2005-04-12_GENERAL DOCUMENTS - M1999002

Home Browse Search

of we1120-12 as a test installation for TDR. Consistent with the failure of the past installations and the ongoing test of TDR post-mining applicability, American Soda proposes to eliminate the requirement for installation of TDR in each well at the time of construction. It is not certain that the TDR system would be functional even in the more limited post-mining application. Therefore, and to prevent a continual revision of permit conditions, American Soda will develop a plan for future TDR installations only if the test proves that post-mining TDR monitoring is viable. The original Subsidence Monitoring Plan (Appendix J, dated December 23, 1999) has been revised to reflect this approach. EPA proposes to replace the original document with the revised August 2004 Subsidence Monitoring Plan as Appendix J. b) Borehole Geophysics -Gamma Logs: Gamma logging is a common technique used to identify geologic formations and the depth at which they occur. Gamma logs essentially read the radioactive signature of the surrounding formation and record the depth at which it occurs. Because each formation has a characteristic signature, gamma logging could potentially be used to determine if subsidence occurs by comparing a baseline gamma log (e.g, conducted at the time of well construction) against later gamma logs (during operations) to evaluate whether baseline formation depths have changed. A gamma log is currently required as part of the geophysical logging completed at the time of well drilling. The existing subsidence monitoring plan also requires a gamma log to be run at each well in service after achieving individual well production levels of 85,000 tons, 150,000 tons, and.after retirement for the purpose of determining if subsurface subsidence has occurred. The subsidence monitoring plan also requires a gamma log to be run if TDR monitoring indicates that subsidence of 6 inches or more has occurred at the Mahogany Zone. Gamma logs run to date in wells have provided limited value. The well bores are subject to mineral and metals buildup (scaling) during the solution mining process. This scaling affects the ability to run and interpret gamma logs, effectively masking the radioactive gamma signature of the formations. The degree of interference from scaling is somewhat variable, but because the gamma log method does not produce consistent data, it is not considered reliable as a means to identify or evaluate subsurface subsidence. Consideration has been given to continuing the gamma logging gamma logging and install radioactive markers in the well bore (7-inch casing) pipe thread at certain locations in the well (e.g., near the A-groove, B-groove, and crown pillar). This option could only apply to new well construction and would not resolve the issue with existing wells. This approach would improve the ability to consistently identify the radioactive markers themselves but would not provide complete formation logging for subsidence monitoring. If movement is indicated by this method, it could provide a relative indication of where the movement occurred but would not provide precise depth or travel information for the event. Further, if casing breaks occur as the result of thermal expansion and contraction, a shift in marker location would not necessarily be related to formation subsidence. This method does not provide a direct measurement of or link to whether subsidence has occurred. Radioactive markers have also been known to dislodge from a well bore and this does create some concerns regarding waste management and liability. Additionally, there is a perception issue associated with the potential for one of the markers to come in contact with food-grade product. At this time, American Soda is not proposing to utilize this technique in new well construction. FINAL Addendum to SOB For Modification No. 9, UIC Area Permit No. C030858-00000 Page 3 of 29