Shale is a very common sedimentary rock, yet little is known about pore connectivity and how fluids flow through it, questions central to its potential as a means of storing radioactive waste in future. We spoke to Dr Maartje Houben about her work in characterising fluid flow pathways in shale rock and the wider implications of her research beyond the academic sector.
Photo by Patrick Hendry on Unsplash
Deeper underground to probe shale permeability
A number of
countries across Europe are investigating the potential of shale rock as a means of storing radioactive waste and captured CO2 underground, while these rocks also host valuable reserves of shale gas. Shale rock itself has a low level of permeability, so it’s difficult to transport fluids through it, yet there is still more to be learned in this respect. “We know that fluids flow through these rocks, but we don’t know how, or where it goes,” explains Doctor Maartje Houben, a researcher at Utrecht University. A deeper understanding of fluid transport within shale rock is clearly essential if radioactive waste is to be stored safely underground, a topic central to Dr Houben’s research. “We aim to image fluid flow in shale rock,” she outlines. “We take rocks from the UK, then we bring them back to the laboratory for analysis.”
Imaging rocks The rocks are placed in a triaxial machine to effectively replicate underground conditions, with researchers aiming to reach a hydrostatic pressure level corresponding to a certain depth. Two main techniques are used to image the rocks, namely ion-beam milling and scanning electron microscopy. “With the milling we’re essentially polishing the rocks, as they’re very fine-grained. We use ion-beam milling – we’re shooting argon or gallium ions onto the surface, which gives us a perfectly flat surface,” explains Dr Houben. A scanning electron
Segmented microCT image of Opalinus Clay, where the arrows indicate possible fluid flow directions through the sample. (a.) Fluid flow parallel to the bedding is biased by the microcracks present parallel to the bedding and the microcracks could in this case form highways for fluid flow. (b.) Fluid flow perpendicular to the bedding does not show any highways because microcracks are not connected, hence fluid flow is expected to go through the matrix following a byway model. (c.) SEM matrix image showing that even at higher resolution pores for isolated islands may or may not be connected through smaller pore throats. (d.) High resolution SEM matrix image showing isolated pores.
transport in her research. “Horizontal transport is a lot faster in shale, so the most likely way for fluids to travel is to go along the bedding,” she says. These are important considerations in terms of both the storage of waste and hydraulic fracturing – fracking – the process by which shale gas is extracted from rocks underground; current fracking methods are relatively imprecise. “The rocks are fractured, and then they see how much gas comes out,” explains Dr Houben. “It would be very helpful to be able to predict what a fracture will look like.”
We aim to identify how fluids flow through shale rocks, and the actual pathways that they use. We want to pinpoint which layers are more permeable than others. microscope is then used to image the surface. “We use electrons to image the polished surface. The scanning electron microscope has a resolution on the nanometre scale, so a very high level of detail,” continues Dr Houben. A variety of different parameters affect the speed at which fluids are transported through a rock, including whether they interact with the rock and any pressure differences between different areas. A fluid could also flow in different directions of course, so Dr Houben is considering both vertical and horizontal
www.euresearcher.com
This depends to a large degree on the bedding direction of the shale. If a rock is fractured parallel to the bedding, then its permeability does not seem to change much; however, it’s very different if it is fractured perpendicular to the bedding. “In this case, the permeability of the rock goes up quite significantly,” says Dr Houben. These rocks might already be fractured, which could affect both a fracking company’s ability to extract gas, and their suitability as a site for the storage of radioactive waste. “Do
these fractures increase the permeability by five orders of magnitude, or by less?” continues Dr Houben. “We aim to identify how fluids flow through shale rocks, and the actual pathways that they use. We want to pinpoint which layers are more permeable than others.” Mapping fluid highways and byways in shales Dr Maartje Houben Geosciences Utrecht University Aardwetenschappen Budapestlaan 4 3584 CD Utrecht T: +31 30 253 5095 E: m.e.houben@uu.nl W: https://www.uu.nl/ staff/mehouben
Dr. Maartje Houben is a postdoctoral researcher at the Faculty of Geosciences at Utrecht University (NL). After her PhD project pioneering a new approach to quantify porosity in clay rich rocks at RWTH-Aachen University (DE), her research now focusses on fluid-flow pathways through low permeable rocks.
25