Cleat in Tj Coal Geol Review

7/23/2019 Cleat in Tj Coal Geol Review

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.International Journal of Coal Geology 35 1998 175207

Characteristics and origins of coal cleat: A review

S.E. Laubach a,), R.A. Marrett b, J.E. Olson c, A.R. Scott a

aThe Uniersity of Texas at Austin Bureau of Economic Geology, 10100 Burnet Road, Austin,

TX 78758-4497, USAb

The Uniersity of Texas at Austin Department of Geological Sciences, Austin, TX 78758-4497, USAc

The Uniersity of Texas at Austin Department of Petroleum and Geosystems Engineering, Austin,

TX 78758-4497, USA

Received 18 December 1996; accepted 16 April 1997

Abstract

Cleats are natural opening-mode fractures in coal beds. They account for most of the

permeability and much of the porosity of coalbed gas reservoirs and can have a significant effect

on the success of engineering procedures such as cavity stimulations. Because permeability andstimulation success are commonly limiting factors in gas well performance, knowledge of cleat

characteristics and origins is essential for successful exploration and production. Although the

coalcleat literature spans at least 160 years, mining issues have been the principal focus, and

quantitative data are almost exclusively limited to orientation and spacing information. Few data

are available on apertures, heights, lengths, connectivity, and the relation of cleat formation to

diagenesis, characteristics that are critical to permeability. Moreover, recent studies of cleat

orientation patterns and fracture style suggest that new investigations of even these well-studied

parameters can yield insight into coal permeability. More effective predictions of cleat patterns

will come from advances in understanding cleat origins. Although cleat formation has been

speculatively attributed to diagenetic andror tectonic processes, a viable mechanical process for

creating cleats has yet to be demonstrated. Progress in this area may come from recent

developments in fracture mechanics and in coal geochemistry. q 1998 Elsevier Science B.V.

Keywords: coal; fractures; brittle deformation; statistical analysis; mechanics; natural gas

1. Introduction

Fractures occur in nearly all coal beds, and can exert fundamental control on coal

stability, minability, and fluid flow. It is therefore not surprising that coal fractures have

)

Corresponding author. Fax: q1-512-4710140; E-mail: [email protected]

0166-5162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. .PII S 0 1 6 6 - 5 1 6 2 9 7 0 0 0 1 2 - 8


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( )S.E. Laubach et al.rInternational Journal of Coal Geology 35 1998 175 207176

. .Fig. 1. Schematic illustration of coal cleat geometries. a Cleat-trace patterns in plan view. b Cleat .hierarchies in cross-section view adapted from Laubach and Tremain, 1991 . These conventions are used for

cleat: length is dimension parallel to cleat surface and parallel to bedding; height is parallel to cleat surface and

perpendicular to bedding; aperture is dimension perpendicular to fracture surface. Spacing between two cleats .of same set is a distance between them at right angles to cleat surface.

been investigated since the early days of coal mining, and that published descriptionsand speculation on fracture origins date from early in the nineteenth century Mammatt,

.1834; Milne, 1839; cited in Kendall and Briggs, 1933 . Although various miners termsfor systematic fractures in coal have been used over the years, they are still generally

.referred to by the ancient mining term: cleat Dron, 1925 .

Cleats are fractures that usually occur in two sets that are, in most instances, mutually

perpendicular and also perpendicular to bedding. Although pre-1990 geologic literature

and current mining usage distinguishes these sets on the basis of factors such as

prominence that are difficult to quantify, abutting relations between cleats generally .show that one set pre-dates the other Fig. 1 . This is a readily quantifiable distinction

between sets in most outcrops and cores. Through-going cleats formed first and are

referred to as face cleats; cleats that end at intersections with through-going cleatsformed later and are called butt cleats Laubach and Tremain, 1991; Kulander and Dean,

.1993 . These fracture sets, and partings along bedding planes, impart a blocky character .to coal Fig. 2 . A hierarchy or perhaps a continuum of cleat sizes typify the

. .population of cleats in a coal bed Laubach and Tremain, 1991 Fig. 1 . This is most

easily recognized in cleat heights, but size variations are also evident in cleat lengths,

apertures, and spacings.

Although fractures in coal are relatively unimportant in strip mining, their signifi-

cance in efficient design and safety of underground coal mines has continued to

command the attention of the mining industry Hanes and Shepherd, 1981; Esterhuizen,.1995; Molinda and Mark, 1996 . In contrast, the characteristics and origin of fractures in

.coal have been relatively neglected in recent 19661996 geological literature. For

example, only two AAPG Bulletin papers specifically on coal fractures have been


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( )S.E. Laubach et al.rInternational Journal of Coal Geology 35 1998 175 207 177

Fig. 2. Cleats in coal outcrop of Cretaceous Fruitland Formation. Photograph, view to northwest. Rock

hammer for scale.

published in that time, and the leading journal on coal geology, the International Journal

of Coal Geology, has only published one article that focused on cleat in the last 10years. Other journals and geology textbooks have also paid little heed to cleat in recent

times.

