Abstract
Sequestration of industrial carbon dioxide (CO2) in deep geological saline aquifers is needed to mitigate global greenhouse gas emissions; monitoring the mechanical integrity of reservoir formations is essential for effective and safe operations. Clogging of fluid transport pathways in rocks from CO2-induced salt precipitation reduces injectivity and potentially compromises the reservoir storage integrity through pore fluid pressure build-up. Here, we show that early warning of salt precipitation can be achieved through geophysical remote sensing. From elastic P- and S-wave velocity and electrical resistivity monitoring during controlled laboratory CO2 injection experiments into brine-saturated quartz-sandstone of high porosity (29%) and permeability (1660 mD), and X-ray CT imaging of pore-scale salt precipitation, we were able to observe, for the first time, how CO2-induced salt precipitation leads to detectable geophysical signatures. We inferred salt-induced rock changes from (i) strain changes, (ii) a permanent ~ 1.5% decrease in wave velocities, linking the geophysical signatures to salt volume fraction through geophysical models, and (iii) increases of porosity (by ~ 6%) and permeability (~ 7%). Despite over 10% salt saturation, no clogging effects were observed, which suggests salt precipitation could extend to large sub-surface regions without loss of CO2 injectivity into high porosity and permeability saline sandstone aquifers.
Highlights
The physical properties of both fluids defined by the temperature, pressure and composition[25,26], brine salinity[7]
The sample was subjected to X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM–EDS) analysis to assess the precipitation of secondary minerals resulting from the CSMe test
Accounting for this micro-porosity, the S NaCl determined from X-ray computed tomography (XCT) scan drops to 3.5%, indicating the salt precipitation was still in an early stage after two days exposed to CO2
Summary
The physical properties of both fluids defined by the temperature, pressure and composition[25,26], brine salinity[7]. Detection of salt precipitation is crucial for timely mitigation strategies and for preserving reservoir integrity. This requirement contrasts with the lack of experimental and modelling studies aimed at the identification of C O2—induced salt precipitation from field-scale monitoring datasets, especially geophysical remote sensing. We integrate elastic and electrical resistivity measurements, X-ray micro-CT imaging, and rock physics modelling, to assess the potential of combined seismic and electromagnetic surveys for early detection of CO2—induced salt precipitation. CSMe aimed at analysing the distribution of C O2, brine, and C O2-induced precipitation of salt crystals in the rock using micro-X-ray computed tomography (XCT) scans. From the XCT, we developed a four-phase segmentation analysis to obtain the C O2, brine, quartz grain and salt volumetric fractions. For the thin section analysis, we developed a three-phase (pore space, quartz grain and salt) segmentation analysis
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