Talk:Darcy's law

From Wikipedia, the free encyclopedia

	Portal This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
???	This article has not yet received a rating on the assessment scale. [FAQ]
???	This article has not yet received an importance rating within physics.
Help with this template Comments: edit – history – watch – refresh I'd like to thank the author for putting the article together. However, the article would benefit from a serious revision. The most important correction is to explicitly include gravity, by using the vertical z coordinate in the main formula, and by replacing the sketch with the one having the pipe at a slope or a vertical pipe. The explanation that gravity can be included within P is not correct, because when the gravity part is added P ceases to be pressure. A further correction concerns the role of Darcy's law in the fundamental equations of porous media flows. Darcy's law provides the relationship between the resistance of the porous material to fluid flow and the fluid velocity (averaged over the Representative Elementary Volume of the porous material). In other words Darcy's law is a parameterisation (valid for small Reynolds numbers) not the fundamental equation itself. Depending on a particular application the fundamental equation may contain just the Darcy's equation or also some other terms such as acceleration or Brinkman's term. Such equations are not 'Additional forms of Darcy's law' as stated in the article. They are various versions of momentum equation which use Darcy's law to parameterise the resistance (i.e. drag or skin friction) term. The Brinkman's correction term accounts for the viscous resistance within the fluid flowing through the porous material, not for the porosity of grains. The article should reference at least one classical textbook such as Bear, Jacob, 1979, Hydraulics of groundwater: New York, McGraw-Hill, 569p. Average58 17:57, 3 June 2007 (UTC) Average58

Contents 1 Reason for Peak Oil Category 2 Article generalization 3 Article consistency 4 Porosity 5 Tensor permeabilities

[edit] Reason for Peak Oil Category

One of the major arguments for Peak Oil is that energy companies will find it increasingly difficult to extract oil from reservoirs.

This is predicated on the understanding that an underground oil field consists of oil co-existing with permeable rock. Once the pressure inside the reservoir drops and electric pumps are used to speed up extraction, lower viscosity oil will be extracted.

Thicker oil passing through permeable rock takes time to get through, hence the reason why Darcy's Law is included as a Peak Oil category. --One Salient Oversight 00:48, 22 Mar 2005 (UTC)

huh? "Once the pressure inside the reservoir drops and electric pumps are used to speed up extraction, lower viscosity oil will be extracted."

Isn't ALL oil pumped from reserviors?

No. TastyCakes 04:37, 9 November 2006 (UTC)

thicker means higher viscosity.

Darcy's law has nothing to do with viscosity, which only tenuously has something to do with peak oil... I could maybe see the label in the permeability article, since that is the only way Darcy's Law is related to viscosity. --kris 03:22, 22 Mar 2005 (UTC)

Viscosity is a factor in darcy's law, it just seems to have been folded in with "K" here, possibly because it's being done for water where viscosity is 1.. some unit I forget. I agree though, its link to peak oil is tenuous at best. TastyCakes 04:44, 9 November 2006 (UTC)

[edit] Article generalization

This whole article has been written from a hydrology standpoint. I have rewritten a bunch of it in a more generalized form, that is one where fluids other than water are involved (as is frequently the case when using Darcy's law in oil field applications). TastyCakes 05:20, 9 November 2006 (UTC)

[edit] Article consistency

In the additional forms section an expression of Darcy's Law common to 'petroleum engineering' uses small 'q' for total discharge (units of volume per unit time). At the start of the article small 'q' is defined as flux, and large 'Q' as total discharge (units volume per unit time).

I can't even see what new information that extra equation for petroleum engineers give to the article. I vote for removal or rewrite of that particular section. Berland 07:45, 29 January 2007 (UTC)

For the sake of clarity, could the petroleum engineering version of the equation be given a large 'Q' too? jac 15:09, 11 January 2007 (UTC)

I'd support that. I also made an article concerning that a few weeks ago, groundwater discharge (which I'm yet to integrate into other articles). I'm not 100% sure about petroleum engineers, however I know the civil/hydro engineers stick to the same 'q' vs 'Q' conventions as hydrogeologists.+mwtoews 00:40, 12 January 2007 (UTC)

This article needs serious revision, to make it both correct and consistent with other articles. First, by not including gravity in the generalized form, the article oversimplifies the Darcy equation. I will fix this if I have time. Definately stick to the q=flux Q=flow rate. Also, put in references to other hydrogeology books to supporrt the generalized forms. Luckymonkey 17:02, 19 February 2007 (UTC)

No, gravity is (or can be) included, in ΔP, in the equation q = − κ / μΔP. It is a vector equation, and κ possibly as a tensor if anisotropy is to be considered. Berland 17:33, 19 February 2007 (UTC)

[edit] Porosity

I think the symbol for porosity should be φ, not n, as in the porosity arcticle. Berland 07:25, 17 January 2007 (UTC)

I know I've seen both, but I forget who uses what (different disciplines have different notations). You might add a sentence to the article saying "Foo engineers use "x" to denote porosity, and bar engineers use "y"." Lunch 18:31, 17 January 2007 (UTC)

Unfortunately, there are many conventions used for the "symbol for porosity" (such as n, φ, β). For the sake of consistency with the porosity article, I suggest using the same φ symbol, as Berland suggested, and perhaps add the sentence about the different conventions in the porosity article. Fetter and Domingo and Schwartz use n, while Dingman uses φ. Personally, I'm a fan of φ for porosity, and n for Manning's roughness coefficient.+mwtoews 19:40, 17 January 2007 (UTC)

I changed to φ. Berland 07:39, 29 January 2007 (UTC)

[edit] Tensor permeabilities

There's no discussion of anisotropic permeabilities; in this case, the permeability term, k, is a tensor. Please could someone add some information on how this tensor and the resulting equation are to be interpreted?

The section 'In 3D' encompasses this, though it is not explicitly mentioned that κ there is a tensor/3 by 3 matrix. --Berland 21:16, 29 July 2007 (UTC)