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Jubilee

How to model temperature change and irrigation

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Hi Jubilee,

 

You can specify the amount of various minerals in a domain in many ways. For example, you can set the fraction or percentage of a nodal block's volume that a mineral occupies, as you've done, as the mass or volume of mineral per volume of porous media or per liter of fluid, etc. When I compare your input (59 vol% Quartz, 3 vol% Gypsum) with your description, however, I think the values you specified are incorrect. There should be 3 g of Gypsum for ever 97 g of Quartz according to your description (meaning the mass of Quartz is 22.3 times greater than that of Gypsum), but when I plot the mass of each mineral in the initial system I get 500 g of Quartz to 22.13 g of Gypsum (the mass of Quartz is only 22.6 times greater than that of Gypsum). You should make sure that you're correctly accounting for the density of each mineral (you can view this info from the Config - Show menu). You should also note that wt% (which you used in an earlier script) is relative to the mass of solution, not to the total mass of minerals. If you have 1 kg of water plus 100 g solute in SpecE8, and Quartz is 100 wt%, that means you'll have 1.1 kg of Quartz in the system.

 

When you use the built-in rate law function in React, X1t, or X2t, you specify a specific surface area for the mineral of interest in cm2/g. As a mineral dissolves or precipitates, its mass will either decrease or increase and the surface area evaluated in the rate law will be calculated from the product of the mineral's mass and specific surface area (you can view the surface area of a kinetically reacting mineral in Xtplot, under the System parameters).The assumption here is that the surface area to mass ratio does not change. If this ratio does change (as it would in a single growing crystal, for example) then you can write a custom rate law. Another option available in GWB10 (which contains transient field variables) is to use the built-in rate law but write an equation describing how the specific surface area changes as a function of the mineral's mass.

 

You can model evaporation by removing solvent water from the system. Chapter 24, Evaporation, in the Geochemical and Biogeochemical Reaction Modeling text, describes how this is done.

 

Hope this helps,

Brian

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Hi Brian

Thanks for your constant support.

I incorporated the changes you suggested in your previous email and obtained a more accurate result. So far, I have identified the dispersion and mineral specific surface area as key drivers for my model. I still have a couple of questions regarding the reactant plane.


On the reactant plane, there are 4 options (simple fixed, kinetic, and sliding). I would like to know the implication of selecting gypsum under “Simple” instead of “Kinetic”. From my observation, “kinetic” gives me a better control as I can specify parameters such as the rate constant, surface area, dissolution or precipitation, and other key parameters that influence my model. However, I can only specify the mineral abundance when using “Simple”.

I would like to know the following:


1. Given that I have no control over the aforementioned parameters when using “simple”, what are the defaults for “simple? Is there a document that gives the default parameters of the model?

2. Not much is said in the GWB guides regarding “xaffin”. What does this parameter mean and what is its significance?


I look forward to your response.

Regards,

Jubilee

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Hi Jubilee,

 

Glad to hear you're getting better results. A simple reactant is just titrated into your fluid gradually over the course of your model. For example, if you chose 100 g of Calcite as a simple reactant in React, the program would add a small amount of Calcite to the fluid at each step of the calculation. By default the program takes 100 steps, so 1 g would be added at each step. If the fluid is undersaturated with respect to Calcite, it will dissolve; if not, it will simply accumulate in the system. See section 3.1 "Titration paths" in the Reaction Modeling Guide for more information.

 

The built-in rate law for a kinetically reacting mineral looks something like this: r = k * A * (1 - Q/K), where k is a rate constant, A is the surface area of the mineral, Q is the ion activity product, and K is the equilibrium constant for a mineral's reaction. With a cross-affinity term, you can evaluate the (1 - Q/K) term for a different mineral, which might be an altered surface layer of a crystal whose bulk composition is that of the first mineral. For more info on xaffin, see section 4.2.3 "Cross-affinity rate laws" in the Reaction Modeling Guide.

 

Hope this helps,

Brian

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Hi Brian,

I am considereing precipitation in X1t using the built in rate law for a kinetically reacting mineral ( r = k+ * A * (1 - Q/K)). Generally, Keq = K+/K- where k+ = dissolution rate constant and k- is the rate constant for precipitation, and Keq is the eqilibrium rate constant. When modeling for precipitation in x1t and i supply a value for the dissolution rate constant (k+), does GWB calculate the k- from the k+ and Keq or precipitation is only determined by the value of Q/K.

