Porous Media for Dummies

Porosity vs permeability

We've noticed over the years that a lot of freelancers seem to think they know simple answers to complex questions but, in reality, know little more than whoever asked the question in the first place. We noticed that again today when we ran across an English major (albeit from Harvard) who seemed convinced that she knew how to answer the deceptively simple question "What Is a Porous Medium?" at WiseGEEK niche site AboutMechanics.com. We're here to tell you that E. A. (Elizabeth) Sanker didn't do a very good job.

The answer is pretty simple: a porous medium is one that contains void spaces as opposed to being a dense solid, like metal or glass. Sanker nibbled around the edges of that definition when she opined that,

"A porous medium is a substance that contains pores, or spaces between solid material through which liquid or gas can pass."

That business about "through which liquid or gas can pass" is quite telling, because it tipped our staffers off to Sanker's ignorance of her topic. You see, the ability for fluids to pass through a medium is not porosity, it is permeability – and the two are different properties. But let's get back to E. A.'s text... Sanker informed us that,

"Examples of naturally occurring porous media include sand, soil, and some types of stone, such as pumice and sandstone."

In truth, Elizabeth, all ;naturally-occurring stone is porous to some degree, whether it's granite or shale. In fact, shale is highly porous but isn't permeable, which is why fracking was developed. Sanker continued to confuse porosity and permeability as whe went along, explaining to her readers that,

"Physically, a porous medium can be distinguished from other materials — including other porous media — by its porosity, or the size of its pores. Materials with low porosity are less permeable and typically have smaller pores..."

No, Elizabeth, porosity is not a measure of the size of the pores, it is a measure of the portion of the volume that is void space. Oh, and permeability is a measure of the interconnectedness of the pores, not their size. Get it right or don't write! As for this claim:

"...materials with high porosity have large pores and are easily permeated... "

Don't make shit up! And while we're on the subject of MSU¹, this is also wrong: "Natural gas trapped in low-porosity stone is known as 'tight gas' and is more difficult to access than other reserves." Tight gas is no more difficult to "access" than other gas, it is more difficult to produce. Sheesh. More bullshit follows, including this rubbish: "Porosity ranges from a low percentage in dense shale and sandstone to about 50% in sand, and up to 70% in clay." We'll agree with the statement that unconsolidated mud and sand have higher porosity than shale and sandstone, but her numbers are ridiculously high. Sand has an upper limit of porosity of about 35%, not the 50% Sanker claims; while only a slurry of water-saturated mud (clay) can even approach the 70% she claims for clay. Perhaps she was confusing water content with porosity? That's not to mention that no one with knowledge of sedimentary geology would ever use the phrase "dense shale."

In other words, this self-described "science writer" didn't know the science she was writing about. What do we call people who do that? We call them the Dumbass of the Day.

¹ MSU: Making Shit Up.

SI - HYDROLOGY