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Modelling fluid flow and sediment deposition within glacial hydrofracture systems: implications for waste water injection from hydrocarbon platforms

Prof Jeff Peakall (SEE), Prof Dave Hodgson (SEE), Prof Quentin Fisher (SEE), Dr Emrys Phillips (BGS)

Project partner(s): British Geological Survey (CASE)

Contact email: j.peakall@leeds.ac.uk

Overview

This project will be of interest to someone looking for a multidisciplinary project including fieldwork In Iceland, and experimental work, in the broad areas of sedimentology, structural geology, and potentially geomechanical modelling.

Scientific Background

Hydrofracture systems (also referred to as water-escape features or clastic dykes) provide clear evidence for the passage of pressurised meltwater beneath both former and contemporary glaciers and ice sheets. They are typically found in subglacial to ice marginal settings and range from relatively minor features just a few millimetres across, through to much larger structures (up to several metres wide) which can be traced laterally for several tens of metres (see van der Meer et al., 1999; van der Meer et al., 2008; Phillips and Merritt, 2008; Phillips et al., 2013; Phillips and Hughes, 2014). The marked fluctuations in hydrostatic pressures that occur during hydrofracturing leads to brittle deformation of the sediment and/or bedrock beneath the ice, accompanied by penecontemporaneous liquefaction and introduction of the sediment-fill. Due to the pressurised nature of the meltwater, this sediment infill can be introduced from structurally above (downward injection) or below (upward injection) the developing hydrofracture system. The introduction of pressurised meltwater beneath glaciers and ice sheets has a profound effect on deformation beneath the ice not only leading to increased forward motion of these ice masses, but also facilitating movement along prominent décollement surfaces within the sediment pile (e.g. Phillips et al., 2002; Kjær et al., 2006; Benediktsson et al., 2008) and/or the detachment and transport of sediment and/or bedrock rafts (e.g. Phillips and Merritt, 2008; Burke et al., 2009; Vaughan-Hirsh et al., 2013).

Despite their importance, many of the processes underlying the formation of hydrofracture systems remain poorly understood. Previous research (e.g. van der Meer et al., 1999; van der Meer et al., 2008; Phillips et al., 2013; Phillips and Hughes, 2014) has revealed that hydrofractures from glacial environments range from simple features in which initial fracture propagation is immediately followed by the injection of the fluidised sediment fill (cut and fill), through to highly complex multiphase systems which are thought to be active over a prolonged period, accommodating several phases of fluid flow and sedimentation. Naturally occurring hydrofracture systems are also known to exploit pre-existing structures (e.g. bedding, faults) within the host rock/sediment, with these inherent weaknesses facilitating the fracking process and thereby fluid migration. However, there is still considerable debate regarding the mode of propagation of these fracture systems and the nature of fluid flow (turbulent vs laminar) and sediment transport within these conduits.

Waste waters from hydrocarbon production on the UK Continental Shelf (UKCS) are frequently reinjected into shallow, glacially-derived sediments. Similarly, there are large quantities of toxic waste products that need to be disposed of as part of the decommissioning process on the UKCS, and again shallow injection of these is a likely scenario. However, the lack of knowledge on hydrofracturing processes in glacial sediments limits the ability to: i) predict and model the pathways these injected fluids may follow, ii) assess the nature and significance of any hydrofractures that may develop, and iii) examine the potential for seal failure and leakage. The proposed PhD research project aims to address this fundamental lack of understanding of glacial hydrofracturing processes and spatial relationships, and to apply this knowledge base to examine the implications for waste water injection on the UKCS.

Aims and Objectives

The main aim of the proposed PhD project is to investigate the processes occurring during the formation and subsequent evolution of hydrofracture systems. The objectives include:

  • To understand the processes occurring during fluid flow, and model sedimentation within hydrofracture systems. This will be achieved using an integrated work programme comprising detailed fieldwork, macro- and microscale analysis, and physical modelling experiments, that aim to investigate sediment transport and dispersal within active hydrofracture systems, as well as the nature of fluid flow (turbulent vs laminar) during the hydrofracturing process; and
  • To investigate the potential of hydrofracture systems to act as pathways for the enhanced migration of fluids (sediment and water) within glacial environments. Granular-scale 2D and 3D volumetric representations of the sedimentary fills will be investigated. There is also potential depending on the applicant’s background, for applying numerical models to simulate intergranular fluid flow through sediment-filled hydrofractures systems, thereby assessing their ability to act as fluid pathways.
  • To examine the implications for shallow water injection of hydrocarbon production-derived waste products into glacial sediments.

These objectives form the foundations of a fully integrated, multidisciplinary research project which will investigate fluid and sediment flow through hydrofracture systems in glacial environments from a macro- to microscale and apply this knowledge to shallow water injection into glacial sediments.

