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Fracking

Issues in Environmental Science and Technology

The Royal Society of Chemistry

Introduction and Overview: the Role of Shale Gas in Securing Our Energy Future

PETER HARDY

ABSTRACT

The phenomenon of shale gas is both topical and controversial. Its proponents claim that it is a clean, environmentally friendly and abundant source of cheap natural gas; its opponents believe the opposite. In several countries it is a fast-growing industry and operations have begun in the UK.

With conventional reserves of natural gas being quickly depleted, gas prospecting is turning to "unconventional resources", one example being gas found in shale. Uncommon technologies, notably hydraulic fracturing and horizontal drilling, are necessary for shale extraction to be economical.

Shale gas has faced some difficulties over concerns regarding environmental pollution. In the US, Gasland, an influential film was released alleging that waste fluid from hydraulic fracturing, "flowback water", was polluting groundwater. While it is possible for methane to enter groundwater through a faulty well completion, in the UK it is the responsibility of the Environment Agency and HSE to ensure regulation is adequate to prevent risks to the environment or human health.

There have been two earthquakes in Lancashire thought to have been caused by shale gas operations. The results of an investigation into these have been accepted as revealing that they were caused by hydraulic fracturing operations and new guidelines are being proposed to reduce the risk of this happening again.

With insufficient public information and sometimes animosity towards shale gas, drillers need to consider developing corporate social responsibility programs tailored to the needs of the communities local to drilling, with especial consideration towards environmental initiatives.

Worldwide, shale gas has had a significant and growing impact on gas production and looks likely to rapidly transform the energy situation.

In Europe, Poland and France have the largest reserves; Poland has embarked on a program to exploit its shale gas reserves. France, on the other hand, has outlawed the hydraulic fracturing technology vital to shale gas on environmental grounds.

The UK's shale gas reserves are unlikely to be large enough to be a "game changer"; however, they would contribute to gas security and the UK's energy mix, as well as being perceived as a lower-carbon alternative to coal-fired electricity generation.

There are already substantial reserves of gas available worldwide; however, the development of these unconventional gases, which are often in more politically stable parts of the world, may provide a greater security of supply to the Western World going forward.

1 Introduction

As the existing conventional gas supplies have started to decline in some parts of the western world, the search has been on for alternative secure sources of supply. One of the most exciting developments in the last 20 years in the natural gas sector has been the development of unconventional gases and, in particular, the exploration and production of shale gas. The existence of shale gas has been known for decades but technological difficulties and substantial financial costs associated with its extraction have up until recently made its exploitation uneconomic. However, increased demand and lagging supply have resulted in the price of gas rising to the point where, along with the development of advances in drilling, shale gas has started to represent a viable alternative to conventional sources of supply. Shale gas is now being produced in large quantities in the USA as their industry has developed over the last decade. Exploitation of reserves is now progressing in other parts of the world, including Poland and Australia. In addition, exploration is starting in other countries including the UK but the development of shale gas production, which often includes hydraulically fracturing of the rock (otherwise known as fracking), is not without its opponents. In America the film, Gasland, raised issues relating to problems associated with fracking which has caused some people to have environmental concerns. In some countries, such as France, an embargo has been placed on fracking and even within the USA some states are not as yet permitting it. There have been reports of ground-water contamination which has resulted in illnesses, gas coming out of water taps, and earthquakes caused by fracking. However, in many parts of the world shale gas is seen as a secure source of hydrocarbon that cannot be ignored. Development is seen in many countries as a way to secure energy supplies that is independent of events in the more volatile parts of the world where most of the existing gas and oil reserves are located.

1.1 History

In 1821 shale gas was produced from a natural seepage in the Appalachian Mountains at Fredonia, New York, USA. It was trapped and piped in hollow logs where it was used to light homes and businesses. The profit margins were small and small local operators exploited it as a "cottage industry".