This neglect partly reflects a historic impasse in the scientific understanding of

certain types of fractures such as those in coal. Cleats most commonly occur without any

observable shear offset, and are thus properly termed opening-mode fractures. Studies of

the origins of opening-mode fractures, such as cleats and joints, are plagued by the

problem of equifinality the situation where several processes may lead to the same

result rendering unique causes difficult to deduce from products. This arises because

.coals and other rocks have low tensile strength, and a wide range of loading paths canlead to propagation of opening-mode fractures Engelder, 1985; Engelder and Fischer,

.1996 . Yet such fractures preserve little evidence of the loading conditions that caused

them. A symptom of this is the variety of plausible scenarios that have been advanced


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for the origins of cleats Kendall and Briggs, 1933; McCulloch et al., 1974; Ting, 1977;.Close, 1993 and the lack of explicit, testable predictions that can be used to identify the

best interpretations.

Owing to the increasing importance of coal beds as gas reservoirs, geologists are

again becoming concerned with the characteristics and origins of cleat. For coalbed

methane extraction, knowledge of the properties of natural fractures is essential forplanning exploration and development because of their influence on recovery of

methane, and the local and regional flow of hydrocarbons and water. New mapping of

cleat patterns, guided by recent conceptual advances in the description and theoretical

understanding of fracture processes, is beginning to bring these patterns into focus. This,

together with more rigorous study of the petrology of cleat development, may resolve

uncertainties about causes of cleat that now hinder predictions of interwell-scale patterns

of fractures in coal beds. This paper highlights recent studies with emphasis on our

research in United States coalbed methane basins, and points to areas of potential future

advancement.

2. Cleat attributes

As many observers have noted, the attributes of cleats closely resemble those offractures termed joints in other rock types Dron, 1925; Kendall and Briggs, 1933;

Williamson, 1967; McCulloch et al., 1974, 1976; Ting, 1977; Campbell, 1979; Laubach

.et al., 1992 . Cleats are opening-mode fractures rather than faults; typically there is noappreciable offset parallel to cleat walls. At surface conditions they generally have

apertures less than 0.1 mm, and these scarcely visible apertures caused some early

workers to mistakenly conclude that no opening had occurred normal to cleat walls.

They also have surface structures such as arrest lines and hackles characteristic of .opening-mode propagation Kulander and Dean, 1990 .

Many of the prominent attributes of cleats and cleat patterns have been known since

the 1930s. Individual fractures are generally planar but sometimes are locally curved in

plan view. Cleats are subvertical in flat-lying beds and are typically oriented at right

angles to stratification even where beds are folded. In many cases cleats are confined toindividual coal beds, or to layers composed of a particular maceral type. Commonly they

are uniform in strike within an outcrop or core and arranged in subparallel sets that have .uniform regional trends Kendall and Briggs, 1933 . Yet they locally show abrupt lateral

.and vertical shifts in strike Dron, 1925 . Coals containing closely spaced faults rather

than opening-mode fractures are apparently rare. This reflects either proximity to large

faults, reactivation by slip on pre-existing cleats, or anomalous burial conditions .Ammosov and Eremin, 1963; Laubach et al., 1993 .

One remarkable attribute of cleat formation is the extent to which they are developed

in many coal beds of nearly all ranks in maturity. Cleats are typically much moreintensely developed than fractures in adjacent non-coal rocks. The great intensity of

cleat development, in comparison to fractures in other rock types, is an important clue to

cleat origins.


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2.1. Cleat size

There are little quantitative data on the size of cleats. Because many cleats are only

centimeters in length and height and typically have nearly indiscernible aperture,

modeling studies usually assume cleats are small in terms of length, height, and aperture.In San Juan Basin, New Mexico, Fruitland Formation coal beds cleat lengths and heights .are reported to range from microns to meters Tremain et al., 1991 . Yet these cleats are

arranged in a hierarchy of sizes that includes fractures of decimeter size master cleats,.Fig. 1 .

.Estimates of in situ cleat widths range from 0.001 to 20 mm Gamson et al., 1993 .

However, most information on cleat width is based on outcrop studies andror micro-

scopic examination of coal samples not under confining pressure. Little reliable informa-

tion is available on cleat apertures in the subsurface. The parallel-plate fracture

permeability model can be used to estimate that cleat apertures range from 3 to 40 mm .Fig. 3 . Where diagenetic minerals have filled cleats, considerably wider fractures are

locally preserved, including fractures with widths as much as 0.5 cm in Cretaceous coal

beds in the western United States. These fossilized cleat apertures show that fractures

much wider than most outcrop- and core-based estimates are locally present in the

subsurface. Because cleat apertures may change as effective stress is altered in coalbed

methane reservoirs, lack of reliable information on in situ cleat apertures is a potential

barrier to effective reservoir management.

Fig. 3. Relation among face-cleat spacing, permeability, and aperture assuming parallel-plate model. Using

measured cleat spacing and permeability for highly productive coalbed methane wells in San Juan and Black

Warrior Basins, shaded area shows range of inferred aperture size.


