Very basic guide to shale gas...
- · Shale is a sedimentary rock,
it is very fine grained and tightly compacted. It’s formed in marine or
lagoon environments over a very long period typically 10’s of millions of
years. It is formed from fine silty deposits as well as the remains of
dead plankton, airborne dust etc. River delta mud often forms shale in due
course. Lots of organic material is buried with the mud that forms shale;
something like 95% of all organic matter that is buried by sedimentation
is in mud that later forms shale. Bacteria is also buried with this organic
‘soup’ and over time convert organic matter to gas and oil: In the oil
industry many shale deposits are often called “source rock” or “the
kitchen”
- · Most of the organics in
shale “migrate up” away from the shale during the formation period as the
density of this material is much lighter than the seawater that is also
present in the pore spaces of the mud (just like oil and vinegar salad
dressing quickly separates)
- If the migrating organic
fluid encounters an impermeable barrier such as a layer of clay, mudstone,
or more shale, it becomes trapped and over time becomes an oil or gas
reservoir, trapped below a “cap rock”, often in the pore spaces of sandstone.
Unlike shale, sand grains create millions of tiny bridges as they are
squeezed together, creating permeable sandstone that holds oil and gas in the
pore spaces that are all interconnected.
- · As the transformation from
sedimentary mud to shale rock occurs over 1000’s of years, some remaining
oil and gas remains trapped in the structure of the shale.
- · Most shale has gas (or oil)
trapped in tiny bubbles but unlike sandstone, the bubbles are nor connected- the particles
of mud are squeezed so tightly together the bubbles are isolated from each
other , so the rock has moderate porosity- full of pores of gas, but zero
permeability- the ability of gas to flow through tiny channels in
the rock.
To labour the point, Sandstone has
good porosity (lots of little bubble of oil, gas or water) and ALSO good
permeability (the gaps between the grains of sand are all connected together so
oil, gas or water can flow through the rock, like water through a water filter)
- · Shale is formed in layers of
sedimentation- it has a natural tendency to fracture horizontally along
the layers. If you drill a hole into shale and pump water into the hole,
it will open horizontal cracks around the bore hole. But the water pressure has to be high enough
to literally lift the entire rock sitting on top of it in order to open a
crack. So the shallower the shale layer is, the easier it is to frack it
horizontally along the bedding plane. As you drill into deeper layers of
shale, it requires more and more pressure to create a horizontal crack.
There is a certain point at which it becomes easier for a crack to ignore
the bedding plane and go vertically upwards, pushing the rock to either
side as the crack is propagated instead of horizontally along the weaker
bedding plane.. THIS IS A VERY IMPORTANT FACTOR in shale gas
development. You have to understand this point to understand the pros and
cons of shale gas.
- During fraccing, secondary
fractures are created because a cold fluid is being pumped into hot rock
(at 6000 ft, the temperature is about 50 centrigrade) which assists the
frac process.
- · When shale such as the USA Marcellus
is fractured, as the water is pumped into the shale it cracks the rock
horizontally because the shale is relatively shallow at (approx.) 4-6000
ft deep. If you stop pumping the water, the horizontal cracks will close
up again.
- · Sand is pumped with the
water into the shale so that when the pumps are switched off, the cracks
are held open by the sand grains. The cracks join the little bubbles of
gas together (creating permeability!) allowing the gas to flow up the
well.
- · When the shale that is full
of gas/oil is very deep in the ground such as 10,000 ft (for example),
when water is pumped into it, the pump pressure is not strong enough to
make horizontal cracks, instead it causes cracks to go vertical- this is
the big problem with fraccing, vertical fracs are pretty useless in a
normal vertical well. In deep wells, all the energy expended in fraccing
the formation, only joins together areas near the well bore- further away,
permeability is not created or improved.
- The huge game changer
in the last 15 years is that horizontal wells are drilled
into the shale. When fluid is pumped into the horizontal well, primary
fractures are forced to radiate outward from the well bore, perpendicular
to the bedding plane of the shale- from these primary fractures, millions
of smaller secondary fractures travel along the weaker bedding planes of the
shale vastly increasing the area of rock being fractures and making shale
gas economically viable by massively increasing the productivity of the
well.
(click on image to enlarge)
- Horizontal wells are now also
fracced in stages, the bottom 200’
of the well is isolated and fracced first, then the system is pulled up
along the hole a few hundred feet and repeated 20-30-40 times so all the hydraulic
horsepower used is focussed on a relatively small volume. 15 years ago it
was common to try and fracc the whole production interval in one go, with
mixed success.
- One of the dangers of fraccing 15 years ago without doing it in stages was that all the hydraulic force would find one weak spot and create a single large fracture instead of 1000’s of small cracks. This fracture could propagate vertically 1000’s of feet without the operator being aware this was happening- it was possible to fracc vertically from the gas bearing shale all the way up to an aquifer 2-3000 feet above. However, this risk has almost been eliminated with today’s technology as the fracc job is carried out is small stages and modern computes with high resolution pressure gauges allow engineers to interpret exactly how the volume of the fracture space is growing /propagating – single vertical fractures show a markedly different pressure response to matrix fracturing and can be identified immediately- during the pumping process, allowing the job to be shut down before any environmental damage occurs.
- · Marcellus shale in the
Eastern USA is perfectly placed to be too deep to contaminate surface
water, but shallow enough to be fracced profitably from horizontal wells
at present day economic gas production levels. (The Marcellus shale covers
vast areas of the USA)
- Shale in other regions were
formed at different historical times, are at different depths. Thousands of
feet of rock have been eroded above the Marcellus in America, making it
easier to fracc. Often shale deposits in other parts of the world are
buried much deeper and do not have the same proven properties as the
Marcellus.
