9
to be validated by actual field data. Similar deep-cover site
data would need to be collected to enhance the data popu-
lation to make the results statistically valid.
CONCLUSIONS
Following the first longwall excavation at a deep-cover
gas well site, where the depth to the Pittsburgh seam is
1,307 feet, the pre-mining modeling prediction of long-
wall-induced casing deformations in the fully cemented
production casing were in excellent agreement with post-
mining Caliper survey results. After second panel mining,
the post-mining Caliper survey revealed a large deforma-
tion near the top of the Pittsburgh seam, which prompted
the acquisition of an on-site gamma log. This on-site
gamma log revealed a thick claystone layer at 23 feet above
the Pittsburgh seam horizon, which was not present in a
core hole 1,500 feet from the study site. Revised FLAC3D
models based on this new geologic detail reveal very high
longwall-induced casing stress near a thick claystone layer
at 23 feet above the Pittsburgh seam. Additional modeling
results indicate that, under a cover depth of 1,000 feet or
less, longwall-induced deformations in fully cemented pro-
duction casing are expected to stay in the elastic range. On
the other hand, strain hardening in fully cemented produc-
tion casing is likely for cover depths up to 1,200 feet and
plastic casing deformation is likely for cover depths greater
than 1,200 feet. Leaving the production casing uncemented
or using softer cementing material is identified as the best
practice. This case study demonstrates that uncemented
production casing or softer cementing material serves to
uncouple longwall-induced deformations and stresses from
the production casing. With proper gas well pillar sizing,
leaving the production casing uncemented or using softer
cementing material would allow for post-mining re-entry
and resumption of gas production, which allows the co-
existence of both coal and gas operations while safeguard-
ing miner safety and health.
DISCLAIMER
The findings and conclusions in this report are those of
the authors and do not necessarily represent the official
position of the National Institute for Occupational Safety
and Health (NIOSH), Centers for Disease Control and
Prevention (CDC). Mention of any company or product
does not constitute endorsement by NIOSH.
REFERENCES
Su, W.H., Zhang, P., Van Dyke, M., and Minoski, T.
2018a. Effects of cover depth on longwall-induced
subsurface deformations and shale gas well casing sta-
bility. In Proceedings of the 52nd ARMA Conference,
Seattle, WA, 20 pp.
Su, W.H., Zhang, P., Van Dyke, M., and Minoski, T. 2018b.
Effects of longwall-induced subsurface deformations
on shale gas well casing stability under deep covers.
In Proceedings of the 37th International Conference
on Ground Control in Mining. Morgantown, West
Virginia, 63–70.
Figure 12. Effect of softer cement between intermediate and production casings on longwall-induced casing stress
at the deep-cover study site
to be validated by actual field data. Similar deep-cover site
data would need to be collected to enhance the data popu-
lation to make the results statistically valid.
CONCLUSIONS
Following the first longwall excavation at a deep-cover
gas well site, where the depth to the Pittsburgh seam is
1,307 feet, the pre-mining modeling prediction of long-
wall-induced casing deformations in the fully cemented
production casing were in excellent agreement with post-
mining Caliper survey results. After second panel mining,
the post-mining Caliper survey revealed a large deforma-
tion near the top of the Pittsburgh seam, which prompted
the acquisition of an on-site gamma log. This on-site
gamma log revealed a thick claystone layer at 23 feet above
the Pittsburgh seam horizon, which was not present in a
core hole 1,500 feet from the study site. Revised FLAC3D
models based on this new geologic detail reveal very high
longwall-induced casing stress near a thick claystone layer
at 23 feet above the Pittsburgh seam. Additional modeling
results indicate that, under a cover depth of 1,000 feet or
less, longwall-induced deformations in fully cemented pro-
duction casing are expected to stay in the elastic range. On
the other hand, strain hardening in fully cemented produc-
tion casing is likely for cover depths up to 1,200 feet and
plastic casing deformation is likely for cover depths greater
than 1,200 feet. Leaving the production casing uncemented
or using softer cementing material is identified as the best
practice. This case study demonstrates that uncemented
production casing or softer cementing material serves to
uncouple longwall-induced deformations and stresses from
the production casing. With proper gas well pillar sizing,
leaving the production casing uncemented or using softer
cementing material would allow for post-mining re-entry
and resumption of gas production, which allows the co-
existence of both coal and gas operations while safeguard-
ing miner safety and health.
DISCLAIMER
The findings and conclusions in this report are those of
the authors and do not necessarily represent the official
position of the National Institute for Occupational Safety
and Health (NIOSH), Centers for Disease Control and
Prevention (CDC). Mention of any company or product
does not constitute endorsement by NIOSH.
REFERENCES
Su, W.H., Zhang, P., Van Dyke, M., and Minoski, T.
2018a. Effects of cover depth on longwall-induced
subsurface deformations and shale gas well casing sta-
bility. In Proceedings of the 52nd ARMA Conference,
Seattle, WA, 20 pp.
Su, W.H., Zhang, P., Van Dyke, M., and Minoski, T. 2018b.
Effects of longwall-induced subsurface deformations
on shale gas well casing stability under deep covers.
In Proceedings of the 37th International Conference
on Ground Control in Mining. Morgantown, West
Virginia, 63–70.
Figure 12. Effect of softer cement between intermediate and production casings on longwall-induced casing stress
at the deep-cover study site