[OLD] Eh confusion

[webmaster]

From: Gregg Jones

Subject: Eh confusion

I sent the message below to the users group on the 20th of January and received several helpful responses. The model ran just fine when I added a command to decouple HS-. Thanks to those who provided guidance. My problem now is with interpretation of the output. My intention was to plot the log activity of the sulfate/sulfide ratio against pH to determine at what mixture of injected fluid and formation water pyrite becomes unstable. I was able to do that but now I'd like to take the Eh and pH values provided in the output to determine pyrite stability. The problem is that for each mixing step, the output provides more than one Eh value. The first Eh value within the data for a given mixing step is always positive no matter what mixing ratio I use. That can't be right because I know pyrite is stable in my formation water so I should get negative Eh values when my mixture is mostly formation water. The other Eh value in a given mixing step is under the heading of Nernst Redox Couples. There are Eh values for 2 equations, one of the equations is: 8*e- + 9*H+ + SO4-- = 4*H2O + HS-. The Eh values for this equation are always negative, even when I have almost pure injection water which is highly oxygenated as you can see from my input. Shouldn't the Eh be positive for the injection water? So there's my dilemma. Any advice? Gregg

 

I am trying to simulate the injection of potable water that is fresh out of a water treatment plant into an aquifer storage and recovery well. The storage zone is in the Suwannee Limestone in the Floridan Aquifer. I am interested in the composition of the mixed fluid. I am assuming the treatment plant water is equilibrated with the atmosphere. I am interested in getting the sulfate/sulfide ratio at each reaction step. I am assuming the sulfide concentration is virtually zero in the water out of the treatment plant. Prior to mixing, the sulfate and sulfide concentrations in the formation water are 1,526 mg/l and 14.5 mg/l respectively. I ran the model first without swapping in HS- and it ran fine. At the pH of the formation water, sulfide is present mostly as HS-. When I swapped HS- for O2(aq), the model didn't run and the message was initial solution is too supersaturated. Am I setting the problem up correctly and why does the model not run when I try to get sulfide into the basis? I'm using release 3. I start by characterizing the injection water from the treatment plant.

Swap O2(g) for O2(aq)

Swap CO2(g) for H+

f O2(g) = 0.2

log f CO2(g) = -3.5

T = 25.8

TDS = 325 mg/l

Ca++ = 88.9 mg/l

Mg++ = 9.5 mg/l

Na+ = 61 mg/l

HCO3- = 81.9 mg/l

S04-- = 114 mg/l

Cl- = 28.7 mg/l

Fe++ = 0.1 mg/l

K+ = 1.9 mg/l

Go

Pickup reactants = fluid

Reactants times 5

Now I characterize the formation water.

T = 26.4

TDS = 2510 mg/l

pH = 6.99

Ca++ = 399 mg/l

Mg++ = 148.5 mg/l

Na+ = 52.7 mg/l

HCO3- = 129.5 mg/l

SO4-- = 1526 mg/l

Decouple HS-

HS- = 14.94 mg/l

Cl- = 97.2 mg/l

Swap HS- for O2(aq)

HS- = 14.94 mg/l

Fe++ = 0.07 mg/l

K+ = 5.2 mg/l

Go

 

From: Craig Bethke

Subject: Re: Eh confusion

The first Eh value represents the redox potential of the couples that you left engaged: O2/H2O, Fe3+/Fe2+, HCO3/CH4, H+/H2, and so on. Since you set these couples in redox equilibrium, they all have the same redox potential. The Nernst Eh for the SO4--/HS- couple, which you have disengaged, reflects the sulfate/sulfide ratio you set for the fluid. You decoupled this reaction, so the redox potential will differ from the first value. This Nernst Eh will invariably be small because you maintain a certain amount of sulfide in the fluid. This sulfide cannot oxidize in your simulation because you have disengaged the redox couple. There is no oxidation reaction available to it. One way to allow the sulfide to oxidize would be to specify a kinetic redox reaction by which it reacts with an oxidant to form sulfate.