|Cuss, R J, Wiseall, A C, Hennissen, J A I, Waters, C N, Kemp, S J, Ougier-Simonin, A, Holyoake, S, and Haslam, R B. 2015. Hydraulic fracturing: a review of theory and field experience. British Geological Survey Internal Report, OR/15/066.|
Context of M4ShaleGas
Shale gas source rocks are widely distributed around the world and many countries have now started to investigate their shale gas potential. Some argue that shale gas has already proved to be a game changer in the U.S. energy market (EIA 2015). The European Commission's Energy Roadmap 2050 identifies gas as a critical energy source for the transformation of the energy system to a system with lower CO2 emissions that combines gas with increasing contributions of renewable energy and increasing energy efficiency. It may be argued that in Europe, natural gas replacing coal and oil will contribute to emissions reduction on the short and medium terms.
There are, however, several concerns related to shale gas exploration and production, many of them being associated with the process of hydraulic fracturing. There is also a debate on the greenhouse gas emissions of shale gas (CO2 and methane) and its energy return on investment compared to other energy sources. Questions are raised about the specific environmental footprint of shale gas in Europe as a whole as well as in individual Member States. Shale gas basins are unevenly distributed among the European Member States and are not restricted within national borders, which makes close cooperation between the involved Member States essential. There is relatively little knowledge on the footprint in regions with a variety of geological and geopolitical settings as are present in Europe. Concerns and risks are clustered in the following four areas: subsurface, surface, atmosphere and society. As the European continent is densely populated, it is most certainly of vital importance to understand public perceptions of shale gas and for European publics to be fully engaged in the debate about its potential development.
Accordingly, Europe has a strong need for a comprehensive knowledge base on potential environmental, societal and economic consequences of shale gas exploration and exploitation. Knowledge needs to be science-based, needs to be developed by research institutes with a strong track record in shale gas studies, and needs to cover the different attitudes and approaches to shale gas exploration and exploitation in Europe. The M4ShaleGas project is seeking to provide such a scientific knowledge base, integrating the scientific outcome of 18 research institutes across Europe. It addresses the issues raised in the Horizon 2020 call LCE 16–2014 on Understanding, preventing and mitigating the potential environmental risks and impacts of shale gas exploration and exploitation.
Study objectives for this report
This report has been produced as a state-of-the-art review of our current knowledge of hydraulic fracturing as part of Work Package 1 of the European Commission M4ShaleGas project. The general objective of the M4ShaleGas program is to provide scientific recommendations for minimizing the environmental footprint of shale gas exploration and exploitation in Europe. The objective of this report is to summarize our current state-of-the-art understanding of the hydraulic fracturing operations of shale gas exploration and production companies.
Aims of this report
This report has been written from the open peer-reviewed literature and from government commissioned research reports as a statement of our current knowledge. However, considerable information has been acquired from industry conference proceedings. The rate of publication on topics related to the extraction of shale gas is high at present and every care has been taken to include as many of the key publications as possible. This report has been written to make statements on our knowledge of the following questions:
- How do hydrofractures form?
- How far do hydrofractures extend during stimulation?
- What dictates where hydrofractures propagate?
- How do hydrofractures interact with the existing fracture network?
- Can the size and distribution of hydrofractures be controlled?
No other aspect of hydro-fracturing is considered.
Knowledge of the environmental footprint from shale gas exploration and exploitation mainly come from US and Canadian experiences. Shale gas development in Europe may benefit from lessons learned in the US. However, population densities, geological settings, and regulations in some areas of European Union Member States are markedly different from those in the US and Canada.
Within the M4ShaleGas project four key gaps in our knowledge related to the potential environmental risks and impacts of shale gas exploration and exploitation will be addressed, as identified from consultations with different stakeholders (i.e. public, regulators, governments and industry). These key gaps are:
- the need for a research-based understanding of differences between Europe, US and Canada resulting from differences in their geological and geopolitical settings;
- the need for quantitative risk assessment and mitigation of risks and impacts that are specific for Europe;
- lack of knowledge on the applicability of US and Canadian best practices to Europe; and
- insufficient research-based knowledge on public perceptions of risks and impacts in Europe.
