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Permalink Reply by Les Bamburg on June 11, 2009 at 7:10

Wolverine, without being too detailed:

Porosity - the amount of space in the rock. ie 10% porosity means 90% of the volume is rock and 10% is open space full of water, oil and/or gas.

Permeability - the ability to flow (natural gas) through the rock. The higher the perm the easier to flow gas to the wellbore. If the space in the rock is not interconnected then there is little or no permeability. You can have a lot of rooms (porosity) in a house but if all the doors are locked you can't walk thru it. The shale matrix has little permeability but natural fractures (cracks) or those created by frac'ing add permeability to the formation.

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Permalink Reply by The_Baron on June 11, 2009 at 7:21

From "Porosity and Permeability." World of Earth Science. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. Gale Cengage, 2003. eNotes.com. 2006. 11 Jun, 2009
porosity-permeability>

Porosity and Permeability

Porosity and permeability are two of the primary factors that control the movement and storage of fluids in rocks and sediments. They are intrinsic characteristics of these geologic materials. The exploitation of natural resources, such as groundwater and petroleum, is partly dependent on the properties of porosity and permeability.

Porosity is the ratio of the volume of openings (voids) to the total volume of material. Porosity represents the storage capacity of the geologic material. The primary porosity of a sediment or rock consists of the spaces between the grains that make up that material. The more tightly packed the grains are, the lower the porosity. Using a box of marbles as an example, the internal dimensions of the box would represent the volume of the sample. The space surrounding each of the spherical marbles represents the void space. The porosity of the box of marbles would be determined by dividing the total void space by the total volume of the sample and expressed as a percentage.

The primary porosity of unconsolidated sediments is determined by the shape of the grains and the range of grain sizes present. In poorly sorted sediments, those with a larger range of grain sizes, the finer grains tend to fill the spaces between the larger grains, resulting in lower porosity. Primary porosity can range from less than one percent in crystalline rocks like granite to over 55% in some soils. The porosity of some rock is increased through fractures or solution of the material itself. This is known as secondary porosity.

Permeability is a measure of the ease with which fluids will flow though a porous rock, sediment, or soil. Just as with porosity, the packing, shape, and sorting of granular materials control their permeability. Although a rock may be highly porous, if the voids are not interconnected, then fluids within the closed, isolated pores cannot move. The degree to which pores within the material are interconnected is known as effective porosity. Rocks such as pumice and shale can have high porosity, yet can be nearly impermeable due to the poorly interconnected voids. In contrast, well-sorted sandstone closely replicates the example of a box of marbles cited above. The rounded sand grains provide ample, unrestricted void spaces that are free from smaller grains and are very well linked. Consequently, sandstones of this type have both high porosity and high permeability.

The range of values for permeability in geologic materials is extremely large. The most conductive materials have permeability values that are millions of times greater than the least permeable. Permeability is often directional in nature. The characteristics of the interstices of certain materials may cause the permeability to be significantly greater in one direction. Secondary porosity features, like fractures, frequently have significant impact on the permeability of the material. In addition to the characteristics of the host material, the viscosity and pressure of the fluid also affect the rate at which the fluid will flow.

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Permalink Reply by Skip Peel - Mineral Consultant on June 12, 2009 at 3:10

Wolverine. Porosity refers to the pores (spaces) between the grains of rock. The higher the porosity, the more space for hydrocarbons to be contained. Permeability is a measure of how connected those pores are one to the next. Those connections allow hydrocarbons to move through the formation. Where there is high permeability, hydrocarbons flow from high pressure to low pressure, the well bore. A vertical well is sufficient to produce such a formation. Where permeability is low, hydrocarbons do not flow easily and need to be fractured.

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Permalink Reply by sesport on June 12, 2009 at 4:21

Skip - Everyday joe example of porosity, lava rocks in the landscape beds are highly porous? :0)

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Permalink Reply by Skip Peel - Mineral Consultant on June 12, 2009 at 4:32

Absolutely, sesport. Ever seen a HS core? Some describe it as looking like "black board" material. It looks so dense it is difficult to imagine it contains so much hydrocarbon. And there have been a number of comments on the site about drillers in the past complaining how it would eat up a drill bit. Dense and hard. The technology to produce the HS is amazing. IMO, the salient fact to take from this discussion is that permeability of a formation may be addressed by the completion design (frac). Porosity is what it is. No way to change it.

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Permalink Reply by sesport on June 12, 2009 at 4:52

So, in everyday joe speak, we want to turn that "blackboard" (I think I remember something like that, lol) matereial into something more like Jello so that the gas & liquid(s) can migrate between the cells? :0)

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Permalink Reply by Skip Peel - Mineral Consultant on June 12, 2009 at 4:58

Not exactly. One of the unintended results of fracing is to "super glue" the well shut. There is a marine shale in south LA. that is notorious for this. In technical reports you will see the phrase, "near wellbore permeability", used. The object is to fracture the formation in a particular direction to an approximate distance without turning the formation immediately in proximity to the wellbore perforations to jello.

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Permalink Reply by sesport on June 12, 2009 at 5:59

Okay, i'm relating this to a construction site in a new subdivision where the crew doesn't clean up nails. :0P

Nails go in tires creating permeability (if one doesn't stop to think and pulls said nails out of tires) for the air to flow out of the tire(s). This is how the fracs create permeability for the gas to flow out of the well?

I think I read in that IPAMS info that 10% of the frac fluid remains behind? Is this then pretty much then "trapped" by the formation as in the chalk example?

Thanks to you both for your patience - :0)

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Permalink Reply by sesport on June 12, 2009 at 4:53

Got that & what's below, thanks. :0)

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Permalink Reply by Grillin' - MmmMmm on June 12, 2009 at 6:38

Actually, lava rocks have very high porosity as when they cool, the pockets form. That's why they are so light. And there is a relationship between porosity and permeability. If a rock has zero porosity, it will have no permeability. But the relationship is not linear. Depends on how much of the porosity is connected. There can be lots of porosity but if it isn't connected, there is no permeability.

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Permalink Reply by Skip Peel - Mineral Consultant on June 12, 2009 at 7:19

I may be porous, but I"m not very permeable. Sorry, Wolverine. We've done it again. Run off the tracks. I hope you'll excuse us as it is Friday afternoon. Somebody cue Jack Blake. Now where'd I put that wine glass?

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Permalink Reply by sesport on June 12, 2009 at 7:24

If this business ever gets cranked up again, it will no more be "poor us." :0)

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