FISSURE FORMATION AND SUBSURFACE SUBSIDENCE IN A COALBED FIRE
1,3
Taku S. Ide1, David Pollard2, and Franklin M. Orr, Jr.3
Dept. of Energy Resources and Engineering, 2 Dept. of Geological and Environmental Sciences
Stanford University, Stanford, CA 94305
e-mail: [email protected]
Abstract
Coalbed fires are uncontrolled subsurface fires
that occur around the world. These fires are believed to
be significant contributors to annual CO2 emissions.
Although many of these fires have been burning for
decades, researchers have only recently begun to
investigate physical mechanisms that control fire
behavior. One aspect of fire behavior that is poorly
characterized is the relationship between subsurface
combustion and surface fissures. At the surface above
many fires, long, wide fissures are observed. At a
coalbed fire near Durango, CO., these fissures form
systematic orthogonal patterns that align with regional
joints in the Upper Cretaceous Fruitland Formation.
Understanding the mechanisms that form these fissures
is important, as the fissures are believed to play vital
roles in sustaining the combustion in the subsurface. In
some of the coalbed fire simulation models available
today, these fissures are treated as fixed boundary
conditions. We argue, using data collected, field
observations and simulation result, that there exists a
relationship between the location and magnitude of
subsidence caused by the fire and the opening of
fissures. The results presented suggest that fissures are
believed to open when subsurface subsidence gives rise
to tensile stresses around pre-existing joints.
Keywords:
coalbed fire, coal fire, subsidence , pre-existing
joints, fissures, numerical modeling, CO2
Introduction
Uncontrolled subsurface fires in coalbeds account
for significant releases of CO2 to the atmosphere. One
of the world’s largest active coalbed fires has been
documented in Wuda, China (Dai et al., 2002), where
the estimated annual loss of coal is around 200,000
tons, equivalent to a yearly emission of ~1.5Mt of CO2
(Kuenzer et al., 2005). Coalbed fires are burning in
many locations in China, Indonesia, India, and the
United States (Stracher and Taylor, 2004). They can be
started naturally by forest fires that burn near an
outcrop, by lightning strikes, by human activities, or by
spontaneous exothermic reactions of pyrites (DeKok,
1986). Forest fires in Indonesia in 1997 and 1998
ignited hundreds of coal fires at outcrops (Brown,
2003). A subsurface fire near Centralia, Pennsylvania,
was started in May of 1962 when the local government
decided to burn an unregulated trash dump in an
abandoned strip mine to reduce trash volume and
control rodents. The fire ignited an anthracite outcrop,
eventually connected to and spread through
underground tunnels, and has been burning since.
Fissures created by the coal fire emit assorted hot gases,
some of which are toxic. A combination of subsidence
and emissions from fissures has caused the town of
Centralia to be abandoned (DeKok, 1986, GAI
Consultants, 1983).
The particular fire examined in this study, called
the North Coalbed Fire, to distinguish it from other
active fires in the region, was discovered in 1998 on the
Southern Ute Indian Reservation when sets of fissures
that are orthogonal to each other—similar to those at
other coal fires around the world—appeared at the
surface (Williamson, 1999). Anecdotal evidences
provided by local Southern Ute Tribe members (Ide,
2007) suggest that the fire may have been smoldering
for decades prior to the reported date of discovery. The
fire continues to burn today. The research effort
described here is an attempt to understand whether coal
combustion followed by subsurface subsidence can
produce fissures with systematic patterns at the surface.
Subsidence can occur when a burned coalseam loses its
structural integrity and collapses under the weight of
the overburden. Understanding the formation of fissures
is important, as they appear to foreshadow the direction
of the combustion front propagation and may play a key
role in sustaining the underground fire.
First, we summarize the San Juan Basin geology,
highlighting key features in the NW section of the
basin, where the coalbed fire is located. Second, we
characterize the geological features and the surface
anomalies in the vicinity of North Coalbed Fire.
Surface topography, images of fissures overlying the
coalbed fire, and measurements of fissure orientations
are presented. We also outline the process of digitizing
features over the North Coalbed Fire and describe how
they were combined with the subsurface information
obtained from the wells drilled in the area. In the third
section, the field data and previous geological surveys
of the area are used to suggest how subsidence can open
pre-existing joints in the area, leading to the formation
of surface fissures over the combustion region. Finally,
we model this phenomenon using a simple boundary
element numerical code, and explore relationships
among key variables that contrast subsidence activities
Stanford Rock Fracture Project Vol. 20, 2009 C-1
and surface deformation. The results and the limitations
of applicability of this simulation model are discussed.
San Juan Basin Geology
The San Juan Basin is an asymmetric, coal bearing
basin that covers approximately 16,800 – 19,400 square
kilometers, stretching approximately 145km west-east
and 160km north-south (Fassett, 2000, Kelso et al.,
1988). It is located near the Four Corners, and spans
across northwest New Mexico and southwest Colorado
as shown in Figure 1. The basin is well characterized
due to the abundance of both coal and coal-bed
methane resources in the Upper Cretaceous Fruitland
Formation (Figure 2). One study has estimated that a
coal-bed methane reserve of nearly 1.4 x 1012 m3 (50 x
1012 ft3) adsorbed onto 219 x 109 metric tons of coal
that underlies the San Juan Basin (Kelso et al., 1988).