Methods

The project will focus on sediment-filled hydrofracture systems exposed within former glaciated regions (Highlands of Scotland, Cumbria) and contemporary glacial environments (Iceland). Fieldwork will be conducted during the first year of the PhD project and will focus upon the macroscale description and analysis of the hydrofracture systems, as well as collection of undisturbed samples for subsequent laboratory analysis. The field data will be used to provide the spatial context of the more detailed microscale analysis as well as allowing the evolution of the hydrofracture system to be linked to the large-scale dynamics of the glacier. Small-scale sedimentary processes during fluid flow and the transport of sediment within hydrofracture systems will be investigated using macro- (sedimentological and structural analysis) and microscale 2D (thin section analysis, SEM) and 3D (micro-CT scanning) analysis. This will enable the construction of detailed 2D and 3D volumetric representations of the structural and sedimentary architecture of natural occurring hydrofracture systems which can be used to design flume tank and pipe-flow experiments to physically model the processes occurring during the transport and deposition of sediments within hydrofractures thereby aiding our understanding of how material is introduced and dispersed within these systems. The results will advance our understanding of the evolution of hydrofracture systems and their sediment-fills, providing important information regarding the changes in the style of sedimentation, sediment supply and fluid flow. Furthermore there is potential, depending on the candidate’s background, to use these 3D volumetric representations of morphology of hydrofractures in numerical modelling experiments aimed at increasing our understanding of the nature of fluid flow (turbulent vs laminar) during the “fracking” process and the changes in flow regime caused by sedimentation.

PhD Schedule, Outputs and Training

This PhD will commence 1st October 2018 and run for 3.5 years. During this period the student will be eligible for all the postgraduate training typically provided to students attending the University as part of the SPHERES Doctoral Training Programme. Through both the BGS and Leeds, the student will receive training in relevant software packages, field based description and analysis of glacigenic sediments, 2D and 3D microscale analysis of glacigenic sediments, technical/scientific writing, etc. The student will be based in the Department of Earth and Environment at the University of Leeds with regular visits to the BGS office in Edinburgh to work with their academic supervisors, as well as interact with other PhD students within. This will provide valuable practical experience of working in both a vibrant and active university department and in the dynamic and world-leading glacial group at BGS. Major deliverables during the first year of the project include a 6 month report, outlining literature, research questions and research plan. Outputs during the subsequent years will include progress reports and the preparation of manuscripts for submission to international scientific journals. The final output of the PhD programme will be a thesis. The student will also be expected to present the results of their research at relevant UK and International conferences.

References

Benediktsson, I.O., Möller, P., Ingólfsson, O., van der Meer, J.J.M., Kjær, K.H., Krüger, J. 2008. Instantaneous end moraine and sediment wedge formation during the 1890 glacier surge of Brúarjökull, Iceland. Quaternary Science Reviews 27, 209–234.

Burke. H., Phillips, E.R., Lee, J.R., Wilkinson, I.P. 2009. Imbricate thrust stack model for the formation of glaciotectonic rafts: an example from the Middle Pleistocene of north Norfolk, UK. Boreas 38, 620–637.

Phillips, E.R., Evans, D.J.A., Auton, C.A. 2002. Polyphase deformation at an oscillating ice margin following the Loch Lomond readvance, central Scotland, UK. Sedimentary Geology 149, 157-182.

Phillips, E., Merritt, J. 2008. Evidence for multiphase water-escape during rafting of shelly marine sediments at Clava, Inverness-shire, NE Scotland. Quaternary Science Reviews 27, 988–1011.

Phillips, E., Everest, J., Reeves, H. 2013. Micromorphological evidence for subglacial multiphase sedimentation and deformation during overpressurized fluid flow associated with hydrofracturing. Boreas 42, 395-427.

Phillips, E., Hughes, L. 2014. Hydrofracturing in response to the development of an overpressurised subglacial meltwater system during drumlin formation: an example from Anglesey, NW Wales. Proceedings of the Geologists’ Association 125, 296–311.

van der Meer, J.J.M., Kjær, K., Krüger, J. 1999. Subglacial water-escape structures, Slettjökull, Iceland. Journal of Quaternary Science 14, 191–205.

van der Meer, J.J.M., Kjær, K.H., Krüger, J., Rabassa, J., Kilfeather, A.A. 2008. Under pressure: clastic dykes in glacial settings. Quaternary Science Reviews 28, 708-720.

Vaughan-Hirsch, D. P., Phillips, E., Lee, J. R., Hart, J. K. 2013. Micromorphological analysis of poly-phase deformation associated with the transport and emplacement of glaciotectonic rafts at West Runton, north Norfolk, UK. Boreas 42, 376–394.

Related undergraduate subjects:

  • Earth science
  • Earth system science
  • Geography
  • Geological science
  • Geology
  • Geophysical science
  • Geophysics
  • Geoscience