Category Archives: Groundwater – geofluids

Mineralogy of sandstones: Porosity and permeability

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porosity in sandstone

Porosity and permeability – the flow of water and other geofluids

Nearly all geological processes require the presence of water in one form or another. Most sedimentation occurs in water (aeolian deposits are the obvious exception). Sediment burial and compaction involves the expulsion of water. Diagenesis would not take place in the absence of water; hydrocarbons would not migrate to traps and minerals would not be concentrated in ore bodies. Aqueous fluids under pressure reduce cohesion and friction promoting rock deformation.  Metamorphism would be painfully slow, even by geological standards if it were not for the transfer of mass in hot aqueous fluids.

All these processes require not only the presence of water, but its continual movement or flow. Below Earth’s surface, the residence and flow of aqueous fluids requires two fundamental rock-sediment properties:
– voids, commonly in the form of intergranular pores and fractures, and
– connectivity among the voids.
The first of these is referred to as porosity; the second as permeability.

There are two main types of porosity: intergranular porosity that characterizes sands, gravels and mud, and fracture porosity in hard rock. Fracture porosity forms during brittle failure of hard rock or cooling of lava flows. Fracture networks that are connected can provide pathways for fluid flow even when the host rock is impervious (e.g. granite, basalt, indurated sandstone). Highly productive aquifers are not uncommon in fractured bedrock.

Intergranular porosity is the void space between detrital grain contacts and is expressed as a percentage of the total sediment-rock volume. It is a dimensionless number (i.e. it has no units of measure). All sediments begin life with some porosity.  Well sorted beach, river and dune sands have initial porosities ranging from 30% – 40%, muds as high as 70%. These values represent the total void space, namely the large pores plus lots of microporosity in tiny nooks and crannies between grains and crystals. Hydrogeologists have found it useful to define effective porosity as that which permits easy movement of fluid. This excludes microporosity where surface tension forces inhibit flow. Effective porosity is always less than total porosity. Follow this link to a simple experiment designed to measure porosity.

As sediment is buried, the grains settle (i.e. they become more closely packed) as they begin to compact.  The reduction in porosity by mechanical compaction continues during sediment burial, in concert with the precipitation of cements (chemical diagenesis).  This is particularly evident during the compaction of mud. The high initial porosity is due to micro-pores between clay particles that have dimensions measured in microns. Compaction compresses the clays and drives off the interstitial water. Compaction (porosity-depth) curves for mud, like the example shown below, typically show a loss of porosity that at shallow depths is almost exponential, becoming approximately linear at depths where shale forms; total porosities in shale are extremely low.

burial sequence for loss of porosity

The conduits for fluid flow (water, oil, gas) from one pore space to another are the narrow connections adjacent to grain contacts. These connections are commonly referred to as pore throats. Pore throats are susceptible to blockage during sediment compaction (lithic sandstones are prone to this) and cementation, particularly clays.

details of pore filling cements

Porosity can also be enhanced during burial diagenesis. The primary mechanism for formation of secondary porosity is the dissolution, or partial dissolution of framework grains like feldspar and carbonate bioclasts. Many of these secondary pores are larger than the associated intergranular pore spaces; this is an important diagnostic clue to their identification. Likewise, carbonate and clay cements may be prone to dissolution, resulting in enhanced post-depositional porosity.

Burial depths and temperatures where formation of secondary porosity is encountered commonly coincide with chemical reactions involving the break-down of organic matter. By-products of these reactions include carbon dioxide (and carbonic acid) and organic acids like acetic acid. There is a fundamental shift in pH and chemical equilibria, particularly for carbonates, and this promotes dissolution.

Secondary porosity can also form during subaerial exposure of rock and by some bioturbation. However, the secondary porosity seen in most ancient sandstones developed during  burial diagenesis.

Permeability measures the ease with which a fluid flows through sediment or rock. The flow of fluid from one part of a rock to another or to a bore hole, depends on the connections among pores and fractures. It is possible for a rock or sediment to have high porosity but low permeability if the connectivity is low – mud and shale are prime examples. In coarse-grained sediments that are devoid of clay, there is a good correlation between porosity and permeability.  This relationship does not apply where there are significant amounts of clay.