The flat, Central Basin is bounded by several key
geologic features, which are described in detail in
previous geologic surveys of the area (Fassett, 2000,
Kelso et al., 1988, Lorenz and Cooper, 2000). The
western and northwestern regions of the basin are
circumscribed by the Defiance and the Hogback
monoclines, respectively, and the Nacimiento uplift
borders the basin on the east side (Lorenz and Cooper,
2000). As Figure 1 shows, the North Coalbed Fire is
located along the Hogback Monocline in the
northwestern portion of the basin. The Hogback
monocline is believed to have formed either due to the
shortening of the Cretaceous strata that induced a rightlateral strike-slip motion along the western and eastern
margins during the Laramide orogeny (Lorenz and
Cooper, 2000), or through reactivation of western
dipping thrust faults underlying the Hogback monocline
that resulted in the uplift (Taylor and Huffman, 1988).
In the former view, the shortening can be attributed to
the Zuni uplift thrusting northward and north
northeastward into the San Juan Basin from the south,
and the San Juan uplift indenting southward into the
basin (Lorenz and Cooper, 2000). Today, only the
forelimb of the Hogback Monocline is exposed and
some of the formation members of the Upper
Cretaceous are exposed on the western side of the
basin. Along the northern perimeter of the Basin,
including areas affected by the North Coalbed Fire,
thick coalseams crop out along the Hogback monocline
(Kaiser et al., 1991).
Formations that make up the Upper Cretaceous
rocks in the San Juan Basin are described by Molenaar
(Molenaar and Baird, 1992). The Fruitland Formation,
which includes the coalbed fire, and adjacent geologic
units are depicted in the stratigraphic column in Figure
2. The left column is representative of the entire San
Juan Basin. The right column shows the top 25m of
rock found directly over the North Coalbed Fire. Above
the coalbed fire, formations above the dotted line—the
Kirtland Shale and most of the Fruitland Formation—
have been removed by weathering and erosion. The
Kirtland Shale and the Fruitland Formation lie atop of
the Pictured Cliffs Sandstone (PC), which was
deposited as regressive marine sands (Condon, 1988)
parallel to the shoreline stretching northwest-southeast
(Fassett, 2000). The Fruitland Formation is a mixture of
mudstones, siltsones, carbonaceous shales and coals
deposited landward and parallel to this shoreline
(Fassett, 2000). Coalseams in the Fruitland Formation
are often referred to as Lower Coal, Middle Coal and
Upper Coal, and the thickest, most continuous coalbeds
are found in the Lower Coal Zone in the northeastern
region of the San Juan Basin (Sandberg, 1988). The
Lower Coal is burning at the North Coalbed Fire.
Figure 1: San Juan Basin and its characteristic
geologic features. The North Coalbed Fire location
is highlighted in the box in the northwestern corner
of the basin along the Hogback Monocline. The
green area denotes outcrops of Pictured Cliffs
sandstone. Figure reproduced from Lorenz and
Cooper, 2003.
At the North Coalbed Fire, 14 boreholes were
drilled in 2007 over an area of 600m x 200m. The high
density of boreholes allowed reliable subsurface
correlations to be made at the site. Both the PC and the
Fruitland Formation rise in a step-like fashion from the
southwest to the northeast with respect to the
isochronously deposited Huerfanito Bed in the Lewis
shale, representing a migrating regression-transgression
cycle over 1.2 million years (Fassett, 1971, Sandberg,
1988). The deposition pattern suggests that the
Stanford Rock Fracture Project Vol. 20, 2009 C-2
subsurface correlation along the shoreline in the
northwest-southeast direction is warranted, as this is the
trend of the long axis of most coal deposits (Fassett,
1988). The echelon geometry of coal deposits can make
subsurface correlations difficult in the transverse
direction (Fassett, 1988), but it has been shown that
Fruitland Formation coalbed correlation was possible
when well logs spaced less than 4km apart were
obtained (Fassett, 1971, 1988).
The San Juan Basin contains several sets of natural
fractures that have been extensively mapped and
documented. Previous studies have offered various
explanations for fracture formations and they are
summarized in Lorenz and Cooper (2000). Ruf (2005)
suggests that the fractures formed due to post-Laramide
extension, while Taylor and Huffman (1998) describe a
Proterozoic crystalline basement with reactivated faults
that may have influenced the orientations of the
fractures in overlying strata. Lorenz and Cooper (2000)
suggest that the orientations of the fractures are most
influenced by the formation of tectonic features such as
the San Juan uplift and the Zuni uplift (cf. Figure 1)
during the Laramide Orogeny.
Despite the competing explanations of the origins
of the fractures, orientation measurements in various
parts of the San Juan Basin are consistent across many
studies. The earliest fracture orientation study of the
San Juan Basin concluded from aerial photography that
northeast (N10E to N60E) and northwest (N15W to
N75W) trends occurred most frequently (Badgley,
1962, 1965, Kelley and Clinton, 1960). Their findings
are generally supported by more recent measurements
(Condon, 1988, 1997, Lorenz and Cooper, 2003, Ruf,
2005, Taylor and Huffman, 1988). The most relevant
study for the North Coalbed Fire was carried out by
Condon (1988), who presents joint orientations and coal
strikes found within the Southern Ute Indian
Reservation. His findings are discussed in detail in the
ensuing section, and they are compared to the fissure
orientations that were measured over the North Coalbed
Fire.
North Coalbed Fire
A satellite image of the area bounded by the red
dotted box in Figure 1 is shown in Figure 3a. The
dotted rectangle depicts the region where surface
anomalies associated with the North Coalbed Fire are
observed. The lack of vegetation over the fire can be
attributed to several factors: a surface forest fire, death
of vegetation due to toxic combustion fumes emanating
from the subsurface, and intentional tree removal to
prevent future forest fires. The North coalbed fire is
contained
between
latitude
N37o01’57”
and
o
N37 02’24” and longitude W108o06’36” and
W108o06’18”, and has an aerial extent of
approximately 600 m x 200m. There are signs of
coalbed fires underlying the bare patch of land south
southwest of the North Coalbed Fire, but only minor
surface deformation is observed; thus this area is not
included in the surveys. The lack of vegetation there is
interpreted as largely due to surface forest fires.
