46th U.S. Rock Mechanics/Geomechanics Symposium,
2012. American Rock Mechanics Association
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Stored supercritical CO2 will acidify native brines in deep saline aquifers and promote mineral dissolution within the storage formation resulting in varying degrees of calcite saturation. Injection overpressure will force these reactive brines into existing and induced fractures in overlying caprock formations. We present examples from our experimental efforts to understand how these fluids might alter fracture geometry and leakage pathway permeability, with the ultimate goal of predicting caprock integrity. We use mineral-specific imaging analysis to correlate changes in fracture geometry with spatial maps of dissolution and precipitation. Synchrotron-based x-ray spectroscopy and diffraction imaging of thin section sub-samples of the cores from the flow-thru experiments are used to connect mineral-specific dissolution and precipitation processes with the geometric changes in fracture aperture observed with CT images. Results of μXRF, μXANES and μXRD analyses reveal that preferential calcite dissolution and the spatial distribution of relatively insoluble dolomite and silicate minerals produced the non-uniform aperture widening. These results clearly point to the need for predictive models of caprock integrity to consider coupled geochemical processes, mineralogical characterizations, and geometric alterations of flow paths.
One of the most important challenges to predicting the long-term integrity of caprock formations overlying geologic CO2 storage reservoirs is evaluating how much CO2 will flow through caprock fractures, potentially damaging other subsurface resources and eventually escaping to the atmosphere. In addition to studies that quantify the spatial distribution and permeability of existing and induced fractures and faults, mounting experimental evidence suggests that predictive models of caprock integrity need to consider how CO2-acidified brines will react with fracture walls and thereby change the permeability of these flow paths with time. This paper contributes to the growing body of experimental work aimed at identifying the conditions of fluid chemistry and caprock mineralogy that will result in fracture erosion.
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