Permeability is expressed in two ways. Henry Darcy’s pivotal experiments with sand-filled tubes (in 1856) established an empirical relationship between hydraulic gradient (that basically is an expression of hydraulic potential energy) and discharge. The proportionality constant in this relationship is called the hydraulic conductivity (K) (a label borrowed from electrical theory), that has units of distance and time (cm/s, feet/s). In mathematical terms, hydraulic conductivity is expressed as a velocity, also known as the Darcy velocity. Hydraulic conductivity is the standard expression of permeability in groundwater studies. Its value depends not only on the connectivity of pores but also on the dynamic viscosity and density of the fluid (viscosity measures the resistance to flow – crude oil is more viscous than water). Thus, for any porous medium the value of K will be different for water and oil, a factor that is important in groundwater remediation.

The hydrocarbon industry deals with fluids of highly variable viscosity (water, oil, gas) and has opted for a standard expression of intrinsic permeability (k) that depends only on the porous medium. The unit is the Darcy that mathematically reduces to units of area (ft2, m2). It is basically a measure of pore size (the oil industry commonly uses the term millidarcy). Frequently used conversions to Darcys are:

1 m2 = 1.013 x 1012 Darcy

1 Darcy = 9.87 x 10-13 m2

Hydraulic conductivity (K) and intrinsic permeability (k) are related by fluid density and dynamic viscosity such that:

k (m2) = K (m/s) x (1.023 x 10-7 m.s) (the time components cancel)

Typical permeability values for unconsolidated sediment and some rock equivalents are shown in the table below.

typical values of permeability

As you can see, the permeability of shale is extremely low. This is the reason why shale beds make good seals to hydrocarbon reservoirs, and aquitards to confined aquifers. Fluid flow in shales and well-cemented sandstones or limestones can be enhanced by hydraulic fracturing. This process (fracing) is front and centre of shale oil production (notwithstanding all the pros and cons of this industrial process). But that is a story for another time.

 

Here are three excellent texts that detail the theoretical aspects of the above:

P.A. Allen and J.R. Allen, Basin Analysis: Principles and Applications. Blackwell 2005

C.W. Fetter.  Applied Hydrogeology, 2001. PrenticeHall

P.A.Domenico and F.W.Schwartz Physical and Chemical Hydrogeology,1998 John Wiley & Sons

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Difficulty breathing: The Atacama salt lakes

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I had the good fortune to work in the Atacama volcanic region a few years ago. It may be the closest I get to walking in a Martian landscape (NASA tests its Mars rovers there)

The mountains of Atacama, also known as the Altiplano-Puna Plateau, is one of the driest places on earth; it is located inland from the coastal Atacama Desert. A parched landscape littered with volcanoes, valleys where the few toughened blades of grass eke out a living, and salars, the salt lakes where there is barely a ripple. The salars are a kind of focal point for local inhabitants – Vicuña that graze on spring-fed meadows, flamingos that breed on the isolated breaks of open water, and foxes that lie in wait for both. It is a harsh environment, but stunning; glaring snow-white lake salt against a backdrop of reds and browns. And overhead, crystal skies, fade to black. Continue reading

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Deciphering a volcano’s moods; predicting volcanic eruptions

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A peaceful morning, zephyrs, whispy clouds wrapping, scarf-like, the towering edifice that is your town’s backdrop. A sudden roar; the clouds are shredded.  The turmoil of a volcano with attitude, a billowing column of ash and rock, tossed effortlessly skyward, reaching heights of 5 to 7 km in a matter of minutes. Not content with this scene, parts of the column collapse into fast-moving, ground-hugging pyroclastic flows that smother and incinerate everything in their path. This was Sinabung a few days ago (February 19, 2018 – the Youtube video is worth watching).

The Indonesian volcano is regarded as active and has been erupting off and on for about 7 years, following a 400-year slumber, although the violence of this event caught many by surprise. Happily, it was short-lived and no lives were lost. But the event does illustrate the fickleness of volcanoes, and like a case of indigestion, a bad-tempered, frequently unpredictable response to rumblings in their internal plumbing. Eruption prediction ideally should provide sufficient warning to all those who live nearby; Sinabung decided otherwise. Continue reading

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Erupting mud volcanoes; We have ignition

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Mud
It supports geological processes.
It flows, subsides, and leads to failure, sometimes catastrophically.
It can be beneficial, forming fertile river floodplains.
It can be a pain in the neck, clogging infrastructure.
It oozes when soft; dries brick-hard
People bathe in it. Pigs love it.

And it erupts, as volcanoes.

