[OLD] Dissolved oxygen

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From: Gregg Jones

Subject: Dissolved Oxygen

I am simulating the injection of potable water from a water treatment plant into an aquifer storage and recovery well. The storage zone is in the Suwannee Limestone in the Floridan Aquifer. Reducing conditions allow minute quantities of pyrite that occur in the limestone matrix to be stable in the presence of formation water. My goal is to determine the ratio of

injection water to formation water that causes pyrite to become unstable. I am producing Eh/pH type diagrams to look at pyrite stability but instead of using Eh as my y axis, I am using log activity of the sulfate-sulfide ratio. I am assuming the sulfide concentration is virtually zero in the water coming 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. When I run the simulation, I get the sulfate-sulfide ratio vs pH for each reaction step as I increase the ratio of injection water to formation water. I then plot this reaction path on the stability diagram and I get the ratio of injection water to formation water where pyrite becomes unstable. For my first attempt, I assumed that the treatment plant water is equilibrated with the atmosphere. This would give me a pH of approximately 5.6 and a dissolved oxygen concentration of approximately 7.5 mg/l. I revised the simulation when I received additional data on the chemical quality of the treatment plant water. The actual pH was 7.6 vs 5.6 and the actual DO was 15.4 mg/l vs 7.5 mg/l. When I ran the simulation using these values, I saw very little change in the reaction path and therefore, the ratio of injection water to formation water where pyrite became unstable changed very little. It seems to me that the higher DO concentration should cause pyrite to dissolve at lower injection water to formation water ratios. Why doesn't the model account for the change or am I missing something? I ran into a similar problem when I conducted additional runs where I systematically lowered the DO to see how low the DO would have to be in the injection water in order for pyrite to remain stable in the limestone. Even when the DO value in the injection water is as low as 0.25 mg/l (the level in the formation water), the reaction path looks the same as the path created when DO in the injection water was 15 mg/l. Also, the resulting species and their concentrations are not very different between the injection waters with 15.4 mg/l DO and 0.25 mg/l DO. Shouldn't the DO concentration have a big effect? I am using Release 3.2.1.

 

I start by characterizing the injection water from the treatment plant.

 

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

PH = 7.6

DO = 15.4 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

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: Dissolved Oxygen

I can't say I follow your logic, but I think there's an easier way to work this problem. If I were you, I'd run the mixing simulation in React and then plot the saturation state of pyrite against the mixing ratio (which is numerically equal to the kg of injected fluid added to the initial 1 kg of formation water). Your results should look something like this (see attached file).