NORTHWESTERN CORPORATION

Incident Narrative

THIS IS A SUPPLEMENTAL FINAL REPORT FOR INCIDENT REPORT 20200107-34393 SUBMITTED ON 10/16/20. PLEASE REFER TO THAT INCIDENT REPORT FOR THE INITIAL NARRATIVE. THE CAUSE OF THE INCIDENT HAS BEEN CHANGED IN PART G FROM EXTERNAL CORROSION TO INTERNAL CORROSION. THIS CHANGE HAS OCCURRED BECAUSE OF DATA THAT WAS GATHERED FROM RUNNING A HIGH RESOLUTION VERTALOG, WHICH CAN DIFFERENTIATE BETWEEN INTERNAL AND EXTERNAL CORROSION. THE INTERNAL CORROSION WAS ALSO VERIFIED WITH A CALIPER LOG THAT WAS RUN DOWN THE CASING WITH THE VERTALOG TOOL. AS NOTED IN THE INITIAL REPORT, THE WELL WAS KILLED AND THEN A COMPOSITE PLUG WAS PLACED AT 4479' JUST ABOVE THE PERFORATIONS. A PRODUCTION CASING WAS INSTALLED FROM SURFACE TO 4431'. THE PRODUCTION CASING WAS CEMENTED IN PLACE. CEMENT WAS RUN DOWN THE INSIDE OF THE INNER CASING AND THERE WERE RETURNS TO SURFACE THROUGH THE ANNULAR SPACE BETWEEN THE NEW PRODUCTION CASING AND THE OUTER CASING. THIS SEALED THE LEAK IN THE OUTER CASING AND PROVIDES A MITIGATION AGAINST ADDITIONAL LEAKS. A CEMENT BOND LOG WAS RUN FOR THE CEMENTING WORK OF THE INNER CASING AND NO DEFICIENCIES WERE NOTED IN THE LOG. A PRESSURE TEST WAS COMPLETED ON THE NEW INSTALLATION AND NO ISSUES WERE NOTED DURING THE TEST. FOLLOWING THE INTEGRITY TESTING, THE COMPOSITE PLUG WAS DRILLED OUT. THE WELL WAS THEN RE-COMMISSIONED ON 12/01/20. THE WELL HAS BEEN OPERATING FOR TWO WEEKS WITH NO IDENTIFIED ISSUES. AS AN ADDITIONAL MITIGATION, LEAK SURVEYS HAVE BEEN COMPLETED AROUND THE WELL AND ALONG THE PREVIOUS INCIDENT PLUME BOUNDARY. THE PLUME FROM THE INCIDENT HAS DISSIPATED SIGNIFICANTLY. READINGS WITHIN THE PLUME AREA ARE UNDER A PERCENT GAS IN BAR HOLES. NWE IS CONTINUING TO MONITOR THE PLUME AND WILL DO SO UNTIL IT HAS DISSIPATED. THERE ARE NO SAFETY OR PUBLIC SAFETY CONCERNS. THIS WILL BE THE FINAL REPORT FOR THIS INCIDENT.