[OLD] Reaction Path Problems

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From: Gregg W. Jones

Subject: Reaction Path Problems

I am simulating 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'm 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. My intention is to plot the reaction path on a plot of 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'm also trying to plot the reaction path between the two end members: the treatment plant water and the formation water. Pyrite is stable in the formation water but as I mix in larger volumes of treatment plant water, the H2S and HS- concentrations reported on the output quickly drop below 1X10-8 molal. The program no longer reports the concentrations of these species below that concentration. This is a problem in plotting the reaction path between the two end members because you don't get very far from the formation water on the reaction path before the H2S and HS- concentrations are no longer reported and your path comes to a dead end long before you get to the treatment plant water. I realize the concentrations of H2S and HS- get extremely small but is there any way to get the program to report them? Another problem is that as the volume of formation water gets very small with respect to the volume of the treatment plant water, the program starts to report larger and larger charge balance errors. Why is that happening and does it invalidate my results?

 

Output is as follows. I'm using the latest release of the program. 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 (of course, this number gets larger and larger as I mix

in more treatment plant water)

 

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: Reaction Path Problems

To cause all species to be listed in React's printed output, regardless of concentration, do a Config -> Print Options, then for "Aqueous species" select "long". Run React again and you will see a complete list, not just those species with concentrations > 10^-8 molal. Regarding the charge balance error, although the absolute number of Faradays of charge imbalance increases, the imbalance per kg of fluid does not. So don't worry, be happy.