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An investigation into the controls of fracture tortuosity in rock sequences and its impact on fluid flow in the upper crust

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Abstract
Fractures are ubiquitous in geological sequences, and play an important role in the movement of fluids in the Earth’s crust, particularly in fields such as hydrogeology, petroleum geology and volcanology. When predicting or analysing fluid flow, fractures are often simplified as a set of smooth parallel plates. In reality, they exhibit tortuosity on a number of scales: Fine-scale tortuosity, or roughness, is the product of the small scale (µm–mm) irregularities in the fracture surface, whereas large-scale (>mm) tortuosity occurs as a result of anisotropy and heterogeneity within the host formation that leads to the formation of irregularities in the fracture surfaces. It is important to consider such tortuosity when analysing processes that rely on the movement (or hindrance) of fluids flowing through fractures in the subsurface. Such processes include fluid injection into granitic plutons for the extraction of heat in Engineered Geothermal Systems, or the injection of CO2 into reservoirs overlain by fine-grained mudrocks acting as low-permeability seals in Carbon Capture and Storage projects. Although it is generally assumed that tortuosity is controlled by factors such as grain size, mineralogy and fracture mode, a systematic study of how these factors quantitatively affect tortuosity is currently lacking. Furthermore, in anisotropic rocks the fracture orientation with respect to any inherent anisotropy is also likely to affect tortuosity. In order to address this gap, we have induced fractures in a selection of different rock types (mudrocks, sandstones and carbonates) using the Brazil disk method, and imaged the fracture surfaces using both digital optical microscopy and X-ray Computed Tomography. Using these methods we are able to characterise both the fine-scale (roughness) and large-scale tortuosity. In order to understand the effect of fracture orientation on tortuosity we have also analysed fractures induced at different angles to bedding in samples of a highly anisotropic mudrock taken from South Wales, UK. Results indicate that fine-scale tortuosity is highly dependent on the fracture orientation with regards to the bedding plane, with fractures normal to bedding being rougher than those induced parallel to bedding. Finally, in order to measure the effect of tortuosity on fluid flow, we have carried out a series of core flooding experiments on a subset of fractured samples showing that fracture transmissivity decreases with increasing tortuosity.

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MLA
Forbes Inskip, Nathaniel, et al. “An Investigation into the Controls of Fracture Tortuosity in Rock Sequences and Its Impact on Fluid Flow in the Upper Crust.” 12th International Conference on Porous Media and Annual Meeting, Abstracts, International Society for Porous Media (Interpore), 2020, pp. 56–57.
APA
Forbes Inskip, N., Phillips, T., Bisdom, K., Borisochev, G., Busch, A., & den Hartog, S. (2020). An investigation into the controls of fracture tortuosity in rock sequences and its impact on fluid flow in the upper crust. In 12th International Conference on Porous Media and Annual Meeting, Abstracts (pp. 56–57). Online: International Society for Porous Media (Interpore).
Chicago author-date
Forbes Inskip, Nathaniel, Tomos Phillips, Kevin Bisdom, Georgy Borisochev, Andreas Busch, and Sabine den Hartog. 2020. “An Investigation into the Controls of Fracture Tortuosity in Rock Sequences and Its Impact on Fluid Flow in the Upper Crust.” In 12th International Conference on Porous Media and Annual Meeting, Abstracts, 56–57. International Society for Porous Media (Interpore).
Chicago author-date (all authors)
Forbes Inskip, Nathaniel, Tomos Phillips, Kevin Bisdom, Georgy Borisochev, Andreas Busch, and Sabine den Hartog. 2020. “An Investigation into the Controls of Fracture Tortuosity in Rock Sequences and Its Impact on Fluid Flow in the Upper Crust.” In 12th International Conference on Porous Media and Annual Meeting, Abstracts, 56–57. International Society for Porous Media (Interpore).
Vancouver
1.
Forbes Inskip N, Phillips T, Bisdom K, Borisochev G, Busch A, den Hartog S. An investigation into the controls of fracture tortuosity in rock sequences and its impact on fluid flow in the upper crust. In: 12th International Conference on Porous Media and Annual Meeting, Abstracts. International Society for Porous Media (Interpore); 2020. p. 56–7.
IEEE
[1]
N. Forbes Inskip, T. Phillips, K. Bisdom, G. Borisochev, A. Busch, and S. den Hartog, “An investigation into the controls of fracture tortuosity in rock sequences and its impact on fluid flow in the upper crust,” in 12th International Conference on Porous Media and Annual Meeting, Abstracts, Online, 2020, pp. 56–57.
@inproceedings{8676249,
  abstract     = {{Fractures are ubiquitous in geological sequences, and play an important role in the movement of fluids in the Earth’s crust, particularly in fields such as hydrogeology, petroleum geology and volcanology. When predicting or analysing fluid flow, fractures are often simplified as a set of smooth parallel plates. In reality, they exhibit tortuosity on a number of scales: Fine-scale tortuosity, or roughness, is the product of the small scale (µm–mm) irregularities in the fracture surface, whereas large-scale (>mm) tortuosity occurs as a result of anisotropy and heterogeneity within the host formation that leads to the formation of irregularities in the fracture surfaces. It is important to consider such tortuosity when analysing processes that rely on the movement (or hindrance) of fluids flowing through fractures in the subsurface. Such processes include fluid injection into granitic plutons for the extraction of heat in Engineered Geothermal Systems, or the injection of CO2 into reservoirs overlain by fine-grained mudrocks acting as low-permeability seals in Carbon Capture and Storage projects. Although it is generally assumed that tortuosity is controlled by factors such as grain size, mineralogy and fracture mode, a systematic study of how these factors quantitatively affect tortuosity is currently lacking. Furthermore, in anisotropic rocks the fracture orientation with respect to any inherent anisotropy is also likely to affect tortuosity. In order to address this gap, we have induced fractures in a selection of different rock types (mudrocks, sandstones and carbonates) using the Brazil disk method, and imaged the fracture surfaces using both digital optical microscopy and X-ray Computed Tomography. Using these methods we are able to characterise both the fine-scale (roughness) and large-scale tortuosity. In order to understand the effect of fracture orientation on tortuosity we have also analysed fractures induced at different angles to bedding in samples of a highly anisotropic mudrock taken from South Wales, UK. Results indicate that fine-scale tortuosity is highly dependent on the fracture orientation with regards to the bedding plane, with fractures normal to bedding being rougher than those induced parallel to bedding. Finally, in order to measure the effect of tortuosity on fluid flow, we have carried out a series of core flooding experiments on a subset of fractured samples showing that fracture transmissivity decreases with increasing tortuosity.}},
  articleno    = {{354}},
  author       = {{Forbes Inskip, Nathaniel and Phillips, Tomos and Bisdom, Kevin and Borisochev, Georgy and Busch, Andreas and den Hartog, Sabine}},
  booktitle    = {{12th International Conference on Porous Media and Annual Meeting, Abstracts}},
  language     = {{eng}},
  location     = {{Online}},
  pages        = {{354:56--354:57}},
  publisher    = {{International Society for Porous Media (Interpore)}},
  title        = {{An investigation into the controls of fracture tortuosity in rock sequences and its impact on fluid flow in the upper crust}},
  year         = {{2020}},
}