Nanoindentation of Horn River Basin Shales: The Micromechanical Contrast Between Overburden and Reservoir Formations. Issue 3 (17th March 2023)

• Record Type: Journal Article
• Authors: Charlton, T. S.; Rouainia, M.; Aplin, A. C.; Fisher, Q. J.; Bowen, L.
• Is Part Of: Journal of geophysical research. Volume 128:Issue 3(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-03-17
• Subjects: nanoindentation -- micromechanics -- shale -- elasticity -- creep -- anisotropy; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2022JB025957
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: We present a micromechanical characterization of shales from the Horn River Basin, NW Canada. The shales have contrasting mineralogy and microstructures and play different geomechanical roles in the field: the sample set covers an unconventional gas reservoir and the overburden unit that serves as the upper fracture barrier. Composition and texture were characterized using X‐ray diffraction, mercury injection porosimetry, and scanning electron microscopy (SEM). Grid nanoindentation testing was used to obtain the mechanical response of the dominant phases in the shale microstructure. Samples were indented parallel and perpendicular to the bedding plane to assess mechanical anisotropy. Chemical analysis of the grids with SEM‐EDS (energy dispersive X‐ray spectroscopy) was undertaken and the coupled chemo‐mechanical data was used in a statistical clustering procedure (Gaussian mixture model) to reveal the mechanical properties of each phase. The results show that the overburden consists of a soft clay matrix with highly anisotropic elastic stiffness, and stiffer but effectively isotropic inclusions of quartz and feldspar; the significant anisotropy of the overburden has been previously observed on a much larger scale using microseismic data. Creep displacement is concentrated in the clay matrix, which is the key phase for fracture barrier and seal applications. The reservoir units are harder and have more isotropic mechanical responses, primarily due to their lower clay content. Despite varied compositions and microstructures, the major phases of these shales (clay/organic matrix, quartz/feldspar, dolomite, and calcite) have unique mechanical signatures, which will aid identification in future micromechanical characterizations and facilitate their use in upscaling schemes. Plain Language Summary: The Horn River Basin is an area of northwest Canada where shale rocks have been used to produce gas. Reservoir shales are targeted for hydraulic fracturing while the overlying shale forms a barrier that stops fractures from propagating upwards. It is important for us to understand the different ways in which the reservoir and overburden shales respond to pressure changes due to fluid injection. This is because shales will be vital in many future subsurface technologies, for example, as top seals in geological CO2 storage. The mechanical response of shale is difficult to measure on core samples as they often break apart. Here, we used a technique called nanoindentation to measure the mechanical properties of the micron‐sized grains that form the shale. We conducted nanoindentation grids on both overburden and reservoir samples. We then carried out chemical analysis of the grids and used an automated clustering procedure to identify the mechanical properties of different minerals in each shale. We found that the overburden is dominated by a soft clay matrix that shows a tendency to creep over time, a process that would help close induced fractures. In contrast, the low‐clay reservoir shales were harder with much less creep. Key Points: The micromechanical properties of overburden and reservoir shales from the Horn River Basin are studied using grid nanoindentation The overburden is dominated by a highly anisotropic clay matrix of low stiffness which shows large creep displacement Reservoir shales have low clay content and are stiffer, more isotropic, and show much less creep than the overburden samples