Coupled Transport, Reactivity, and Mechanics in Fractured Shale Caprocks

Shales are low‐permeability caprocks that confine fluid, such as CO2, nuclear waste, and hydrogen, in storage formations. Stress‐induced fractures in shale caprocks provide pathways for fluid to leak and potentially contaminate fresh water aquifers. Fractured shales are also increasingly considered...

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Bibliographic Details
Published in:Water resources research 2024-01, Vol.60 (1), p.n/a
Main Authors: Murugesu, M. P., Vega, B., Ross, C. M., Kurotori, T., Druhan, J. L., Kovscek, A. R.
Format: Article
Language:English
Subjects:
Aquifers
Brines
caprocks
Carbon dioxide
Carbon dioxide fixation
Carbon sequestration
Computed tomography
Dissolution
Dissolving
Electron microscopy
Energy recovery
fines migration
Fluid flow
Fluids
Fracture surfaces
fractured shales
Fresh water
Freshwater
Geomechanics
Inductively coupled plasma mass spectrometry
Inland water environment
Kinetics
Mass spectrometry
Mass spectroscopy
Mechanics
Medical imaging
Microcracks
multimodal imaging
multiscale visualization
Permeability
Porosity
Radioactive wastes
reactive transport
Risk assessment
Rocks
Scanning electron microscopy
Sedimentary rocks
Shale
Shales
Storage
Tomography
Tracking
Water pollution
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Summary:Shales are low‐permeability caprocks that confine fluid, such as CO2, nuclear waste, and hydrogen, in storage formations. Stress‐induced fractures in shale caprocks provide pathways for fluid to leak and potentially contaminate fresh water aquifers. Fractured shales are also increasingly considered as resources for CO2 sequestration, enhanced geothermal, and unconventional energy recovery. Injecting reactive fluids into shales introduces chemical disequilibrium, causing an onset of a series of dissolution, precipitation, and fines mobilization mechanisms. The reactions have rapid kinetics and significant impact on porosity and permeability; consequently, flow and storage properties of caprocks. While previous research has explored the separate effects of these reactions, this study aims to uncover their simultaneous occurrence and collective influence. This study unveils these highly coupled transport and reactivity mechanisms by tracking and visualizing the reaction‐induced alterations in the matrix, microcracks, and fractures of shales over time. We conducted brine injection experiments sequentially at pH 4 and 2 in a naturally fractured Wolfcamp shale sample while simultaneously imaging the dynamic processes using X‐ray computed tomography (CT). CT images are validated by finer resolution images obtained using micro‐CT and scanning electron microscopy. We also tracked the sample permeability and fluid chemistry using brine permeability and inductively coupled plasma mass spectrometry, respectively. Findings show that fluid primarily flowed through fractures, dissolving reactive minerals and mobilizing fines on fracture surfaces. Dissolution of fracture asperities under confining stress resulted in the closing of fractures. Clogging in narrow fracture pathways, caused by fines accumulation, diverted fluid flow into matrix pores. Plain Language Summary Geological formations have significant potential to store CO2 and other fluids. Overlying low‐permeability shale caprocks prevent the upward movement of such fluids. Naturally occurring or induced fractures in caprocks may serve as potential leakage routes. Our research employs a range of experimental approaches to investigate the behavior of reactive fluids within shale rock fractures. Our findings indicate that, in the rock‐fluid system studied, fractures undergo a self‐sealing process due to a combination of geochemical and geomechanical phenomena. This discovery enhances our ability to assess risks and
ISSN:0043-1397
1944-7973
DOI:10.1029/2023WR035482