Figure 2: Stratigraphic columns representative of
the San Juan Basin (left) and the top 25 meters of
the lithology over the North Coalbed Fire (right).
Over the coalbed fire, formations above the blue
dotted line have been removed due to weathering.
Stratigraphic column on left adapted from Molenaar,
1977. (Figure not drawn to scale).
A cross-section, A-A’ is drawn (Figure 3b) by
superimposing a USGS geological survey map over the
satellite image in Figure 3a. The cross-section line is
roughly perpendicular to the strike of the Hogback
monocline. The cross-section shows that the Fruitland
Formation crops out along the Hogback Monocline
limb (cf. Figure 1). To the northwest of the Hogback,
only the Lewis Shale—containing the Huerfanito
Bentonite Bed—is observed. The region affected by the
coalbed fire is located near the coal outcrop along the
Hogback, and is circumscribed by the dotted box. In
this region, the local topography slopes between 5 and 9
degrees to the southeast, and the coal layer dips 6 to 15
degrees in the same direction (Condon, 1988). Both the
surface topography and the coalseam flatten towards
the southeast in the direction of the Central Basin (cf.
Figure 1). The continuous and low permeability
Kirtland Shale Formation, which is absent over the
North Coalbed Fire, caps the Fruitland Formation to the
southeast.
Many fissures are exposed on the surface overlying
the North Coalbed Fire. The fissures are distinguished
from regional joint sets in the same strata because
Stanford Rock Fracture Project Vol. 20, 2009 C-3
fissures typically have widths on a decimeter scale,
whereas joints have apertures less than 0.5 cm (Condon
1988). Some of these fissures emit high temperature
combustion gases, indicative of the active fire below,
while others are at ambient temperature.
displacement and surface sediment layer rotation on
one side of the fissure. The other side of the fissure
does not show much displacement. Figure 4c shows an
example of molehill fissures, where surface layers of
sedimentary rock are rotated to form an apex. At
molehill fissures with visible fractures at the surface,
combustion gases with temperatures as high as 290oC
(550oF) have been recorded. The temperatures at the
fissures were measured using a thermal gun, Raynger 3i
Series, made by Raytek. Figure 4d is an example of
narrow fissures, many of which are located in the
northern most portion of the field, and these emit the
hottest exhaust gases recorded in the field at about
1000oC. Both the molehill and narrow fissures are
about 0.15m in width. All of the fissures appear to be
opening Mode I fractures (Pollard and Aydin, 1988), as
displacements are dominantly orthogonal to the fracture
surfaces.
Figure 3: a) A satellite image over the North Coalbed
Fire. The red dotted box outlines the region affected
by the underlying fire. A cross-section line A-A’ is
used in Figure 3b. Satellite image is provided by
Googlemaps. b) Cross section A-A’ showing surface
topography
and
representative
subsurface
stratigraphy in the vicinity of the North Coalbed
Fire. The coalbed fire is located near the coal
outcrop inside of the red dotted box. Kl = Lewis
Shale, Kp = Pictured Cliffs Sandstone, Kf = Fruitland
Formation, Kkl = Kirtland Shale.
Four types of fissures have been observed.
Examples are shown in Figure 4: gaping fissures,
plateau/offset fissure, molehill/buckling fissures, and
narrow fissures. The gaping fissure in Figure 4a is wide
enough for an adult to climb inside. Typical gaping
fissures are 0.15~0.3 m wide at the surface and are
often wider below the soil level. Based on observations
made inside of the gaping fissure in 4a, many fissures
may be connected to each other in the subsurface.
Gaping fissures are at ambient temperatures. The
surface sediment layers do not show significant rotation
around the edges of the fissure. Rather, the fissure
appears to have been pulled apart from either side.
Figure 4b is an example of a plateau fissure. Plateau
fissures have similar apertures at the surface as gaping
fissures but fissures of this type show significant
Figure 4: a) a gaping fissure with an adult inside b) a
plateau fissure, c) a molehill fissure with a 0.15 m
aperture at the apex, d) a narrow fissure venting
O
exhaust gases exceeding 900 C.
Orientations of the fissures are systematic, and they
often form orthogonal patterns at the surface. The
directions and the lengths of 165 fissures are
represented on a rose diagram in Figure 5. The length
has been made dimensionless with respect to the
longest fissure in the field, which is 75m. The diagram
shows that there are three main fissure directions over
Stanford Rock Fracture Project Vol. 20, 2009 C-4
the North Coalbed Fire and that the longest and most
frequently occurring fissures, F1, have azimuths
approximately in the N50E direction. The next most
prominent set, F2, strikes in the N35W direction,
roughly perpendicular to the first set. The third set, F3,
is directed towards the North, and these have similar
lengths to the N35W set. The fissures frequently occur
together in approximately orthogonal pairs, including
members of the N50E and N35W sets.