Regards,

Jubilee.

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Hi Brian,

Based on your previous response regarding the use of the titration model for simple reactants, i would like to know if this implies that the reaction term will be eliminated from the reactive transport equation. If not, what expression serves as the substitute for the reaction term in the equation.

Jubilee.

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Hi Jubilee,

 

The Geochemist's Workbench uses a technique called operator splitting in which it separately solves the transport equations from the equations describing chemical reactions. Equation 21.2 in the Geochemical and Biogeochemical Reaction Modeling text is a version of the "advection-dispersion-reaction equation", but the R term just describes the net addition to or loss of solute from solution resulting from a system of chemical reaction equations (including equilibrium reactions, reactions defined by kinetic rate laws, etc.).

 

Hope this helps,

Brian

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Hi Brian,
I have yet to come up with a mathematical expression for the equilibrium reaction equation when using the simple reactant in my model. Unfortunately, the geochemical and biogeochemical reaction text does not give sufficient information about this. Although, I had a clue of what is going on with the AsK+(1-Q/k) term of the equation when a simple reactant is chosen, my assumption broke down when I discovered that the dimension analysis (unit) of the dispersion and advection terms of Eq 21.2 is different from that for the reaction term. Is there a reason for this? My understanding is that the dimensions of the RHS of the equation should be equal to that of the LHS, and that all additive terms of the RHS and LHS should have the same dimensions.
Regards,
Jubilee

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Hi Brian,

I am still expecting your response to my previous post.

Regards,

Jubilee

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Hi Jubilee,

 

Apologies for not responding sooner. I'm currently traveling back from our Applied Geochemical Modeling workshop in Japan.

 

There isn't a simple expression for the "R" term (for chemical reactions) that appears in equation 21.1 and 21.2 in the GBRM. If it helps, Chapter 13 describes in general form how simple reactants can incrementally change the composition of a system. After each increment, however, the program will recalculate the equilibrium state. Chapter 3 describes the set of governing equations which are part of the "R" term and Chapter 4 their numerical solution.

 

As far as I can tell, units for the advection, dispersion, and reaction terms in equations 21.1 and 21.2 are all consistent. There is no reason they should be different.

 

Hope this helps,

Brian

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Dear Brian,

 

May i know which of the GWB packages is most suited for carrying out thermodynamic simulations on reservoir rock cores.

 

I am looking to find out the effect of changes in pressure and temperature (reservoir conditions, 3500psi, 130oC) on the concentration of injected fluids during water flooding.

 

The end game is to determine if there is precipitation of the minerals in the injection fluid before they react with the formation.

 

Kindly let me know if GWB is capable of this kind of simulation.

 

Jubilee.

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Hi Jubilee,

 

Depending on what exactly you're trying to do you could use React, X1t, or X2t. With React, you could set up a simple polythermal path in which you heat your injection fluid to formation temperature. That way, you could see whether increasing the fluid's temperature causes minerals to precipitate. If you're interested in where the reactions take place (i.e. at the injection point or somewhere in the formation) then you'll want to set up a reactive transport model. A radial domain in X1t, a standard rectilinear grid with a well in X2t, or an axisymmetric domain in X2t would all be very good options for your example. Depending on whether the domain is controlling the migrating fluid's temperature or it's the other way around, you might consider the "constant temperature" option. See section 2.15 (Polythermal simulations) in the Reactive Transport Modeling Guide for more info.

 

The default GWB thermo dataset is compiled from 0 - 300 C along the steam saturation curve. At 130 C this is about 2.7 bars, or 39 psi. You can use a thermo dataset compiled at the pressure of interest, but hydrothermal chemists not uncommonly assume the effects of confining pressure are small compared to the uncertainty in determining log Ks and activity coefficients. Note, however, that gas partial pressures are almost invariably significant. You account for the partial pressure of a coexisting gas by setting its fugacity.

 

Hope this helps,

Brian

 

P.S. This question isn't really related to your earlier posts in this thread. If you have a new question, please try to create a new topic.

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