Andrew Wigston

Hello, I am trying to create a stability diagram for calcite in pure water as a function of the partial pressure of CO2(g). I think this should be relatively straight-forward, but I don't think I have quite got it. In React I was able to make an X-Y plot of the solubility of calcite as a function of the partial pressure of CO2 (g), where the solubility is represented on the Y-axis by the concentration of Ca++ in fluid. I figure this is a pseudo-stability diagram, as regions above the curve are where calcite is stable (i.e. the fluid would be supersaturated with respect to calcite) and regions below the curve are where it is unstable (i.e. the fluid is undersaturated). But, I can't figure out how to do something similar using Act2. I can plot the stability of calcite as a function of CO2(g) fugacity, but I have to set the pH at a fixed value. I was thinking (hoping) that the CO2(g) fugacity could be allowed to determine the pH, as it did in React. I have attached a Word document with a number of screen shots of what I have done (both the basis inputs and resultant plots). Thanks for any help you can provide. Sincerely, Andrew Calcite stability diagrams using React and Act2 .docx

Hi Jia, thanks. I will blame some faulty course notes for my erroneous understanding. Cheers, Andrew

Hello, it turns out I need more help understanding units! A common unit for the chemical composition of fluids is ppm. My understanding is that this is mg / kg of solvent, where the solvent is pure water (at least it is for aqueous geochemistry). However, I can't seem to find this unit in GSS or React. mg/kg is an available unit, but based on the answers to my previous question, I assume this is mg of solute per kg of solution...not solvent, and thus it isn't ppm. Molal is a unit, so I could always convert from ppm to molal: molal = ppm / (gram formula weight * 1000). But, this is annoying to do. So, am I wrong in my interpretation of units, or is ppm not an available unit in GWB? Thanks, Andrew

Hi, I am using GWB 2021 to investigate the potential for halite to form in a brine used for a geothermal project. The brine is extremely saline (~340,000 mg/L). So far I have only used an equilibrium approach. I am interested in incorporating kinetics just to see the effect. I understand that halite is a very fast reacting mineral. I am using React. In the Reactants pane, I require values for rate constant (or preexp and activation energy) and surface area (and others that are to be set by me). I have only been able to find a rate constant for halite (link below). The log k is -0.21 mol * m^-2 * s^-1 at 25C (which I calculate, for the purposes of matching the units required in GWB's React, is 6.17 x 10^-5 mol * cm^-2 * s^-1). I know that React can calculate a temperature-variable rate constant if I provide the program with values for the pre-exponential and activation energy parameters. However, the reference below only provides the rate constant and activation energy. https://pubs.usgs.gov/of/2004/1068/pdf/OFR_2004_1068.pdf But, I can't find any info on the surface area. Does anyone have a reference that has this info or any insights? I understand that the duration of the simulation is important (as set on the Basis pane). I'm not sure how long the brine would be in circulation in the system, but I think it is likely on the order of minutes to tens of minutes. The temperature change would be around 125C to around 60C. Thanks, Andrew

Hi Jia, Thanks very much. Makes sense. Cheers, Andrew

Hello, I ran a very saline water that was supersaturated with respect to a few minerals. I allowed them to precipitate as I cooled the water. The output text file from React shows what and how much precipitated in the "Minerals in system" portion of the file for each temperature step. The units are: moles, log moles, grams, and volume. Are these reported per kg of solvent? When I plot the results using Gtplot with temp as the X-axis and Minerals (g/kg) as the Y-axis, I get smaller values than in the output text file. The solvent mass is 1 kg and the solution mass is much greater (around 1.3-1.4 kg). Thus, I assume that the results in the output text file are per kg of solvent, and those in the Gtplot are per kg of solution. Is this correct? Thanks, Andrew B5 S1_hal eq at 125_precip on_125-25_phrqpitz.rea

Hi Jia, Thanks for the answer. You are correct. It is working fine now. Thanks, Andrew

Hello, I have a number of samples in GSS. I would like to calculate the saturation index (SI) of some minerals for all the samples for a few temperatures...i.e. calculate the SI of calcite for all samples at 25C, then 50C, etc. I have GSS set up so that calcite SI is an analyte to be calculated. I know I can manually enter the temperature for each sample in GSS and re-calculate the calcite SI. But, I am hoping that I can use a header or trailer command (under Analysis>Options) so that the new temperature applies to the SI calculation for all samples. I didn't find anything in the manual or GWB Forum, and I tried myself using various commands such as Temperature = 80, but no luck. Thanks, Andrew Wigston Ottawa, Ontario, Canada.