The azimuths of the fissures were compared with
observations of joint orientations reported in Condon,
1988. Condon measured 1,600 joints and coal cleats at
37 different outcrop locations on the Southern Ute
Indian Reservation. Of the 37 measurement stations, 8
of them are located along the Hogback Monocline and
are spaced approximately 2km apart. Most of his
measurements were for fractures found in formations of
the Upper Cretaceous, the majority of which are in the
Kirtland Shale, Fruitland Formation and the Pictured
Cliffs Sandstone. Four dominant joint sets, labeled J1
through J4, were described. Their stereonets are
reproduced in Figure 6. A comparison of Figures 5 and
6 shows that F1 corresponds to J3, F2 to J4, and F3 to
J2 based on similarities between the fissure orientations
and joint orientations. Typically, the joints occur in
pairs—a J1-J2 pair and a J3-J4 pair—much like the
fissures F1 and F2 that form orthogonal pairs above the
North Coalbed Fire. Condon classifies the J1~J4 joints
as extension joints, due to the lack of features such as
slickenside striations that would suggest lateral shear
movement and the presence of plumose structures,
arrest lines, and twist hackle features that indicate
extension joints (Condon, 1988).
Figure 5: A rose diagram showing the orientation
and the lengths of the fissures found above the
North Coalbed Fire. The characteristic length scale
is ~100m.
Figure 6 (bottom): Four stereonets reproduced from
Condon, 1988. a) J1 joint b) J2 joint c) J3 joint d) J4
joint.
While J1, J2 and J3 are stratigraphically continuous
through multiple beds, J4 fractures only cut through the
sandstone in interbedded sequences of sandstone and
shale. Their orientation ranges are as follows: J1
(N4E~N23E), J2 (N72W~N83W), J3 (N41E~N64E)
and J4, (N24W~N49W). Joints J1, J2, and J3 have
exposed lengths of roughly 1 to 5m, and are spaced
0.15m to 6m. J4 exposed lengths are less than 2m, and
the spacings are more inconsistent (Condon, 1988).
The fissures above the North Coalbed Fire were
mapped using a pack-mounted GPS receiver in order to
place them with respect to the topography of North
Coalbed Fire site. In addition, the GPS was used to
digitize the topography and to mark the locations of
boreholes that have been drilled in the area. The GPS
device used in the survey was a Trimble ProXH, and
the points recorded had better than 1 meter accuracy,
with most having better than 0.5 meter accuracy after
differential correction. Figure 7a shows the digitized
representation of the surface overlying the North
Coalbed Fire. The contour map approximately
represents the region bounded by the red dotted box in
Figures 1 and 3a. The left edge of this map traces the
contact line between the Fruitland Formation and the
Pictured Cliffs Sandstone (cf. Figures 3a, 3b). The
dominant N50E trending fissures are nearly parallel to
the local strike of the Hogback Monocline. The black
lines represent narrow fissures that have been grouted
using a specialized concrete produced by Goodson and
Associates (Williamson, 1999). The concrete was
injected into identified openings in 2000 in an attempt
to smother the fire. Boreholes were drilled around the
Stanford Rock Fracture Project Vol. 20, 2009 C-5
grouted fissures, and thermocouples were installed to
allow monitoring of temperature changes. These
boreholes are shown as open white triangles in Figure
7a. Although the attempt to extinguish the fire was not
successful, the driller’s logs from 2000 provide
valuable insight into the subsurface. This particular
extinguishing method failed mainly because it was not
possible to locate and fill all existing fissures in the
region. Fourteen additional boreholes were drilled in
2007. These boreholes are marked with solid triangles
in Figure 7. For these new boreholes, driller’s logs were
obtained, and most of the boreholes were logged using
caliper, density and gamma ray logs. In one of the
boreholes, borehole 7, an 80ft core was obtained.
The surface information in Figure 7a can be related
to the subsurface information by creating a crosssection along the line A-A”. This cross section shows
that the depth to coal is approximately 20 meters. The
cross-section is approximately perpendicular to the
prominent N50E fissures. Any boreholes or fissures that
lie close to this line are plotted along with the surface
topography in Figure 7b. Where available, a
combination of driller’s-logs and well-logs were used to
identify the depths and thicknesses of void, ash and
coalseams at each intersecting well. If data were
missing at a well, lithologies below it are left blank in
Figure 7b. Fissures that intersect the cross-section line
A-A” are represented using red circles. It is worth
noting that in borehole 11, no signs of coal combustion
were apparent. The last set of fissures occurs up dip of
borehole 11, and there are no fissures down dip of this
borehole. A black line in Figure 7b connects the bottom
of the coalseam.
Stanford Rock Fracture Project Vol. 20, 2009 C-6
Figure 7: a) A contour map of the North Coalbed Fire site, fissures and wells created using a packmounted GPS. Red lines are gaping fissures, green lines are plateau / offset fissures, magenta lines are
molehill/buckling fissures, and blue lines are narrow fissures. The cross-section is created along A-A”.
b) A cross-section of the North Coalbed Fire site along A-A”. Red circles indicate locations of fissures
that intersect the line. Numbers above the surface represent well numbers.
Stanford Rock Fracture Project Vol. 20, 2009 C-7
Formation of Fissures – A Conceptual
Model
We hypothesize that fissures are created from preexisting fractures in the overlying sandstone and shale
that widen when subsidence occurs. Subsidence results
when the burned coal loses structural integrity and
collapses under the weight of the overburden. In Figure
7b, the occurrences of surface fissures coincide with
regions where void and ash were encountered during
drilling. For example, borehole 4 (solid triangle, Figure
7b) located near the peak of the topography contains
only ash and coal. The lack of a void in this well
suggests that subsidence occurred, and thus both the ash
and void are fully compacted. Fissures located between
boreholes 4 and 5 may have resulted from this
subsidence. Similarly, some of the void space may have
been compacted at well 5, causing a fissure to open-up
down dip of this borehole.