In the late 1960s and early 1970s it was clear that the political situation in the Middle East was changing. There had been Arab–Israeli wars in 1967 and 1973 and the situation led to dramatically increased prices for oil as well as supply shortages. The Organisation of the Petroleum Exporting Countries (OPEC) also rose to international prominence during the 1970s, as its Member Countries took control of their domestic petroleum industries and acquired a major say in the pricing of crude oil on world markets. On two occasions, oil prices rose steeply in a volatile market, triggered by the Arab oil embargo in 1973 and the outbreak of the Iranian Revolution in 1979. OPEC broadened its mandate with the first Summit of Heads of State and Government in Algiers in 1975, which addressed the plight of the poorer nations and called for a new era of cooperation in international relations in the interests of world economic development and stability. This led to the establishment of the OPEC Fund for International Development in 1976. Member Countries embarked on ambitious socio-economic development schemes. It was against this background of volatile oil prices and trying to ensure security of supply that, in 1976, the United States Department of Energy initiated the Eastern Shales Project at a cost of up to $200 million to evaluate the geology, geochemistry and petroleum production engineering of non-conventional petroleum, including shale gas. Important reports established findings from what was then the only shale gas production in the world, based on the Devonian and Mississippian shales in the Appalachian basin. These reports led to the establishment of the Gas Research Institute and also stimulated research at Imperial College in the United Kingdom looking at evaluating potential resources. The geology of the plate tectonics of the Atlantic Ocean implied that the continuation of the Appalachian basin extended across into the UK and on into mainland Europe (see Figure 1). Imperial College focussed on the US paradigm of "cottage industry" and reviewed potential shale gas extraction from throughout the rock strata. The study concluded that Pre-Cambrian and Lower Palaeozoic shales were generally too metamorphosed to be potential reservoirs and most Mesozoic and younger organic-rich shales and mudstones were deemed too immature to be considered. Carboniferous shales, in general, and Namurian shales, in particular, were found to be ideally suited, both in terms of maturity and in degree of natural fracturing (see Figure 2). During the late 1970s, profit made from gas extraction was subject to both Corporation Tax and Petroleum Revenue Tax, meaning that production was nowhere near economic.

The conclusions of the Imperial College study on shale gas potential in the UK were presented to the UK Department of Energy in 1985. They were met with polite interest but the chances of shale gas being exempt from Petroleum Revenue tax was not countenanced. Subsequent attempts to inform the wider world failed and no reputable scientific journal would publish papers on the UK's shale gas resources. Finally, conclusions of the research were published in the US. Meanwhile in the US, shale gas development was continuing in the Appalachians, from a geographic perspective as well as from the study of rock strata, especially the distribution, deposition and age of sedimentary rocks, and various technological advances were also being looked at. The Appalachian basin, from New York State through Ohio to Kentucky and Illinois, was the main historic area for shale gas production, but there had been other basins where the gas was produced, such as the Williston Basin. This is a large intracratonic sedimentary basin in eastern Montana, western North Dakota, South Dakota and southern Saskatchewan where the Bakken Shale had produced gas since 1953. Stimulated by the Department of Energy and the Gas Research Institute, shale gas areas were found in the Cretaceous Lewis Shale of the San Juan Basin, the Mississippian Barnett Shale of the Fort Worth Basin and the Devonian Antrim Shale of the Michigan Basin. Geochemical studies revealed that the gas was not produced by microorganisms that generate heat within organic waste (thermogenic) but was produced by bacterial anaerobic respiration (methanogenesis). The bacteria had entered the fractured shale from ground water percolating down from the glacial drift cover. This second process for gas generation opened up new areas for exploration: areas where the source rock was previously deemed immature or over-mature for thermogenic gas generation.

The shale gas renaissance was also brought about by improved methods of well drilling and advances in completion technologies. The ability to drill multiple wells off a single pad was both financially and environmentally rewarding. The ability to drill wells horizontally as well as vertically, together with the ability to steer the drill along "sweet spots", enabled permeable gas-changed zones to be tapped into. This was coupled with more dramatic hydraulic fracturing techniques. Seismic techniques, which could use the fracturing process as an energy source, enabled gas-charged "sweet spots" to be mapped in three dimensions.

In the US the development of shale gas expanded dramatically from the mid 1990s, with the number of gas rigs in operation increasing from around 250 in 1993 to over 1500 by 2007. This has seen production of shale gas in the US increase from 1293 billion cubic feet in 2007 to 7994 billion cubic feet in 2011. This has resulted in the US natural gas wellhead price falling from $8.01 per thousand cubic feet in January 2006 to $2.89 per thousand cubic feet in January 2012. These are similar prices to those seen in the US in the early 1980s. However, the reduction in the price of natural gas seen in the US is unlikely to be repeated to the same extent in Europe. This is due to the limited export market that the US has for gas at the present time as it has no export terminals for shipping the gas globally as liquefied natural gas (LNG). The largest exporter of LNG worldwide, Qatar, has six operational export terminals, whilst Australia, which is rapidly increasing its export of LNG, has three operational export terminals with another seventeen projects either under construction or being planned. Worldwide there are thirty-two operational export terminals with another sixty-nine under construction or in planning.

