Wellbore-plugging materials are threatened by challenging plugging and abandonment (P&A) conditions. Hence, the integrity and resilience of these materials and their ability to provide sufficient zonal isolation in the long-term are unknown. The present work focuses on investigating the potential to use zeolites as novel additives to the commonly used Class-H cement. Using four different zeolite–cement mixtures (0%, 5%, 15% and 30%, by weight of cement) where samples were cast as cylinders and cured at 90 °C and 95% relative humidity, the unconfined compressive strength (UCS) testing showed a 41% increase with the 5% ferrierite addition to the Class-H cement in comparison to neat Class-H cement. For triaxial compression tests at 90 °C, the highest strength achieved by the 5% ferrierite-added formulations was 68.8 MPa in comparison to 62.9 MPa for the neat Class-H cement. The 5% ferrierite formulation also showed the lowest permeability, 13.54 μD, which is in comparison to 49.53 μD for the neat Class-H cement. The overall results show that the 5% ferrierite addition is the most effective at improving the mechanical and petrophysical properties based on a water/cement ratio of 0.38 when tested after 28 days of curing in 95% relative humidity and 90 °C. Our results not only demonstrate that zeolite is a promising cement additive that could improve the long-term strength and petrophysical properties of cement formulations, but also provide a proposed optimal formulation that could be next utilized in a field trial.
Deformation Rate Analysis (DRA) was carried out on rock samples from a characterization well at the former FutureGen2 site in Morgan County, Illinois. Although the experience with DRA reported in the literature is typically focused on shallower formations encountered in mining applications, the method was explored here using triaxial compression experiments for core retrieved from 1150-1350 meters depth. Consistent with past manifestations of the DRA method, core samples were subjected to two identical cycles of loading that started below the likely range of in-situ stress magnitudes and peaked above them, after which the core was unloaded in an identical manner to form a “saw-tooth” load/unload path. The inflections in the difference between the stress-strain relationship comprise the focus of the analysis. The inflection stresses closely follow vertical stress inferred from integrating the density logs for the vertically-oriented core plugs. The horizontal minimum stress inferred from horizontal core plugs matches well with bounds obtained by hydraulic fracture stress testing and corresponding to an average minimum tectonic strain of 170 microstrain. The maximum stress is found to track between the minimum horizontal and vertical stress in the sedimentary formations, implying a normal faulting regime. With a similar level of implied tectonic strain (average of about 350 microstrain), the maximum horizontal stress in the stiffer Precambrian Basement exceeds the vertical stress and implies strike-slip stress regime. Hence, the DRA method is found to be useful for bounding all three principal stress magnitudes and for detecting a shift in tectonic regime between units that was suspected but unable to be verified prior to these experiments.
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