The notion of subsurface compaction leading to
surface deformation and fracturing is not new. It is
explored in Whittaker and Reddish’s work on
subsidence related to long-wall coal mining
(Whittakker and Reddish, 1989). They describe surface
profiles associated with various subsurface subsidence
configurations. Their work is based on examples from
various long-wall coal mining sites and includes field
observations, experimental, and numerical results. In
long-wall mining, the excavation front advances much
like we envision the combustion front may move
through the coalseam in a coalbed fire. Figure 8 is a
conceptual model of subsidence near a long-wall
mining process (Whittaker and Reddish, 1989). Here,
tensile fractures develop in strata immediately
overlying the collapse. This figure can be also be used
to illustrate an empirical relationship presented in their
work, which shows that the ratio of the length of coal
excavated (L) to the depth (d) of excavation must
typically exceed 1.4 for maximum subsidence to occur
(Whittaker and Reddish, 1989). Adjacent unmined parts
of the coalseam and a natural arch that develops above
the coal removal site may be capable of supporting
most of overburden when L/d is less than 1.4
(Whittaker and Reddish, 1989). There are two key
differences between their study and the work presented
in this paper. First, the North Coalbed Fire is burning
~20m below surface, whereas longwall mining
typically occurs at much greater depths (Whittaker and
Reddish, 1989). Second, pre-existing vertical joints and
their response to subsurface compaction are not
discussed in Whittaker and Reddish.
Figure 8: A conceptual model of subsidence
associated with long wall mining. Tensile stress
fractures associated with the collapse are shown. L
signifies the length of coal excavated, and the d the
depth below the surface of the seam being mined.
(Whittaker and Reddish, 1989)
Previous studies of coalbed fires have also
suggested that subsurface subsidence leads to the
formation of fissures at the surface (Buhrow et al.,
2004, Cao et al., 2007, Chen, 1997, Dunrud and
Osterwald, 1980, Gielisch and Kuenzer, 2003, Kuenzer,
2007a, 2007b, 2008, Litscheke, 2005, Sokol and
Volkova, 2007, Wessling, 2007, Wessling et al., 2008,
Zhang, 2007). Figure 9 is from one such study of a
coalbed fire in China, where subsidence apparently
played a significant role in opening surface fissures
indicated by the arrows. Most of these studies did not
examine whether fissures resulted from the widening of
pre-existing joints in the region. In Chen’s work (Chen,
1997), it is shown that fissure orientations coincide with
joint orientations in the sandstones overlying the
coalbed fire in Ruqigou, China. In this study, however,
relationships between variables such as the location and
magnitude of subsidence and the widths of surface
fissures were not established (Chen, 1997). In a
coalbed fire combustion simulation model presented by
Huang et al. (2001), the fissures were modeled as fixed
boundary conditions—through which exhaust gases can
escape and fresh oxygen can enter—irrespective of the
location of the combustion front. Similarly, in the
numerical model of Wessling et al., mechanical
processes such as subsidence and fissure openings were
not considered (Wessling et al., 2008). By establishing
a first order relationship between combustion front
location and fissure opening width as a function of
governing variables such as depth, length of collapse,
proximity of preexisting fissures, and the stiffness of
the overburden rock, we hope to aid future numerical
modeling of coalbed fires.
Stanford Rock Fracture Project Vol. 20, 2009 C-8
Figure 9: A picture of a subsided area and fissures
nearby (indicated by arrows) in a coalbed fire at the
Wuda Syncline, Inner Mongolia Autonomous
Region, China. (courtesy of Chris Hecker, ITC,
2008)
Observations at an outcrop about 1km north of the
North Coalbed Fire shows how subsurface subsidence
can cause pre-existing fractures to open at the surface.
This outcrop exposes a fossilized coalbed fire,
subsidence, extension fractures and fissures. At this
outcrop, shown in Figure 10a, a coalseam is overlain by
10m of sandstone, shale, and siltstone. A person 1.5m
tall standing to the right is used as a scale.
There are two prominent features at the outcrop: the
fissure that runs down the middle of the outcrop, and
the deformed ash layer towards the bottom of the
outcrop. The fissure down the middle of the photograph
is labeled as Fissure 2, and this fissure has an opening
of around 0.5m at the surface. When the coalseam was
consumed by a combustion front moving from the right
to left, we suggest the ash deformed by compaction
under the weight of the overburden. The maximum
collapse recorded at the outcrop is ~1.5m.
Features at this outcrop such as Fissure 2 and the
subsided ash layer were mapped using a laser
rangefinder produced by LaserCraft Inc. (LaserCraft,
2007). The digitized version of the outcrop is
juxtaposed next to the photo of the outcrop in Figure
10b. Note how the tabular coalseam is deformed due to
collapse of the ash layer. Above the collapse, opening
fractures, much like those depicted in Figure 8, were
observed. In addition, four fissures with more modest
openings were mapped over the collapse. In subsequent
sections, when we compare numerical solutions to our
measurements at this outcrop, we assume that Fissure 2
in Figure 10b widened largely due to the collapse of the
combusted coal layer and that the weathering process to
expose the outcrop did not significantly enhance the
opening.
Figure 10: (left) A picture of an outcrop near the North Coalbed Fire with an exposed fossilized coalbed
fire, subsidence and associated surface fissures. (right) Some features from the same outcrop
mapped using a Laser Range Finder. The major features are depicted using thicker lines.