In the UK, shale gas as a potential industry did not develop at all until the British Geological Survey (BGS) noted the potential for its production in 1995. Shale gas was not mentioned in reviews of future UK petroleum resources published in 2003 by the Oil and Gas Directorate of the Department of Trade & Industry. The 6th Petroleum Geology Conference on the Global Perspectives of North West Europe was held later in the same year. The three-day programme concluded with a session on non-conventional petroleum. This included a presentation on the UK's shale gas resources and provided a platform to disseminate updated conclusions of the Imperial College research of some 15 years previously. The advances in US shale gas exploration and production technology could now be applied to the UK. In 2008 the British Geological Survey began to review UK shale gas resources and delivered a presentation on their results at the 7th Petroleum Conference in March 2009. Subsequently, the Department of Energy & Climate Change commissioned the BGS to prepare a report on The Unconventional Hydrocarbon Resources of Britain's Onshore Basins – Shale Gas.

The result of this was that several companies started to look at shale gas sites within the UK at the time of the announcement of the 13th onshore round of UK licences in 2006. In 2008 Wealden Petroleum Developments Ltd was awarded a license that covered large parts of the Weald, an area in South East England situated between the parallel chalk escarpments of the North and the South Downs, for exploration. Additionally, Cuadrilla Resources Corporation was awarded a licence that includes areas of the North West of England.

2 Shale Gas Production and Reserves in the UK

2.1 Overview

2.1.1 Shale Gas Production Process. As noted, horizontal drilling and hydraulic fracturing are the two technologies that together have the potential to unlock the tighter shale gas formations.

Hydraulic fracturing (also known as "fracking") is a well-stimulation technique which consists of pumping a fluid and a propping agent ("proppant"), such as sand, down the wellbore under pressure to create fissures in the hydrocarbon-bearing rock. Propping agents are required to "prop open" the fracture once the pumps are shut down and the fracture begins to close. The ideal propping agent is strong, resistant to crushing, resistant to corrosion, has a low density and is readily available at low cost. The products that best meet these desired traits are silica sand, resin-coated sand (RCS) and ceramic proppants. The fractures start in the horizontal wellbore and can extend for several hundred metres while the sand holds the fissures apart, allowing the gas to flow into the wellbore. Recovery of the injected fluids is highly variable, depending on the geology, and ranges from 15 to 80%.

Horizontal drilling allows the well to penetrate into the hydrocarbon rock seam which can be typically 90 m thick in the US, but can be up to 1000 m thick in some of the UK shale gas seams. Horizontal drilling maximises the rock area that, once fractured, is in contact with the wellbore and therefore maximises the volume of shale gas that is released.

Horizontal drilling is performed with similar equipment and technologies to that which has been established over decades for vertical drilling and, in fact, the initial drilling of the vertical bore is almost identical to a conventional well. However, the well development and gas extraction processes differ widely between conventional and unconventional gas production. Whilst some conventional wells have been stimulated by hydraulic fracturing in the past, horizontal drilling and hydraulic fracturing are key requirements to make the exploitation of shale gas deposits economically viable.

The requirement for horizontal drilling and hydraulic fracturing also results in differences in the distribution of wells above the shale gas formations. The process involves locating several individual wells on a single "multi-well" pad. Normally 6–10 horizontal wells radiate out from the centre well pad; these then are drilled in parallel rows, typically 5–8 m apart. Each horizontal wellbore is typically 1–1.5 km in lateral length, although they can be considerably longer.

As the array of wells drilled from each pad only enables access to a discrete area of the shale formation, several multi-well pads in a geographic area are required in order to maximise shale gas extraction. In the US they typically locate a maximum of nine pads per square mile. In the UK, Composite Energy has estimated that about three pads per square mile should be sufficient for the UK setting. However, the geological and above-ground constraints will also impact on the location of well pads.

The differences in the production process between conventional and unconventional gas production also results in differences in the level of effort required to extract shale gas. It also affects the amount of resources used and the corresponding volume of waste products generated.

2.1.2 Well Pad Construction. As already stated previously, the pads used for multi-well drilling require an area of land sufficient to accommodate fluid storage and equipment associated with the hydraulic fracturing operation. This utilises larger equipment for horizontal drilling than that required for vertical drilling only. This results in between 0.4 and 1.2 ha (1–3 acres) of land being required for a multi-well pad.

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Excerpted from Fracking by R.E. Hester, R.M. Harrison. Copyright © 2015 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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