Hi Jia, I have a follow up question. I ran the same water as above and did the same swap (dolomite for H+) but this time I raised the temperature from 25C-120C. The amount of dolomite at 120C in the system (0.012976 moles, as specified in the "Minerals in system" portion of the output file) is more than the initial amount (0.01222 moles). The solution mass is only slightly different (1.2224 vs. 1.2222 kg). I was under the impression that when I swap dolomite for H+ that no dolomite precipitates or dissolves. Can you shed some light on what is going on? Thanks, Andrew B4 DST8WDw_as is_dol for H+_25-120C.rea

Hi Jia, Thanks very much. Makes perfect sense now. Andrew

Warning – long post! My question concerns how to determine the pH and partial pressure of CO2 (PCO2) in brine samples. I tried two different methods: 1) swapping a carbonate mineral in in place of H+; and, 2) titrating in CO2(g) till I get a saturation index (SI) of 1. I am wondering which is the most appropriate method (and some other questions). Background: I have a number of samples from a brine. The water is an Na-Cl type brine: Na+ ~70,000 mg/L (~3.2 molal) and Cl- ~115,000 mg/L (~3.5 molal) (there is ~ a few % charge imbalance). The pH was measured in the lab. The formation T is 120C. The formation contains dolomite, but no calcite or other carbonate minerals. The samples are oversaturated with respect to dolomite at the formation T. I believe that the samples may have degassed CO2 during sampling and that this accounts for the oversaturation. Method 1: Swap out H+ for most abundant carbonate mineral I have seen in a number of places in the Geochemical and Biogeochemical textbook that pH measurements of brines are typically erroneous and that a common method to determine the actual pH is to swap out H+ for the most abundant carbonate mineral. In my case it is dolomite. At the formation temperature of 120C dolomite is very oversaturated (saturation index of ~1200). I swapped in dolomite in place of H+ using React and the thermo.tdat database, and raised the T to 120C. The resultant PCO2 was 0.2632 bar and the pH was 5.632. Question: What is React doing? My thought on what is happening when I swap dolomite in for pH is that the program essentially looks at the reaction: CaMg(CO3)2 + 2 H(+)= Ca(++) + Mg(++) + 2 HCO3(-) and says I know the Ca(++) and Mg(++) activity from the input, and I know the HCO3(-) activity from the alkalinity, and FUTHERMORE I know that the solution is in equilibrium with dolomite. That means that the only unknown is the H(+) activity, and it solves for it. However, when I compared the starting and final amounts of Ca++, Mg++ and HCO3- (as given in the text file under “Original basis, In fluid) there was a slight difference (starting / ending mg/kg): Ca++ 7,704 / 7,990; Mg++ 265.8 / 275; HCO3- 157.4 / 163 (the same relative differences are seen with the moles unit). What is going on? It would appear that some amount of dolomite is dissolving to account for the increase in Ca++, Mg++, and HCO3-. Or does it have something to do with how React calculates fluid density (and this then carries over into concentration units)? Or is it something else? Method 2: Titrate in CO2(g) till a SI of 1 is reached for dolomite I believe another way to try and determine the pH and in situ PCO2 is to titrate in CO2 until the solution is in equilibrium with dolomite (i.e. SI = 1). So, I tried this with the same sample using the React program, again raising the T to 120C. The only difference being that I started with the lab-measured pH of 6.41 and that I reacted 25.0 mmol of CO2(g) (which I settled on after trying different amounts). This amount results in an SI for dolomite of pretty close to 1 after the full amount has been added. The resultant pH is 5.053 and the PCO2 is 2.759 bar. There is no change in the amount of Ca++, or Mg++, although the final concentration of these is slightly greater than the starting one, but I figure this is likely due to a change in density of the fluid (as I think this effects concentration values). React is obviously doing something different than in the first scenario. Question: What is React doing? My thought on what is happening when I add CO2(g) is: CaMg(CO3)2 + 2H20 + 2CO2 = Ca(++) + Mg(++) + 4 HCO3(-) Since Ca++ and Mg++ are fixed, the only thing that can change is the amount of HCO3-. I find the carbonate system complex but my take on the overall effect of adding CO2 is that the effect of creating acidity (and increasing the solubility of carbonates) is greater than the effect of generating HCO3- (which would serve to increase the reaction quotient, Q). [This has to be the case as when I add CO2 the SI of dolomite becomes ~1 and the pH goes down.] Is my understanding correct? Question: Which approach is more appropriate - swapping in dolomite for pH, or titrating in CO2 till I get an SI of 1 for dolomite? Question: Is there a way to do this for multiple samples at the same time? For example: Suppose you had 230 samples, all of which were in equilibrium with dolomite at 70 bars CO2 pressure but that through some exchange reaction they had quite different alkalinities. It would be very onerous to have to do the titration exercise individually on each sample. I have attached two files - one for each approach. Thanks in advance for you help. B4 DST 8W Dw_as is_dol for H+_120C.rea B4 DST 8W Dw_as is_+25 mmol CO2_120C.rea

