a fixed activity buffer, but it's my understanding that this function fixes an activity that is specified in the basis pane whereas I'd like to fix the activity of the CO2 in brine that is flushing the rock (i.e., the brine in the react pane). I hope this explanation makes sense; the file I posted earlier this week (Kaszuba input-flush model) illustrates my problem.

 

Thank you for your help

 

 

John

[Brian Farrell]

Hi John,

 

The composition of the flushing fluid remains constant, so you don't need to do anything to maintain the CO2 activity in the pre-saturated brine. If I run your "Kazuba input-flush model" and plot CO2(aq) activity as a function of the amount of mass reacted, I see the CO2(aq) activity of the system approaches and levels off at a value of 3.114, which is just about the same activity as that calculated for the flushing fluid in " Kaszuba input-equilibrate with CO2". The activity of CO2(aq) in the system changes because it's a mixture of the initial porewater and incoming fluid, in addition to the effects of reaction with the rock matrix. The activity asymptotically approaches the value in the flushing brine, though, as more and more brine is flushed through.

 

Hope this helps,

Brian