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.