Not the magmatic kind, with 1000oC lavas or explosive ash columns, but eruptions nonetheless. Most mud volcanoes are much smaller than their magmatic counterparts; some only a metre high, others 10s of metres. Eruptions may be the quiet, oozy kind where mud flows, slithers and slides down slope, or more violent, shooting sticky stuff 10s of metres into the air (or water); some even ignite in a cascade of fireballs. And yes, they do form on the sea floor.  One example in 2015 along the Sea of Azov coast (land-locked between Russia and Ukraine), sent mud and water several metres into the air; you can see the muddy jetsam gradually expanding across the sea surface.  Continue reading

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Nitrate in excess; a burgeoning global contamination problem

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A “Nitrate timebomb”.  Last week’s media metaphor (Nov 10, 2017), was no doubt intended to create visions of dire deeds. After all, it seems that explosions are not in short supply these days. The actual story though is more droll, based as it is on the slow leakage of excess chemicals called nitrates, into the global environment. No fireworks; only leakage. The headline in several media outlets, only lasted a day or two, barely scratching our collective consciousness. Perhaps the problem is too big, or too remote – a candidate for the too-hard-basket. As Mark Twain might have said, “I guess so, I dunno”.

Nitrogen itself is not a concern; every breath we take contains 80% N2. It’s what we do with nitrogen that is causing problems, particularly in natural systems like soils, surface waters, groundwater aquifers, and ultimately, the oceans. The scientific paper that caused these brief media conniptions was published this month in Nature Communications (it is Open Access). Continue reading

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Springs and seeps

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It rains quite a bit on Mamaku Plateau, the tableland underlain by volcanic debris that was  deposited violently 240,000 years ago; an eruption that also gave rise to Lake Rotorua caldera (central North Island, New Zealand). Some of that rain seeps into the myriad fractures, nooks and crannies, and heads west as groundwater. Fifty to 100 years later that same water emerges, chilled to a cool 11oC, at Blue Springs (about 40km west of Rotorua). Spring water here flows at 42 cubic metres per minute (9,240 gallons per minute), enough to maintain a decent-sized stream (Waihou Stream). Continue reading

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The brutality of Surtsey’s laboratory

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I recently came across a local newspaper article describing a new volcanic island, rising from its own ashes above the sea floor, off the coast of Tonga.  The subtlety of memory returned me to 1963, and an announcement over our morning radio, of the birth… of a volcanic island off the coast of Iceland. Images, arriving a couple of days later (this was 1963 after all), gave witness to a natural brutality I had not seen before; the sea in boiling turmoil, torn by erupting columns of rock and steam. Beautiful, in an awe-filled way.

It has been fifty years since the cessation of volcanic activity. Surtsey has become home to plants and birds, a laboratory for the adaptable, the dispersible, and the colonial.  The only sounds that resonate now are noisy gulls and pounding North Atlantic waves. Continue reading

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Contrails, analogues, and visualizing groundwater flow

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Analogues and analogies.  Standard dictionaries define these as a comparison, correspondence, or similarity between one thing and another, that can apply to concepts, ideas or physical entities. They are tools, used to illustrate concepts, particularly abstract ideas, to help explain phenomena or theories. Science makes frequent use of analogies. It does so because many phenomena that it attempts to investigate and explain extend beyond normal human experience, beyond what is visible to the unaided eye, beyond what we can touch.  Well-chosen analogies can help us understand the universe without, and the universe within. Continue reading

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A misspent youth serves to illustrate groundwater flow

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Groundwater is always on the move. Under some conditions, in fractures or other large conduits, it can move quickly; almost at a walking pace. Under other conditions it moves inexorably slowly, like fractions of a millimeter a year. Regardless, it is always compelled to move. Movement requires energy.  Where does this energy come from?  What drives the flow of groundwater?  Answers to these questions provide the foundations to the science of hydrogeology. Continue reading

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Stabilisation of an architectural icon; the Leaning Tower of Pisa

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Sunday in Pisa proved to be a welcome change from the usual tourist-cramped, shoulder-barging throngs of popular attractions in Tuscany.  No problem finding a seat in a decent café, en route to the Piazza del Miricoli.  Cross the street, turn a corner and there – the massive, white-marbled Pisa Duomo, Romanesque grandeur with a veneer of 21st Century scaffolding.  But the sense of balance normally attributed to cathedrals, is disrupted by the stand-alone bell tower that leans precariously, like a drunk looking for a lamppost.  The Leaning Tower of Pisa has been looking for a lamp-post for almost one thousand years.  And for a thousand years, people have been drawn to the tower not because it is particularly beautiful, but because it looks like it is about to fall over. Continue reading

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