help with unknown Master/solution data

[rm]

Dear phreeqc community,

I have a solution data from hydraulic fracturing (HF) flowback water. I am interested in a mixing reaction of the acidic HF fluid with nearby surface water to see the extent to which pH gets buffered. I have a well constrained composition of surface water but the HF fluid composed of hydrocarbons, polymers, and acids could not be found in the available phreeqc databases. Therefore all concentrations were set to zero (see attached warnings!). We also do not know the pH/alkalinity of the HF water.

How difficult would it be to find thermodynamic properties of these compounds and assign their master/solution species in the input file? Any leads would be highly appreciated. In the event mixing appears to be a rather difficult route, I would probably just react the strong acid (HCl) with my solution to get a reasonable answer. Thanks in advance!

[dlparkhurst]

You should not try to define all of the organic compounds as aqueous species of carbon (C), first because they are not thermodynamically stable compounds. Even if you input thermodynamic constants the result would show that they would, thermodynamically, decompose to C(4) and C(-4) aqueous species. Second, it would be a painful process for little result.

For starters, you could use the pH and assume it is an HF solution that mixes with your surface water.

To get more complicated, you could look at an acidity titration of your back-flow water. I'm not exactly sure how you would do it, but you could then consider the combined effect of your organic acids by adding one or more organic acids that attempt to replicate the acidity titration. These would be fictive acids like OrgaH, OrgbH, with associated pKs and concentrations determined by fitting the acidity titration. Then you could add this HF/Organic acid solution to your surface water.

[rm]

Hi David,

thank you! I agree with you - it is not worthwhile to consider all organic compounds as aqueous species. I am offering two approaches here to get the same result: (i) titration approach with known HCl concentration and (ii) mixing approach where I estimated the pH of the HF solution (3.3) based on the amount of HCl (0.0005 moles). Since the concentrations of the org. acids are over an order of magnitude lower, I am assuming adding those would not change the results. Please let me know if there is room for improvement. Thanks!

(i) Titration approach

Code: [Select]

SOLUTION 1 Surface water
    temp      20
    pH        7
    pe        4
    redox     pe
    units     ppm
    density   1
    Alkalinity 40
    Cl        40
    Na        20
    Ca        20
    -water    1 # kg

Use solution 1

REACTION 1
    HCl        1
    0.0005 moles in 3 steps

SAVE solution 1

SELECTED_OUTPUT 1
    -file                 selected_output_1.sel
    -pH                   true
    -molalities           HCl
    -activities           H+

USER_GRAPH 1
    -axis_titles            "HCl added" "pH" ""
    -chart_title            "HCl concentration"
    -axis_scale y_axis      auto auto auto 0.1
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x

  -start
10 GRAPH_X RXN
20 GRAPH_Y -LA("H+")

  -end
    -active                 true
END


(ii) Mixing approach

Code: [Select]

SOLUTION 1 Surface water
    temp      20
    pH        7
    pe        4
    redox     pe
    units     ppm
    density   1
    Alkalinity 40
    Cl        40
    Na        20
    Ca        20
    -water    1 # kg

SAVE solution 1

SOLUTION 2 HF Fluid
    temp      20
    pH        3.3
    pe        4
    redox     pe
    units     ppm
    density   1
    -water    1 # kg

SAVE solution 2

MIX 1
    1    0.5
    2    0.5

SAVE solution   3

USER_GRAPH 1
    -axis_titles            "Simulation#" "pH" ""
    -chart_title            "evolution of pH"
    -initial_solutions      true
    -connect_simulations    false
    -plot_concentration_vs  x
  -start

10 GRAPH_X SIM_NO
20 GRAPH_Y -LA("H+")

  -end
    -active                 true
END


[dlparkhurst]

Just a couple comments.

(1) The following generates an HF solution of pH 3.3:

Code: [Select]

SOLUTION 2 HF Fluid
    temp      20
    pH        3.3
    F         1 charge
END
(2) SAVE only applies to the final "reaction" solution in a simulation. A simulation is everything up to an END or the end of the file. So, I think only SAVE solution 3 applies, and then only to the solution after REACTION (first input file) or MIX (second input file). Initial SOLUTION definitions do not need a SAVE statement.

[rm]

Thank you! I think there is one misunderstanding about the HF solution - I am actually referring to hydraulic fracturing (HF) flowback water.

[dlparkhurst]

Different worlds.