Faults in Coal

Lead Research Organisation: University of Strathclyde
Department Name: Civil and Environmental Engineering

Abstract

The project will look to address the clear gap in understanding of the mechanical and hydrological properties of faults cutting coal-bearing sediments, with the aim of informing predications of their properties at depth. Uncertainty in the mechanical and fluid flow properties of faults cutting coal presents a major risk for unconventional oil and gas, mine safety (water inrush accidents and gas blowouts), mine waste water drainage and fault reactivation. Improving the understanding of the mechanical and hydrological properties of faults cutting coal will aid prediction, prevention and management of the above problems. This is of upmost importance in deep underground mine safety where faults remain the single greatest risk leading to subsurface fatal accidents.

The major questions that will look to be addressed, and that will enable prediction of faults cutting coal include:
- What are the main processes governing fault development in these rock types?
- What processes act to enhance or reduce fault zone permeability and/or strength?
- Which aspects of the resulting fault rocks can be quantified from site-scale data and which can only be characterised in a statistical, risk-based way?
- What data could be collected to quantify risk of the hazards associated with faults cutting coal and how can this data be used to reduce the risk of the hazard?

The use of field mapping, analysis of microstructure, analysis of core and the investigation of mine records (e.g. mine maps) will be used to improve models of sub-surface fluid flow applicable to a number of industry applications (e.g. Mine safety, fault reactivation related subsidence, coal bed methane, carbon capture and storage).

A number of key sites will be used to gain an understanding of faults cutting coal bearing sediments including the well exposed fault traces at Spireslack surface coal mine, other UK outcrops and outcrops in Utah. Field studies will be used to constrain the key parameters that control the fault architecture (e.g. thickness and mechanical properties of host rock layering). Lab based analysis of samples collected from the field will constrain mechanical and permeability properties along with characteristic microstructures to enable permeability modelling. The temporal element of fault evolution will also be investigated through field and microscopic analysis of coal bearing faults.

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509760/1 01/10/2016 30/09/2021
1857013 Studentship EP/N509760/1 01/10/2016 30/06/2020 Billy Andrews
 
Description • Large, but consistent, variability exists between geologist when fracture data is collected in the field or from field photographs. This can have a large effect on the fracture statistics used for fracture modelling.
• Systematic biases towards the study of normal faults has led to the under-appreciation of strike-slip tectonics in the Northumberland Basin, which appears to have been dominated dextral deformation in the Westphalian similar to that observed in the Midland Valley of Scotland.
• Both pre-existing weaknesses (e.g. early joints) and later mineralised faults can greatly effect the growth of faults and connectivity of the network.
• Lithology and sub-bed heterogeneities combine to cause a strong, scale dependent, mechanical stratigraphy that strongly effects fracture properties. In heterolithic units, fractures are found to be oversaturated due to the presence of strength heterogeneities.
• The high proportion of organics in the system strongly effects the spatial distribution of fracture and formation of fault rock. Thin smears of organics are often observed along the PDZs of small to medium offset faults, and in the fault core of larger faults.
Exploitation Route Many further work options have been suggested from this work, some of which are summerised below:
1) How widespread is the under-representation of strike-slip faults, and what effect will this have on basin-history models? Can the orientation of fault features over multiple scales be used to inform regional tectonics in areas where normal faults are misinterpreted?
2) Can the normalised distance from a fault-zone be used to predict the change from lens-development and increased fracturing, to the grinding processes observed close to local PDZs at all scales?
3) How do the findings related to mine geothermal (e.g. Andrews et al, in review) differ from observations from the UKGEOS research site, and how can field evidence be used to help de-risk potential mine geothermal targets?
4) At both field sites used in this study the proportion of coal (c. 5%) and rank were relatively low. How would the results at this study site compare to a study undertaken in a succession with thicker, higher rank coal seams?
5) What is the effect of subtle changes in sedimentology on fault and fracture networks in different geological settings (e.g. deep water systems) which display interbedded competent and incompetent layers?
6) How do the identified differences effect up-scaled permeability estimates and which fracture attribute is most prone to propagate subjective biases? How do the identified biases effect the interpretation of age relationships based on
the mapped features? These questions make the backbone of a potential PhD project assessing Biases in fracture modelling.
Sectors Energy,Environment,Other

 
Description Geological Society Research Grant
Amount £876 (GBP)
Organisation Geological Society of London 
Sector Learned Society
Country United Kingdom
Start 04/2018 
End 04/2018
 
Description NPIF innovation placement
Amount £3,752 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 02/2020 
End 05/2020
 
Description Sapienza Università di Roma 
Organisation Sapienza University of Rome
Country Italy 
Sector Academic/University 
PI Contribution A day long workshop (June 23rd) was held at the University of Strathclyde where collaborators from Sapienza Università di Roma and members of the Faults and Fluid Flow (University of Strathclyde) gave a combination of talks and demonstrations. This was followed by B. Andrews leading a four-day fieldtrip (24th to 27th July 2018) to my primary field area in Whitley Bay where discussions on the field area and the collection of field data was undertaken. Field data collected during this trip, along with additional field data collected my B. Andrews, is being used for the preparation of a manuscript which covers looks at the effect of sampling strategy on the generation of Discrete Fracture Network models. B. Andrews and members of the project will assist in the preparation of this manuscript, however, collaborators from Sapienza Università di Roma will lead the process. During the final day of the trip (27th July) data was collected and field discussions undertaken which, along with follow up workshops (held both at the University of Strathclyde and Sapienza Università di Roma), underpinned a manuscript which has been submitted to Solid Earth titled 'How do we see fractures? Quantifying subjective bias in fracture data collection' which is currently in review for Solid Earth.
Collaborator Contribution Collaborators contributed by hosting B. Andrews for a research visit to the Sapienza Università di Roma including leading field trips to the Central Apennines, Latera Caldera and Sardinia to observe faults and fractures at a variety of geological locations. These discussions included the discussions of observations and field techniques used by the collaborators research group, which is directly applicable to this project. Provided discussions, edits and feedback during the preparations and submission of the manuscript submitted to Solid Earth and have been leading the preparation of a manuscript which investigates the effect of sampling strategy on the generation of Discrete Fracture Network models.
Impact Preparation and submission of Andrews, B.J., Roberts, J.R., Shipton, Z.K., Bigi, S., Tartarello, M.C., Johnson, G.O. in review. How do we see fractures? Quantifying subjective bias in fracture data collection. Solid Earth.
Start Year 2018