Shale Gas and the fracking debate
On 30th June 2014, British Geological Survey published their eagerly awaited report; "The Carboniferous shales of the Midland Valley of Scotland: geology and resource"; a study commissioned to assess the potential for gas or oil production in the area, using modern technologies such as hydraulic fracturing. This report is available in full from the BGS website.
The report provides a welcome up-to-date summary of the geology of the area. It first reviews the various carbon-rich strata across the area, and identifies those sufficiently "mature" to potentially yield gas or crude oil.
The report considers the depth of past coal or oil shale mining activity and for workings deeper than 500 m discounts any potential shale gas or shale oil reserves lying within 305 m vertically (though the report notes specific Midland Valley of Scotland geomechanical and fracture growth height studies are required to give a more robust figure for an appropriate vertical separation). In areas with no past coal or oil shale mining workings below 500 m, a depth cut off of 805 m below Ordnance Datum was used.
This leaves only a restricted area in which oil and gas production might be practical, and estimates potential reserves at 80 trillion cubic feet of gas and 6 billion barrels of oil; very considerably less than the reserves earlier identified in northern England.
The report also points out that the nature of rocks in the Midlands of Scotland, and their complex geology, would offer substantial obstacles to the easy and efficient extraction of gas or oil.
The following account was written before publication of "The Carboniferous shale of the Midland Valley of Scotland" and some points within in may have been superceded by publication of the report.
A brief introduction to fracking in Scotland
The current debate about “fracking” - (the production of methane following hydraulic fracturing of rocks) often raises questions about how this new quest for “shale gas” relates to Scotland's former shale oil industry. The short answer is that the two activities have very little in common. They are likely to exploit different geological resources and employ totally different technologies
In Scotland, any production of shale gas is likely to be confined to the Midland Valley – the belt of sedimentary rocks running SW to NE across the centre of Scotland between the older volcanic rocks of the Scottish highlands and those of the southern uplands. Most of the rocks of the Midland Valley were laid down around 300 million years ago when the area lay at the edge of a giant continent. As the local environment and geography changed, different types of silt and sediment were deposited by the action of rivers, or beneath swamps, lakes, or shallow seas. When environmental conditions inhibited the breakdown of organic materials, this carbon-rich material was also deposited. Over the course of tens of millions of years, a complex succession of different sediments were laid down, which through the passage of time have become sandstones, mudstones, shales, limestones, coals, oilshales, and other types of sedimentary rock.
This layer-cake of minerals was once the foundation of Scotland's wealth; particularly the seams of coal, ironstone and fireclay that were deposited during periods when the land lay beneath tropical swamp. The oilshales and cannel coals exploited for oil production were found over more restricted areas than most seams of coals and are thought to result from blooms of single-cell algae that occurred in discrete freshwater lakes. Within the wide spectrum of sedimentary rocks found in the Midland Valley many contain carbon materials – but only the thicker and richer seams of coal or oilshale ever justified the danger and expense of mining them.
Past onshore exploration for oil and gas
No significant reserves of liquid petroleum or unbound natural gas have yet been found in the Midland Valley, although a few small-scale seepages of oil are known, probably the result of localised heating of coal or oilshale by intrusions of volcanic rocks. Methane and other inflammable gases were commonly found in association with coal seams (and oilshales to a lesser extent), and mines required powerful ventilation systems in order to control the ever present risk of explosion.
1n 1919 the Ministry of Munitions funded exploratory bores for petroleum at West Calder (which proved fruitless) and at Cousland in Midlothian, (which yielded only seven tons of oil). Further exploration of oil reserves in the Cousland and Dalkeith areas took place from 1937, and subsequently the development of the Cousland gas field in 1957 provided Scotland's first supply of natural gas for domestic use. Production continued until 1965. Elsewhere in Midlothian, close to the site of the former Pentland and Straiton shale oil works, substantial exploratory drilling for oil and gas took place in 1984, but met with little success. Unproductive exploratory bores in the former shalefield were also drilled at Pumpherston (in 1962) and Easter Pardovan (in 1945). In the coalfield districts of Lanarkshire, bores have been drilled in the Salsburgh area ( in 1945, 1964 and 1985) , and at Bargeddie in 1989, where a substantial gas reserve was detected. None of these bores remain in production.
