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October 11, 2011 Page 14
<br />Pennsylvania is currently in use there and in several other states. It involves specific pillar sizes
<br />and layouts for the protection of wells, based on the depth of cover, but only extends to a cover
<br />depth of 700 ft. For cover depths of up to 700 ft, the recommended protective pillar zone is
<br />200 ft- square with entries allowed through such a zone, which translates to a 40,000 ft— square
<br />bearing area around the well being protected. A 300 ft- square solid pillar will amount to a
<br />bearing area of at least 90,000 ft- square, which is larger than double the maximum well
<br />protection recommended in the Pennsylvania study. The study guidelines, as reproduced by Peng
<br />(2006),4 are presented in the Appendix.
<br />A back - analysis of 77 cases of mining- related well failure by Peng et al. (2003)5 showed
<br />that 90% of the failures were located within the coal pillar or within 34 ft of the roof or floor
<br />line. Of those, most failures occurred in the floor, followed by the pillar and the roof. Of the
<br />remaining cases, all were in the roof within 34 ft to 100 ft of the seam. Peng et al. (2003 )5
<br />further analyzed 41 of the above - mentioned well - failure cases. They chose cases that had 75%
<br />or more pillar recovery adjacent to the protective pillars. This study compared the failure
<br />frequency of the wells to the ARMPS6 estimated SF values of the protective pillars, and found a
<br />direct correlation between pillar failure, subsequent roof/floor deformations, and well failures.
<br />The study concluded that the angle -of -draw concept had little bearing on gas well failure, as long
<br />as the pillar design is adequate. Out of the 41 high- extraction well - failure cases, only two pillars
<br />had ARMPS SFs of more than 1.5. The underlying premise of gas well protection is that pillar
<br />stability around the well needs to be maintained in order to protect the well. A copy of the
<br />aforementioned publication is presented in the Appendix.
<br />Similar findings have been reported in cases of subsidence - induced oil well failures due
<br />to withdrawal of fluids from oilfields (Bruno 1992) .7 Several failure mechanisms have been put
<br />forth to explain such well failures. Casing compression and buckling failure is attributed to
<br />compaction, subsequent transfer of large axial loads from the formation to the casing, and
<br />absence of lateral constraint. This failure mechanism appears to be consistent with the
<br />observations of Peng et al. (2003 )3 in that the well failure locations are close to the mining
<br />horizon. Casing shear and bending failures are triggered by large shear stresses and horizontal
<br />deformations, which typically occur much above the production levels and towards the flanks of
<br />the field. Such a mechanism may explain some of the outliers (well failure locations far from the
<br />mining horizon) in the study performed by the Commonwealth of Pennsylvania.2 A copy of the
<br />publication is presented in the Appendix.
<br />3Commonwealth of Pennsylvania (1957), "Gas Well Pillar Study," Department of Mines and Mineral Industries, Oil
<br />and Gas Division, 14 p.
<br />PPeng, S. S. (2006), Longwall Mining, Department of Mining Engineering, West Virginia University, Morgantown,
<br />West Viriginia, 621 p.
<br />5Peng, S. S., K. Morsey, Y. Q. Zhang, Y. Luo and K. Heasley (2003), "Technique for Assessing the Effects of
<br />Longwall Mining on Gas Wells —Two Case Studies," Transactions of Society of Mining, Metallurgy, and
<br />Exploration, Inc. Annual Meeting, vol. 314, pp. 107 -115.
<br />'Mark, C., F. E. Chase, and A. A. Campoli (1995), "Analysis of Retreat Mining Pillar Stability," Proceedings of the
<br />14`" International Conference on Ground Control in Mining, Morgantown, West Virginia, pp. 63 -71.
<br />Bruno, M. S. (1992), "Subsidence- Induced Well Failure," Journal of Drilling Engineering, vol. 7, no. 2, pp. 148-
<br />152.
<br />Agapito Associates, Inc.
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