Stanford Rock Fracture Project Vol. 20, 2009 C-9
Numerical Modeling
Numerical models were employed to examine
whether pre-existing joints could pull open to form
fissures when subjected to the stresses due to the
overburden weight and those induced by a subsurface
collapse. The mechanical effects of subsurface
subsidence on the jointed strata overlying the coal were
modeled using a Boundary Element Method (BEM)
formulation for a line source of displacement
discontinuity in an elastic half plane. This problem
formulation is an adaptation of the displacement
discontinuity method (Crouch and Starfield, 1983). The
BEM code is a modified version of Martel’s Matlab
BEM code (Martel, 2003), which in turn is based on the
original Fortran code presented in Crouch and Starfield
(Crouch and Starfield, 1983).
Several key assumptions are made in this model,
including an elastic homogeneous medium with a
reduced stiffness coefficient, infinitesimal strain, a state
of plane strain, and a flat traction free surface. The rock
above the collapsing coalseam is modeled using a
reduced stiffness coefficient in place of explicitly
modeling each fracture and joint in the overburden. The
use of reduced stiffness coefficients compared to the
values measured in experiments is justified in the
previous literature on fractured rock deformation
(Berest et al., 2008, Sanz, et al., 2008). Hooke’s Law is
used to relate stress and strain, while the infinitesimal
strain assumption dismisses higher order displacement
derivative terms in the relationship between strain and
displacement (Malvern, 1960). The infinitesimal strain
assumption admits the use of the method of
superposition, which is used to calculate stress and
displacement distributions in the domain and to create a
half-plane surface. The plane strain assumption restricts
any displacement perpendicular to the plane of the
model (Crouch and Starfield, 1983). Finally, a flat
surface is modeled rather than the actual topography
over the coalbed fire outcrop for simplification.
Although these assumptions lead to a model that, at
best, approximates the deformation of jointed rock over
a coalbed fire, it nevertheless helps to build an intuitive
understanding between subsidence and fissure opening,
which has not been explored in today’s coalbed fire
modeling literature (Huang, 2001, Wessling, 2007,
2008).
In the BEM code, discretized horizontal elements
are used to model the coalseam, and discretized vertical
traction free elements are used to model pre-existing
joints in the overburden. The infinite plane is
transformed into a half-plane by introducing the
principle of superposition to create a traction free
boundary condition along the x-axis (Crouch and
Starfield, 1983). Stress boundary conditions are
perturbed along the horizontal elements to simulate a
collapse as the coal burns, and the elastic domain is
deformed as a result of this perturbation. Stresses and
displacements that arise at any point in the domain can
be calculated by combining the contribution of stresses
and displacements from each element (Crouch and
Starfield, 1983). The stress distribution in the elastic
material is a function of the location and orientation of
the boundary elements and the boundary conditions on
them.
Figure 11 defines the variables and applicable
dimensionless groups used in this modeling. E is
Young’s modulus, and σzz is the normal compressive
stress defined along the horizontal elements to simulate
the downward pressure due to the overburden. These
variables both have units of stress (MPa). All other
variables have units of length (m), and they are defined
as follows: fd is the height of the vertical fracture, fl is
the distance between the vertical fracture and the edge
of the horizontal collapse, d is the depth, and a is the
horizontal length of the collapse.
Figure 11: Definitions of variables and
dimensionless groups used in the BEM model. E
and σzz have units of MPa, while fd, fl, a and d have
units of m.
We first introduce a domain with no vertical joints
in order to illustrate the stress distribution that arises as
a result of collapse of a continuous overburden. We
then introduce a vertical fracture, and compare how it
reacts to a subsidence event when located in regions of
induced tensile stress. This is followed by a sensitivity
analysis to demonstrate the behavior of vertical joints
with respect to various model variables. Finally, a BEM
model is constructed from the outcrop mapped using
the laser range finder (cf. Figure 10b), and simulation
results are compared to field observations.
In the first example, a 12m horizontal line of
elements that is located 10m below the surface is
deformed by applying a uniform compressive stress of
0.25MPa, which is exerted by the weight of the
overlying rock. The elastic modulus of the overburden
is 10MPa, and a maximum compaction of 1.5m is
induced at the horizontal elements. Figure 12 depicts
Stanford Rock Fracture Project Vol. 20, 2009 C-10
the distribution of the horizontal component of normal
stress, σxx, in response to the inward directed
displacement discontinuity on the horizontal elements.
The sign of σxx at the surface is indicated by the words
tensile (+) and compression (-). The blue solid line
along the bottom of the figure indicates the horizontal
elements subject to subsurface subsidence. We suggest
that this inward directed relative motion is similar to
what would occur as compaction of the coalseam
developed during burning. Directly above the elements
at the surface σxx is compressive. The greatest
concentrations of surface tensile stresses emanate
diagonally upward from the ends of the line of collapse.
A modification to the first simulation investigates
the effects of the collapse on traction free vertical
joints. The setting and the parameter values are the
same as the first simulation (cf. Figure 12), except
vertical elements are introduced to simulate the joint.
The vertical elements are placed at x = -12m, where
tensile stresses found in the first case (cf. Figure 12).
Figure 13 shows the model geometry and the resulting
normal horizontal stress (σxx) distributions when a
horizontal collapse occurs near the vertical fracture. A
comparison of Figures 12 and 13 shows that if a vertical
joint exists off to the side of the compaction zone, σxx
relaxes and becomes less tensile as the joint opens.
Figures 14 compares the horizontal displacements
between the two cases discussed in this section, the
model without vertical fracture and the model with the
vertical fracture. The horizontal displacements at the
surface have been made dimensionless by the
maximum vertical subsidence induced along the
horizontal elements. Here a positive displacement
signifies a movement to the right, and a negative
displacement indicates a movement to the left.
