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October 11, 2011 Page 15 <br />Assuming no mining will occur within 150 ft of the projected surface location of a gas <br />well, resulting in a 300 -ft square pillar, AAI used the Mark- Bieniawaski8 pillar strength formula <br />to estimate SFs for the scenarios modeled for subsidence. This case represents the anticipated SF <br />for solitary protective pillars with just mined -out gob areas surrounding them. For a mining <br />height of 5 ft at a depth of 600 ft, the protective pillar SF is 9.7. Even at a depth of 1,200 ft, a <br />300 -ft pillar is estimated to have a SF of 2.2. Thus, a 300 -ft square protective pillar is likely to <br />remain stable in the long term. <br />Given the lack of relevance of the angle -of -draw concept to the gas well failures, the <br />critical nature of pillar design to well protection, and long -term SFs associated with 300 -ft square <br />pillars over a range of cover depths, a 150 -ft barrier between a gas well and retreat line is <br />considered adequate for gas well failure prevention, independent of the retreat mining depths <br />anticipated. <br />4.1.1 Gas Well Subsidence Control Plan <br />No mining (development or retreat) should take place within 150 ft of the seam -level <br />projection of the surface well location. This should protect against (1) accidental underground <br />breech of the well, (2) protective pillar failure, and (3) well damage. <br />4.1.2 Gas Well Monitoring Plan <br />To ensure that the gas well subsidence control plan is effective, AAI recommends the <br />following monitoring plan: <br />• Well owners should be informed as mining approaches and data generated during <br />monitoring should be shared with them. <br />• Accurate location and pad elevations for the gas wells should be obtained by survey prior <br />to mining. <br />• At least two subsidence survey monuments should be installed at the center of two <br />adjacent sides of each well pad. The subsidence monuments may be surveyed by optical <br />or GPS devices. A baseline survey should be performed before development of the panel <br />hosting the well. These surveys should be performed to National Geodetic Survey (NGS) <br />Second Order Class II or better standards. <br />• Monthly survey of the monuments should be carried out while retreat mining is active <br />(when the retreat line is within a 40- degree ( °) vertical offset angle9 from the well, until it <br />is determined that subsidence is no longer active). Annual surveys should be conducted <br />up to 3 years after the undermining has taken place. <br />• If lateral displacements of more than 1 ft or tension cracks within 25 ft of the pad are <br />observed, investigation into well integrity should be undertaken. <br />'Mark. C. (1999), "Empirical Methods for Coal Pillar Design," Proceedings, 2nd International Workshop on Coal <br />Pillar Mechanics and Design, Vail, CO, NIOSH IC 9448, pp. 145 -154. <br />9Measured as the angle from vertical between the edge of the panel at seam level and the surface structure <br />considered. <br />Agapito Associates, Inc. <br />