Hi Jia, Thanks for the clarification. Andrew

Hi Jia, Thanks for your answer. I misunderstood the Elemental composition section of the output file. I thought it was the composition of the system after I had made changes to it (e.g. swapping in quartz for SiO2(aq). However, I believe it is just the original composition – but in elemental form as you noted. Is this correct? I understand from your answer that I can get this info in Gtplot. And to see if I’m really getting this: Assuming that the solvent mass is 1 kg, I would think that the moles in the In fluid section would be molal (i.e. moles per kg of solvent), correct? Thanks, Andrew

Hello again, Follow up question to my initial post (and happy holidays!) In my work I would like to compare the concentration of elements in a fluid before and after I make changes to the system. For example, I would like to see how the concentration of dissolved silicon changes due to swapping in quartz in place of SiO2(aq). I understand that I can go to the Elemental Composition section of the text file output and get data there, which includes total moles, and in the fluid moles, and mg/kg (and there is also a bit for sorbed concentrations). My understanding from your answer to my first question is that total moles is the total moles of a thermodynamic component. In other words the total amount in the system in solids, in the fluid, sorbed, and I suppose any gas phase buffer (if I am correct about the gas buffer, where is it reported? As part of the total moles?). In fluid has the following categories: moles and mg/kg. If I understand correctly from your answer, moles is the amount of the element in the total amount of the fluid, and mg/kg is the amount of the element in 1 kg of the fluid (i.e. solution, not solvent (aka water)). Is this correct? I think it would be helpful to have molal unit (i.e. moles of solute per kg of water) and perhaps mg / kg water as well. Thoughts? Thanks, Andrew Wigston

Hi Jia, thanks very much. Clear now. Andrew

Hello, newbie user here, so please excuse my ignorance regarding what I think may be a simple problem. I am using GWB 14. I have a sample of brine. It has a lab-reported value for Na of 83,900 mg/L. I entered it and all the other analytes in GSS (I did not enter a solvent mass). Out of curiosity, I converted it to some of the various other units available and here is what I get (using the thermo.tdat database) mg/L mg/kg Mol (no units listed) Molal (no units listed) mol/kg mol/L 83,900 68,719 4.19 4.19 2.989 3.649 Questions: 1) what is "kg"? Is it kg of solvent or solution? 2) Is mol just the total number of moles of Na in the system? If so, is the system 1 kg of water plus the mass of all the solutes, or just 1 kg of solution? 3) My understanding is molal is mol/kg solvent. Is this the case here? Then I ran the water in SpecE8 (I didn't make any changes, like charge balancing). Some outputs for Na from the "Original Basis" portion of the text file were: Moles: 4.19 Moles in fluid: 4.19 mg/kg in fluid: 6.77E04. Questions: 1) What is "fluid" in the "Moles in fluid"? Is it 1 kg water + solutes? 2) Is moles the amount in the system? 3) Why would the mg/kg value from SpecE8 be different from that reported in GSS when I converted (they are close, but not exact)? I have attached the GSS file. The sample in question is Border-01 S2. Thanks for your help. Sincerely, Andrew Wigston Border-01.gss