Horizontal surface displacements are continuous when
there are no vertical fractures since the domain is
modeled as an elastic medium. In contrast, surface
displacements are perturbed during the subsidence
when a fracture is located within the tensile region. The
right side of the fracture—the edge closer to the
induced subsidence—displaces towards the region of
the collapse horizontal elements, while the left side
does not displace as much, so the model fracture opens.
This result shows how pre-existing joints in tensile
regions may widen to form fissures.
A sensitivity study was undertaken to explore how
the opening of vertical joints are influenced by the
governing variables presented in Figure 11. Four
dimensionless groups are chosen to represent the
relationships between the variables. Π1, or E/σxx, is the
ratio of Young’s modulus of the rock to the stress
imposed along the horizontal elements to induce
subsidence. Π2, or fd/d, is the ratio of the height of the
vertical fracture to the depth at which compaction
occurs. Π3, or a/d, is the ratio of the horizontal length of
subsidence to the depth. Finally, Π4, or fl/d, is the ratio
of the distance between the vertical fracture and the
edge of the collapsed region to the depth. These groups
are plotted against a dimensionless length scale,
UmaxOpening / UmaxCollapse, which relates the
horizontal displacement of the joint at the surface to the
maximum vertical subsurface subsidence along the
horizontal elements. Here, a negative dimensionless
length means that the edges of the vertical elements
displace away from each other, or in other words, the
joint opens. Simulation results show that this
dimensionless length does not vary with respect to Π1,
and thus the following analyses are limited to
Figure 12: Subsidence along horizontal elements (blue solid line, bottom center) and resulting stress
distributions in the domain. Tensile stresses emanate diagonally upwards from the edge of the
horizontal elements. Colorbar in MPa.
Figure 13: A vertical joint located to the left of the collapsed region. Tensile stresses near the vertical
joint are relaxed due to the traction free elements.
Stanford Rock Fracture Project Vol. 20, 2009 C-11
Figure 14: Horizontal displacements at the surface for cases with no fracture (solid line) and with a
fracture (dotted line). The model with a fracture in the domain shows a displacement discontinuity
indicating an opening.
demonstrating the dependence of the dimensionless
length scale on Π2, Π3, and Π4. The results presented
from the sensitivity analyses can be used, on a first
order basis, to estimate the location and the magnitude
of the subsidence when the only the widths of the
surface fissures are known.
Figure 15 is a plot of the relationship between the
dimensionless opening (Umax Opening / Umax Collapse)
and Π4 (fl/d). Each line represents a different value of
Π2 (fd/d). In the following discussion, the maximum
vertical subsidence length, UmaxCollapse, depth, d, and
length of subsidence, a, will be fixed to simplify our
analysis. As a consequence of fixing both d and a, Π3,
the ratio between the two variables is constant.
Variables a and d are specified such that Π3 = 1.0.
Figure 15 shows that for a constant value of Π2, or for a
fixed height of the fracture, a maximum horizontal
displacement at the surface is observed when Π4~0.8.
The fissure width reaches a maximum when it is
located diagonally above and to the side of the zone of
compaction, consistent with where a concentration of
tensile stresses was observed in Figure 12. The fissure
opening decays to 0 as the vertical joint moves farther
away from the region of subsidence regardless of the
height of the fracture. This result is reasonable since the
stresses associated with the subsidence decay with
increasing distance. Figure 15 also shows that when Π2,
or the height of the pre-existing joint, increases, the
magnitude of the opening also increases. Based on the
results, when Π3 = 1.0, the maximum fissure opening is
observed when the pre-existing joint is located at Π4 ~
0.8 and is stratigraphically continuous down to the
collapse horizon, or Π2 = 1.
Figure 16 is a similar plot. It shows how the
dimensionless length scale (Umax Opening / Umax
Collapse) depends on Π4 (fl/d) for varying values of Π3
(a/d) with both the maximum subsidence distance
(UmaxCollapse) and depth (d) fixed. In addition, fd, the
height of the fracture, is fixed and defined such that Π2
(fd/d) is 1.0. The figure shows that for a fixed value of
Π3, which represents the length over which the collapse
occurs, the fissure opening again depends strongly on
the location of the vertical joint. The fissure opening
decays to 0 far away from the subsidence and is the
widest between 0.5<fl<1.0 depending on the value of
Π3. As Π3 increases, the horizontal displacement at the
surface increases, which makes sense since a longer
subsidence length leads to a greater tensile stress
emanating upwards towards the surface. The location
where the maximum fissure opening is observed moves
closer to the edge of the horizontal compaction when
Π3 increases. In other words, the region of tensile
stresses that extends upwards towards the surface lies
more directly above the horizontal compaction as the
length of subsidence increases when depth is constant.
Figure 15 (above): Dimensionless length vs. Π4 (fl/d).
Each line represents a different value of Π2 (fd/d),
while Π3 (a/d) is kept constant at 1.0.
Stanford Rock Fracture Project Vol. 20, 2009 C-12
Figure 16 (below): Dimensionless length vs. Π4 (fl/d).
Each line represents a different value of Π3 (a/d),
while Π3 (fd/d) is kept constant at 1.0.
We investigate whether the relationship between
the subsidence and fissure opening at the outcrop in
Figures 10a and 10b can be predicted using this
numerical model. In this simulation only the most
prominent fissure at the outcrop, indicated on Figure
10b, is explicitly modeled using traction free elements.
