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Possible Cause A: The design of the DS -4 well may be the cause of its failure. Unlike <br />most groundwater and subsidence monitoring wells in NS's inventory, the DS -4 is cased <br />to TD, which penetrates the Boies Bed with the bottom of casing in the upper R-5 Zone. <br />A bridge plug was set in the Interval in the Upper Boies Bed, and the casing was <br />perforated in the DS Aquifer interval for groundwater monitoring. This design means the <br />101-1-1 was mining directly around the casing. In the past, NS utilized vertical injection <br />wells to mine in Panel 1 and Panel 2. These vertical injection wells were prone to casing <br />failure, similar to the DS -4. In part, this is due to the local tectonic stress in the <br />formation. The vertical well creates a small focal point where formation stress is <br />concentrated. The removal of material around the casing introduces a volume allowing <br />stress release from the formation to occur in the immediate vicinity of the casing, <br />therefore compromising the casing. NS does not see casing failure in the other <br />monitoring wells due to the fact they are cased above the DS monitoring interval, and <br />the monitored interval is open bore hole to the formation. If any borehole failure occurs, <br />it does not damage the casing. Casing failure does not occur in the horizontal wells <br />because the stress is alleviated across a much longer distance. <br />Possible Cause B: The water level and chemistry variance changes occur shortly after <br />bringing the 10H -1V recovery well online and recommencing injection in the 101-1-I. The <br />10H mining interval is unique in the NS current mining inventory in that the injectate <br />travels one way, while the other mining intervals the injectate must exit the production <br />tubing, return through the mining interval, and be recovered through the recovery well. <br />The production tubing in the 10H-1 parted near the intermediate casing shoe, allowing <br />injectate to dissolve nahcolite immediately near the end of the 10H-1 intermediate <br />casing. The DS -4 directly monitors the area above the 10H-1 and 101-1-11 casing shoes. The <br />formation shift indicated by the GR/CCL comparison and the mechanical separation of <br />the casing are the result of this dissolution of nahcolite near the 10H-1 intermediate <br />casing shoe. <br />Conclusion and Plan Forward <br />A mechanical failure in the casing has occurred, and the GR log comparison indicates <br />possible formation shift proximal to the DS -4 well bore location and with limited vertical <br />extent, there appears to be a downward movement at a depth of 1848.5 ft. for an <br />approximate 5-6 ft. The failure is confined to the L-5 Zone, which is a zone of oil shale <br />that has undergone full dissolution of pre-existing saline minerals at this location and is <br />comprised of dissolution features, vugs, and fractures making it more susceptible to <br />collapse. The DS -4 ground water/subsurface subsidence monitor well may be <br />salvageable. NS plans to plug back the lower, damaged portion of the well to the base of <br />the B -Groove Aquifer with cement, followed by casing perforations across the B -Groove <br />Aquifer, and install sampling equipment so that it may become a B -Groove Aquifer <br />monitor well. NS will consider replacing the subsurface subsidence monitoring role of <br />the DS -4 by considering drilling a new, dedicated subsurface subsidence monitoring well. <br />SPECIALIZING IN PROFESSIONAL GEOLOGICAL, ENVIRONMENTAL, HYDROLOGICAL, GEOTECHNICAL AND PERMITTING SERVICES <br />