6
casing deformation at the 642-foot horizon and the plastic
casing deformation at the 1,284-foot horizon, as revealed
by a downhole camera.
CAUSES OF PLASTIC CASING
DEFORMATION AT THE 1,284-FOOT
HORIZON
A few FLAC3D models were constructed to investigate the
causes of plastic casing deformation at the 1,284-foot hori-
zon. Figure 9 shows the longwall-induced casing stress at
the deep-cover study site, where the depth to the Pittsburgh
seam is 1,307 feet. Figure 9 only shows the high stress por-
tion of the casing, which is located near the thick clay-
stone horizon 23 feet above the Pittsburgh seam. Clearly,
this longwall-induced casing stress exceeds the rated yield
strength of the P-110 casing and substantial plastic cas-
ing deformation occurs, which helps to explain the plastic
casing deformation observed by the downhole camera at
the claystone horizon 23 feet above the Pittsburgh seam.
To investigate under what cover depth this type of plastic
casing deformation may occur, three additional FLAC3D
models were constructed and analyzed. Figure 10 shows
that, under a cover depth of 1,000 feet or less, longwall-
induced deformations in fully cemented production cas-
ing are expected to stay in the elastic range. On the other
hand, Figure 10 also indicates that strain hardening in fully
cemented production 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.
EFFECT OF CASING CEMENTING
ALTERNATIVES ON LONGWALL-
INDUCED PLASTIC CASING
DEFORMATION
Since Figure 10 indicates that fully cemented production
casing under more than 1,200 feet of cover could be subject
to high longwall-induced stress and plastic casing deforma-
tion, it is then imperative to find engineering solutions to
prevent such plastic casing deformation under deep cover.
Figure 11 shows that, with uncemented production cas-
ing, longwall-induced casing stress is well below the rated
yield strength of the P-110 casing under a cover depth of
1,307 feet. Similarly, with a gel-type softer cement between
the intermediate and production casings, Figure 12 shows
that longwall-induced casing stress is again well below the
rated yield strength of the P-110 casing under 1,307 feet
of cover.
RESEARCH LIMITATION
Since this study represents the only gas well study site
deeper than 1,200 feet, more data at similar deep-cover gas
well sites are needed for definitive conclusions. FLAC3D
simulations are purely modeling predictions, within the
variance of the variables and parameters used, and need
Figure 8. Measured production casing deformations (via 40-arm Caliper) after second panel mining and images of
moderate and plastic casing deformations
casing deformation at the 642-foot horizon and the plastic
casing deformation at the 1,284-foot horizon, as revealed
by a downhole camera.
CAUSES OF PLASTIC CASING
DEFORMATION AT THE 1,284-FOOT
HORIZON
A few FLAC3D models were constructed to investigate the
causes of plastic casing deformation at the 1,284-foot hori-
zon. Figure 9 shows the longwall-induced casing stress at
the deep-cover study site, where the depth to the Pittsburgh
seam is 1,307 feet. Figure 9 only shows the high stress por-
tion of the casing, which is located near the thick clay-
stone horizon 23 feet above the Pittsburgh seam. Clearly,
this longwall-induced casing stress exceeds the rated yield
strength of the P-110 casing and substantial plastic cas-
ing deformation occurs, which helps to explain the plastic
casing deformation observed by the downhole camera at
the claystone horizon 23 feet above the Pittsburgh seam.
To investigate under what cover depth this type of plastic
casing deformation may occur, three additional FLAC3D
models were constructed and analyzed. Figure 10 shows
that, under a cover depth of 1,000 feet or less, longwall-
induced deformations in fully cemented production cas-
ing are expected to stay in the elastic range. On the other
hand, Figure 10 also indicates that strain hardening in fully
cemented production 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.
EFFECT OF CASING CEMENTING
ALTERNATIVES ON LONGWALL-
INDUCED PLASTIC CASING
DEFORMATION
Since Figure 10 indicates that fully cemented production
casing under more than 1,200 feet of cover could be subject
to high longwall-induced stress and plastic casing deforma-
tion, it is then imperative to find engineering solutions to
prevent such plastic casing deformation under deep cover.
Figure 11 shows that, with uncemented production cas-
ing, longwall-induced casing stress is well below the rated
yield strength of the P-110 casing under a cover depth of
1,307 feet. Similarly, with a gel-type softer cement between
the intermediate and production casings, Figure 12 shows
that longwall-induced casing stress is again well below the
rated yield strength of the P-110 casing under 1,307 feet
of cover.
RESEARCH LIMITATION
Since this study represents the only gas well study site
deeper than 1,200 feet, more data at similar deep-cover gas
well sites are needed for definitive conclusions. FLAC3D
simulations are purely modeling predictions, within the
variance of the variables and parameters used, and need
Figure 8. Measured production casing deformations (via 40-arm Caliper) after second panel mining and images of
moderate and plastic casing deformations