The structure of the M4ShaleGas program is based on the main areas of potential impact:
|WP1||Subsurface; Impact of subsurface activities: Hydraulic fracturing, induced seismicity and well integrity;|
|WP2||Surface; Impact of surface activities: Water, soil and well site activities;|
|WP3||Atmosphere and climate; Impact on air quality and global climate;|
|WP4||Society; Public Perceptions of the Environmental Impacts; and|
|WP5||Integration, stakeholder engagement and dissemination|
The specific objectives of each Work Package will focus on:
- Measuring the environmental impact of shale gas exploration and exploitation in Europe
- Monitoring the environmental impact of shale gas exploration and exploitation in Europe
- Mitigating the environmental impact of shale gas exploration and exploitation in Europe
- Managing the environmental impact of shale gas exploration and exploitation in Europe
Measurements, monitoring, mitigation and management relates to environmental risks and impacts as well as public perceptions on risks and impacts.
Work Package 1 of the M4ShaleGas project is targeted at the impact of subsurface activities; including hydraulic fracturing, induced seismicity and well integrity. Within the work-package there are five areas of research:
|WP1.1||the subsurface impact of hydraulic fracturing;|
|WP1.2||risks of reactivating natural faults and inducing damaging seismicity;|
|WP1.3||seismic monitoring of hydraulic fracturing and gas production;|
|WP1.4||risks of leakage along wellbores; and|
|WP1.5||drilling hazards and well integrity.|
A sixth sub-package task (WP1.6) will integrate the findings from WP1.1–1.5.
WP1.1: The subsurface impact of hydraulic fracturing
The main objectives of this sub work package are to quantify the impact and scale of hydraulic fracturing in the subsurface, and provide recommendations to minimise the subsurface impact of hydraulic fracturing. The work package will address the following main topics:
- Propagation mechanisms of hydraulic fractures, extent of stimulated reservoir volume, subsurface influence of operations.
- Analysis and mitigation measures for leakage risks along fractures, potentially allowing contamination of shallow groundwater.
- Knowledge transfer from ongoing hydraulic fracturing and gas migration experiments, and upscaling from lab- to field-scale.
- Numerical simulations of hydraulic fracturing to analyse the variation and uncertainty in fracture propagation and extent of the stimulated reservoir volume.
Understanding the potential extent of fractures arising from wellbore stimulation allows an estimate of the likely extent of subsurface influence of shale gas extraction (the fractured disturbed zone or stimulated reservoir volume) to be made. Fracture networks will be investigated by a staged approach, involving a combination of scientific review with limited laboratory experimentation and upfront predictive modelling. Insight will be gained into the potential for fractures to propagate between shale targets and neighbouring rock formations adjacent to the wellbore. Numerical simulations of hydraulic fracturing will be performed to make upfront analysis of variation and uncertainty in fracture propagation and extent of the stimulated reservoir volume. This will inform discussions and other Work Packages’ within the M4ShaleGas project that are concerned with the potential for man-made pathways to be created between shales and surface and subsurface receptors, such as shallow aquifers used for drinking water supply.
Structure of the report
This report represents a literature review of the current state-of-the-art knowledge on hydraulic fracturing during shale gas operations. It is made up of eight chapters:
- Chapter 1: Introduction: This chapter outlines the M4ShaleGas project and the aims and objectives of the current study;
- Chapter 2: Hydraulic fracturing: This chapter outlines how hydraulic fracturing is conducted by the industry. These are important considerations when performing laboratory experiments or numerical analysis of the process of hydrofracturing;
- Chapter 3: Shale variability: This chapter briefly outlines the considerable variability seen in shale units in terms of sedimentology, organic content, gas content, and strength properties;
- Chapter 4: Fracture initiation: This chapter introduces the mechanisms responsible for the formation and initiation of hydraulic fractures following perforation of the well casing;
- Chapter 5: Fracture propagation: This chapter outlines how far hydraulic fractures will extend in the sub-surface and the appearance of the hydrofractures;
- Chapter 6: Induced vs natural fractures: This chapter examines the inter-play of the pre-existing fracture network found in natural shale units and the induced hydrofractures created during hydraulic fracturing;
- Chapter 7: Engineering considerations: This chapter discusses the engineering considerations introduced in Chapter 2 and how these can dictate the extent of the fracture zone and/or the yield from a shale gas play;
- Chapter 8: Knowledge gaps: This chapter summarises all the knowledge gaps identified within the previous chapters and makes recommendations on how we may increase our understanding of the shale gas system.
- EIA (2015). Annual Energy Outlook 2015 with projections to 2040. US Energy Information Administration (www.eia.gov).