|There is a lot of confusion regarding the terms Shale Gas and Coal Bed Methane, along with an array of other terms.
Here, I attempt to clear up some of the confusion regarding the terms and associated issues.
Shale Gas is defined as a natural gas produced from shale. Shale has low permeability, so gas production in commercial quantities requires fractures to provide permeability. Shale gas has been produced for years from shales with natural fractures; the shale gas boom seen in the USA in recent years has been due to new technology in hydraulic fracturing (especially directional drilling and frack fluids) to create extensive artificial fractures around well bores. It is sometimes referred to as tight gas. Shale is by far the most common rock associated with tight gas, but others include certain sandstones.
Tight gasis natural gas held in rocks with pores up to 20,000 times narrower than a human hair, such that the gas will not flow freely into a well without fracturing.
Coal Bed Methane (CBM), also sometimes known as sweet gas, coalbed gas, or coal mine methane (CMM), is a form of natural gas extracted from coal beds. To extract the gas, a steel-encased hole is drilled into the coal seam (100 to1500 meters below ground). Often, pressure within the coal seams brings water and gas to the surface readily enough. As the pressure within the coal seam declines, due to natural production or the pumping of water from the coalbed, stimulation by hydraulic fracturing is used . Unlike shale, coal is frequently very porous and permeable, and therefore often has a high water content. It generally needs to be de-watered before any gas can be extracted and collected. The ‘produced water’ is either re-injected into isolated formations, released into streams, used for irrigation, or sent to evaporation ponds. It is often contaminated with all manner of dissolved ingredients from the coal beds and associated rocks.
All the above types of gas extraction fall under the category of Unconventional Gas. One way of defining unconventional gas is that can only be produced economically by using hydraulic fracturing, horizontal drilling, or other techniques to expose more of the reservoir to a borehole in order to gain access to the gas.
HYDRAULIC FRACTURING (FRACKING)
- Shale/tight gas requires intensive fracturing at regular intervals, as essentially gas can only be extracted from fractured rock.
- Shale gas requires the use of a wide range of chemicals due to the extreme conditions (‘tightness’/porosity, depth/pressure, length of boreholes)
- Shale gas requires directional drilling into and along the beds, with explosives set of at regular intervals to fracture as much rock as possible and free as much gas as possible.
- Shale gas requires extraordinarily high hydraulic pressures to be generated in the fissures created, in order to open them up and create the flow of gas. It needs to exceed the rocks strength in order to fracture it. 15000 psi is not unheard of.
- Coal Bed Methane can involve everything involved above in extreme cases, but will generally be used mush less frequently, with less use of frack fluid chemicals; possibly without the use of directional drilling along the seams; and without such extreme pressures being needed.
- Coal Bed Methane is often relatively close to the surface and, being porous and permeable, frequently needs de-watering; producing huge quantities of water, before any gas production is possible. Coal is often an aquifer but the water will invariably be ‘dirty’ to some extent. Very often, this produced water contains excessive levels of sodium bicarbonate that make it unsuitable for direct discharge into water courses or for agricultural use. There are also often issues with salinity and high levels of sulphur compounds and objectionable minerals such as barium.
- Any chemicals used in fracking for CBM cannot be ‘secured’ at all. Rarely is more than 50% of frack fluid recovered, and allied to the porosity and permeability of the geology, will mean that the chemicals will have all manner of ways of ‘escaping’ into the wider environment.
- Shales rarely have any significant water content therefore do not require de-watering.
- It is a myth that the often great depth of shale formations, and the associated distance between shale beds and aquifers (water bearing rocks), means there is no chance of aquifer contamination – for at least 2 good reasons:
- The first of these is that although shale normally has little or no permeability, it may lie adjacent to various conduits that could allow contaminants and fracking fluids to migrate in unexpected ways. Leaky dykes, undetected faults, faults created in the fracking process, and sand channels are all known examples of such conduits.
- The second known issue is effectively the man-made conduits that are the boreholes. The issues of poor well casings are well documented and very common. Even good initial casings degenerate with use and time, such that academic studies have indicated that at least 50% of boreholes will leak in their lifetime.
- The leaks can release the frack chemical nasties into the environment, and/or allow gas migration. They will also allow leached materials contained with the shales to get free into the environment. Some of these can be very nasty, such as radioactive isotopes. Shales are responsible for most natural radioactivity, so gamma ray logs are often used as a good indicator of the presence of such rocks. Radioactive Radon is a particular issue here in South Wales, but other isotopes commonly found include Potassium 40K , Thorium 232Th and Uranium 238U.
- It is not just water that gets contaminated. It is soil and air too. The majority of methane that migrates out of the shale or coal beds, or leaks from fracking operations either underground or at the surface, will leak straight into the atmosphere (not into water supplies), where it is potent greenhouse gas. It is impossible to quantify the totality of these leaks, but they are likely to be enormous.
I hope this is helpful.
If you want to know anything else, please ask via the reply facility (Leave a Comment) or the Facebook page. If I don’t know, I will do my best to find out.