In Fife, an exploratory well was bored near Rosyth in 1940, and further wells drilled in the area during the 1960's and 1980's. A number of wells were also drilled near Leven in the late 1930's with further exploration in the area taking place nearby at Milton of Balgonie during the 1980s. Traces of oil and gas were found in many of these wells, but none ever went into production.
Image developed from "Unconventional Hydrocarbon Resources of Britain's Onshore Basins - Shale Gas" (Department of Energy and Climate Change, 2013 ). Cream boxes represent current onshore licenses, pink areas relate to the14th round licence areas under consultation, markers show conventional wells drilled (not coal bed methane or mine gas)
Modern drilling techniques and borehole technologies enable drilling bits to be accurately steered along a chosen route, and for conditions to be manipulated along the length of the well in ways never previously possible. This includes induced hydraulic fracturing or “fracking”; the process of injecting liquids at high pressure to induce microscopic cracks in the rocks which are then held open by injection of a fine sand. This increased permeability allows release of methane held in spaces between the individual grains of the rock. The process is widely employed throughout the oil and gas industry and has had a major economic impact.
Advanced drilling techniques are of value both in the exploitation of coalbed methane, and in the production of methane or "shale gas" from mudstones and similar rock types.
"Coalbed methane" exploits the inflammable gas associated with seams of coal that once presented a major hazard to mining operations. In the past, (notably at Cardowan Colliery in Lanarkshire), mine gas vented from active workings was used as a fuel, and there may remain opportunities to harvest gas from networks of disused mine workings that underlie much of central Scotland. Drilling techniques now provide the opportunity to reach seams of coal at far greater depths than were practical to work by mining. and having reached these deep seams, the wells can be directed horizontally to follow a seam of coal through the ground. Once a network of wells has been created, ground water within the seams is pumped to the surface, releasing the methane present in the coal seam. As methane is held within natural fractures in the coal seam, this process does not require hydraulic fracturing (fracking) in order to release the gas.
"Shale gas" is the term used for methane associated with mudstone or other fine-grained sedimentary rocks. While the drilling technologies used to reach seams exploited for shale gas are generally similar to those used to extract coalbed methane, additional measures are required to release the gas from the rock. These include hydraulic fracturing or "fracking".
Several licences have been issued for the exploitation of coalbed methane in Scotland. Four of these are held by Dart Energy who currently produce coal bed methane from the area around Airth in Stirlingshire.
The government are now considering the issue of additional licences in Scotland, which might include licences for the production of shale gas. In Britain the hydraulic fracturing process appears to offer most potential for gas production from various sedimentary rocks in England; notably in Wessex and the Weald, East Midlands, Cleveland and West Lancashire areas. The British Geological Survey has undertaken studies to assess the potential for unconventional gas recovery in most of these areas and is currently (February 2014) completing a study into the potential for onshore gas production in Scotland. Until this is published, it is difficult to predict if and where fracking might be practical and worthwhile in Scotland.
What is the difference between shale oil and shale gas ?
"Shale oil" was produced in Scotland from oilshale. The carbon content of oilshale - known as "kerogen" - consists of a complex mix of high molecular-weight organic compounds, thought to have originated from the breakdown of algae and other microscopic creatures. This kerogen is chemically bound to the mineral matrix of the rock. The extreme heat of the retorting process is required to break these chemical bonds, enable other molecular rearrangements, and so release a crude oil vapour. Seams of oilshale occur fairly close to the surface, and were mined in days when the conservation of the natural environment was less of a public concern. Consequently the scars of the industry remain evident in the landscape today.
"Shale gas" is the term currently used for methane produced from mudstone and other fine-grained sediment; many of which are "fissile" (ie they split into sheets) and therefore termed shales. Methane produced from organic matter present when the rock was formed is held in the microscopic pores between the individual grains of mineral matrix. This is released when the mechanical structure of the rock is affected, such as through hydraulic fracturing. Rocks suitable for shale gas are likely to lie deep beneath the surface and are unlikely to have been the subject to earlier mining activity. Present day concerns about custodianship of our environment should ensure that any production of shale gas would be closely controlled and have minimal environmental impact. It seems unlikely that seams of oil shale would be suited for the production of shale gas.
The website of the British Geological Survey contains many studies relating to unconventional hydrocarbon sources and a wealth of other authoritative information.
Prospecting for oil and gas at West Calder in 1919. Image courtesy of British Geological Survey (ref C2887)