ICGCM Papers:
Pillar Design and Global Stability
 
 
Effects of Longwall-Induced Stress and Deformation on the Stability and Mechanical Integrity of Shale Gas Wells Drilled Through a Longwall Abutment Pillar
35th International Conference on Ground Control in Mining
Effects of Longwall-Induced Stress and Deformation on the Stability and Mechanical Integrity of Shale Gas Wells Drilled Through a Longwall Abutment Pillar
by
Daniel W.H. Su, National Institute For Occupational Safety and Health, Jefferson Hill, United States
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[Conference] 35th International Conference on Ground Control in Mining
[Price] Free  [Comments] 0
[Topical Area] Pillar Design and Global Stability
[Author] Daniel W.H. Su, National Institute For Occupational Safety and Health, Jefferson Hill, United States
[Abstract] 
Key Conclusions:
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Once validated by the field instrumentation results, the 3D finite element models were employed to demonstrate that larger coal protection casing thickness had little effect on the lateral deformation, although it reduced the magnitude of casing inelastic deformation, and under deeper cover (~1,100 feet), lateral casing deformation was smaller, although inelastic casing deformation near the seam level was slightly larger. The 3D finite element models were also employed to evaluate various casing and cementing designs, which indicated that (a)when all casings were grouted to the surface, the intermediate and production casings showed deformation but little inelastic strain, (b)when all grouted to the surface except the production casing, the intermediate casing showed deformation and inelastic strain, while the production casing showed little deformation and no inelastic strain, and (c)when all grouted to the surface except the intermediate and production casings, both strings showed little deformation and no inelastic strain.
Key Findings:
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Comparisons of the pre-mining 3D finite element simulations, the subsidence surveys, the coal pillar pressure measurements, the four monitoring wells measurements, and the 60-arm caliper surveys showed that measured test well and IPI casing deformations and profiles were in reasonable agreement with those predicted by the 3D finite element models, and measured surface subsidence and pillar pressure were in excellent agreement with those predicted by the 3D models, which served to validate the 3D finite element models. Also, subtle changes in deformation were observed among the four test well designs.
Objective of the Paper:
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The primary objective of this study is to demonstrate that a properly constructed gas well in a standard longwall mining gate road pillar can maintain mechanical integrity during and after mining operations. The successful designs would allow for the safe recovery of both coal and gas reserves.
Problem Statement:
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Due to a recent shale gas boom, which may enable the United States of America to become energy independent, over the past 10 years, many gas wells have been drilled through active and future coal reserves in Pennsylvania, West Virginia, Ohio, Virginia, and Tennessee. These shale gas wells penetrated many coal seams such as Sewickley, Pittsburgh, Upper/Middle/Lower Kittanning, and Upper/Lower Freeport seams, which are either actively mined or planned to be mined in the near future. These shale gas wells, whether tapped into the Marcellus or Utica Formations, contain very high gas pressure. Longwall and high extraction induced stresses and strata deformations could induce high stresses and deformations in the shale gas well casings, which may seriously compromise the mechanical integrity of the production, intermediate, and coal protection casings. Such a compromise of mechanical integrity of the production, intermediate and coal protection casings would seriously jeopardize underground coal miner health and safety, since the compromised casings may allow high-pressure (2000~3000 psi) gas to migrate into the mine workings to trigger gas explosion which undoubtedly may result in serious injuries and fatalities. Under current MSHA and State regulations, all five casings of a gas well are required to be cemented to the surface, which allows the longwall induced stresses and deformations to be completely coupled to the casings. As a result, coal operators are forced to change gateroad designs to accommodate gas wells and to satisfy the safeguard distance, which will undoubtedly reduce efficiency and reserve recovery. Coal and gas operators are also forced to work together to permanently plug wells, which results in significant loss of revenue for the gas operators.