This fissure at the outcrop is slightly oblique and
appears to be stratigraphically continuous down to the
depth of the collapse. The bottom of this fissure is
located approximately 2.5m left of the edge of the
collapsed zone. All other fissures and tensile stress
fractures at the outcrop are incorporated into the model
by reducing the bulk stiffness of the rock to 10MPa,
which is one to three orders of magnitude lower than
published elastic moduli of various shales and
sandstones. The length of subsidence is approximately
12m at the outcrop, although the exact length is not
known due to limited exposure at the outcrop.
The subsidence occurs approximately 10m below
the surface. This ratio of length of collapse / depth is
close to the critical extraction value of 1.4 observed for
collapses associated with long-wall mining operations
(Whittaker and Reddish, 1989). The collapsing ash
layer is modeled using tilted elements with
appropriately defined stress boundary conditions. The
depth where the collapse occurs is approximately 10m,
maximum subsidence is approximately 1.5m, and a
0.5m surface opening of the vertical fracture was
observed at the outcrop.
When a collapse is induced in the numerical model,
tensile stresses are relaxed around the vertical fissure by
opening. Figure 17a shows the geometry of the model
and Figure 17b is the σxx stress distribution map
resulting from the collapse when a downward stress of
0.25 MPa is defined along the horizontal elements.
Figure 17: a) Geometric representation of the two
prominent features found at the outcrop (cf. Figure
10). b) Tensile stresses dominate around the
diagonally oriented joint, which is stratigraphically
continuous down to the depth of collapse. fd~10m,
d~10m, a~12m, fl~2.5m.
Figure 18: A conceptual model depicting the mechanism of how pre-existing joints above the North Coalbed
Fire open up to form a fissure.
Stanford Rock Fracture Project Vol. 20, 2009 C-13
The dimensionless opening, UmaxOpening / Umax
Collapse is around 0.23 when these parameters are used
to simulate the fissure opening and the collapse.
Alternatively, this value could have been obtained by
calculating appropriate dimensionless variables, and
using Figure 16 to obtain the dimensionless length
scale. Based on the model assumptions, appropriate
dimensionless values are calculated as follows: Π2 =
fd/d ~ 1, Π3 = a/d ~ 1.2, Π4 = fl/d ~ 0.25. Although this
method approximates the coalseam and the traction free
fracture to be horizontal and vertical, respectively, it
nevertheless gives a dimensionless length scale of
approximately 0.23. Both of these values are consistent
with the UmaxOpening / UmaxCollapse observed at the
outcrop. At the outcrop, UmaxOpening = 0.5m and
UmaxCollapse = 1.5m, giving a length scale ratio of 0.3.
The discrepancy is attributed to relatively simple
assumptions associated with this numerical model. In
future modeling efforts, these assumptions will be made
more realistic.
The results from the numerical simulation suggests
that pre-exisiting joints that are located above existing
coalbed fires can open when they are in regions of
tensile stress induced by the subsidence. Figure 18 is a
conceptual model of how the propagation of the
combustion front at the North Coalbed Fire can lead to
opening of fissures at the surface. The figure accounts
for the local geology, geometry and the findings from
the numerical investigations. In the figure, the lithology
above the coalbed fire is characterized as either shales
or sandstones. At the site, shales are often softer than
the sandstones. In this conceptual model, the coalseam
is transformed into a layer of ash as the thin combustion
front propagates through the lower coal. The overlying
strata collapse, and a pre-existing joint opens up to form
a surface fissure. The underlying Pictured Cliffs
sandstone remains intact. Opened fissures above the fire
may act as conduits that connect the surface and the
coalseam. These fissures allow combustion gases to
escape from the combustion zone, and enable fresh
oxygen to reach the coalseam in order to keep the
combustion alive.
Conclusions
At the surface above the North Coalbed Fire, which
burns along the Hogback Monocline in the San Juan
Basin, numerous fissures form orthogonal patterns.
Some of these fissures vent hot exhaust gases from the
subsurface, an indication of a burning coalseam in the
subsurface. A combination of available geologic data
from previous surveys, observations and measurements
from the field allows identification of a mechanism for
the formation of surface fissures. The hypothesis is that
pre-existing joints in the strata overlying the North
Coalbed Fire widen to form fissures when the
underground coalseam combusts and then compacts as
its structural integrity is lost. Previous literature has
suggested or described relationships between surface
deformation and subsurface subsidence, but no work
has established first order functional relationships
between variables that govern fissure widening and
subsurface subsidence in a coalbed fire. In this study, a
simple BEM model was formulated to simulate the
collapse of the coalseam and the opening of preexisting vertical fractures. Results show that the
aperture of the fissures at the surface depends strongly
on where the vertical fracture is located with respect to
the subsurface subsidence. The sensitivity analyses
performed using this simulator also demonstrate the
relationships amongst the governing variables defined
in this study. Those relationships can be used to
estimate the location and subsidence magnitude based
on the fissure locations and width measured at the
surface. The model was tested using a dataset obtained
from a near by outcrop that showed evidences of
subsidence in a combusted coalseam and an opening of
a vertical fracture above. Many assumptions were made
in the simple numerical simulation, and thus there is
some discrepancy between the model results and
measured values.
Acknowledgments
The authors of this paper would like to
acknowledge: Bill Flint of the Southern Ute Indian
Tribe for facilitating fieldwork details and his help in
securing funding, the Southern Ute Indian Tribe for
their gracious hospitality, allowing us to access their
land and their continued support, Jonathan Begay, Kyle
Siesser and Ashley Neckowitz for their help in the field,
and the Stanford Global Climate and Energy Project
and its contributors for their funding to make this
research possible.
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