Solubility of methane in borehole groundwater

[mukherjeeindrani09]

I have measured different cations, anions, EC/TDS, temperature, pH and Eh from the 20 mBGL depth of a borehole. I need to calculate how much methane gas is present in the gas and the dissolved phases in the borehole water.

How can I calculate this using PHREEQC modelling? The diameter and depth of the borehole are known. However, I did not measure the amount of methane dissolved in water and how much amount of methane is present in the headspace of the borehole in the gas phase.

Can someone help me with this?

[dlparkhurst]

Here is a script that you can study. It makes a formation water at 60 C and 100 atm in equilibrium with halite and a methane partial pressure of 100 atm.

Then the water is cooled from 60 to 40 and the pressure is decreased from 100 atm to 1 atm, allowing methane to escape into a gas phase.

You will have to estimate your formation water under in situ conditions, and consider how temperature and pressure change in the well bore. You may need to consider other gases in SOLUTION and GAS_PHASE, and possibly precipitation of scale minerals with EQUILIBRIUM_PHASES in the borehole simulation.

I have used phreeqc.dat and Mtg for methane; Mtg avoids any redox reactions of methane with other constituents.

Code: [Select]

SOLUTION 1
-temp 60
-pressure 100
EQUILIBRIUM_PHASES
Halite 0 10
Mtg(g) 2 10
SAVE solution 2
END
USE solution 2
GAS_PHASE 2
Mtg(g) 0
REACTION_TEMPERATURE
60 40 in 10 steps
REACTION_PRESSURE
100 1 in 10 steps
USER_GRAPH 1
    -headings               TC Methane(aq) Methane(g)
    -axis_titles            "Temperature, Celsius" "Dissolved methane, molality" "Methane gas, moles"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X tc
20 GRAPH_Y TOT("Mtg")
30 GRAPH_SY GAS("Mtg(g)")
  -end
    -active                 true
END


[mukherjeeindrani09]

Thank you @dlparkhurst.

From the above script, it looks like I need to know the equilibrium condition of the groundwater system (e.g., it is in equilibrium condition with respect to methane or halite or not. Is there a way to determine that?) and the partial pressure of the dissolved gases. Unfortunately, these data are unavailable.

Is there still a way to model the gases (CH4, CO2, H2S, NH3, O2 etc., mainly CH4) in the dissolved phase and gaseous phases?

I have the below data -

Code: [Select]

REACTION_TEMPERATURE 1
    17  22  27  32  37
TITLE Borehole groundwater sample collected on 24/10/2023 at 10:30
SOLUTION 1
    temp      12.6
    pH        6.4
    pe        4
    redox     pe
    units     mg/l
    density   1
    Alkalinity 125
    Cl        94
    S(6)      228
    Ca        106
    Mg        54
    Ba        0
    Na        32
    K         8
    Mn        0.8
    B         0
    N(-3)     0.3
    Al        0.07
    Si        0
    Fe        6.41
    -water    1 # kg

END

[dlparkhurst]

You can calculate saturation indices with SOLUTION, which may give clues as to the saturation state of the formation water. If you have no measurements of methane or other gases, then any additional modeling will be based on assumptions. The script that I provided is an assumption-based calculation.