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Fig. 4. Cleat aperture size distribution for combined face and butt cleats in cores from San Juan Basin. Data .represent volumetric 3D sampling. Least squares fit made to data in open symbols.

Size distribution of opening-mode fracture apertures have been studied quantitatively .in a variety of rock types Marrett, 1997 , but there is little published data for cleats to

permit such an analysis. One set of observations, summarized by Close and Mavor .1991 , was collected from six wells in the San Juan Basin. Aperture of a typical cleat in

each lithotype layer was measured microscopically without confining stress. From this

.we calculate cumulative volumetric 3D frequency of cleats as a function of each core .Fig. 4 . This presupposes that all cleats in a specific lithotype layer have the same

aperture, probably a poor assumption.

Apertures observed in three coal cores range from 0.01 to 0.2 mm, although cleats .with smaller apertures were not measured. In each core, cumulative frequency f of

cleats having apertures of e or larger follows a power law:

fsbeyc 1 .

where b is a general measure of the cleat intensity and c is a constant referred to as the

.fractal dimension . A power law forms a line on a loglog plot of f versus e, and theslope of the line is the negative of the fractal dimension. Values of c determined from

.volumetric sampling of cleat apertures 2.742.82 are larger by ;2 than values . .determined from scanline 1D sampling of apertures in non-coal rocks Marrett, 1997 .


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.This is expected assuming aperture, length, and height are linearly proportional due to . . the difference between volumetric 3D and scanline 1D sampling Marrett and

.Allmendinger, 1991; Marrett, 1996 . Thus, size distribution of cleat apertures is consis-

tent with size distribution of fracture apertures in non-coal rocks. .Reports summarized by Close and Mavor 1991 provide the only systematic core

data available on cleat heights. Among cleats with a specific aperture value, heights varysignificantly. However, cleats with large apertures tend to have large heights. Average

.height h of all cleats having a specific aperture value increases approximately linearly

with aperture:

hsae 2 .

where a is about 1000 for cleats in one well. This linear aperturerheight relationship is

consistent with linear elastic fracture mechanics predictions for opening-mode fractures .where fracture height is shorter than fracture length Pollard and Segall, 1987 .

. .Combining results of Eqs. 1 and 2 implies that cleat heights follow a power law sizedistribution. No systematically collected data on cleat lengths are known. Butt cleats are

limited in length by spacing of face cleats, so butt cleat lengths should depend on face

cleat spacings. We might anticipate that face cleat lengths will follow power law size

distributions, by analogy with size distribution of fracture lengths in non-coal rocks .extension fracture data in Marrett, 1997 and references therein . Positive interference

between wells shows that interconnected pathways in cleated rocks can be as long as 1

km.

2.2. Cleat spacing

.Cleat spacing is sufficiently close on the order of centimeters that numerous visible

fractures are typically present in coal cores. However, core observations rarely distin-

guish the hierarchy of fracture sizes that are present, so published cleat spacing

information is difficult to compare. Spacing measurements reported from core may

count all visible fractures, whereas outcrop and mine studies typically choose fractures

of a given size to measure, but this choice is rarely explicitly stated. Recognizing the

wide range of cleat sizes present in coal, quantitative statements of spacing are only

.meaningful with reference to the sizes of cleats included in an analysis Fig. 5 . An .example where a size criteria was specified is the study of Tremain et al. 1991 . In this

.study, fractures of equivalent height defined by layer thickness were compared.

Measurements on long mine highwalls show that spacing between individual cleats of .similar size ranges from microns to more than a meter Fig. 6 . Yet within a given bed,

average spacing of fractures of similar size is remarkably uniform over distances of . .hundreds of meters Fig. 7 Tremain et al., 1991 . Sampling procedure needs to be

specified in order to obtain meaningful fracture spacing measurements. .Variability in spacing or intensity of fracture development of cleats in coal beds has

.long been recognized Kendall and Briggs, 1933; Macrae and Lawson, 1954 . Contrastsin coalbed methane well production also may reflect variable development of cleats, as

several operators have speculated. Methods for identifying and mapping such variability .Esterhuizen, 1995 are important in mining conditions because of implications for


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Fig. 5. Cleat spacing and aperture data from Northeast Blanco Unit No. 403, San Juan Basin. Plotted spacings

represent averages of all measured spacings having a specific aperture measurement. Least squares fit made to

average spacings of face cleats.

Fig. 6. Cleat spacing versus bed thickness of medium-brightness coals, northern San Juan Basin adapted from.Laubach and Tremain, 1991 .


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Fig. 7. Cleat spacing versus traverse distance in a bed of uniform dip, thickness, and composition, San Juan

Basin, New Mexico. Center half of data at each station is shown by box, and median by bar. Dashed line and

shaded area are mean and one standard deviation of measurements from all stations.

.quality of mined coal and mine stability Brady and Haramy, 1994 . These methods

typically rely on visual estimates of fracture intensity, but recent advances in automated

imaging methods and image analysis promise to make these approaches more quantita- .tive Djahanguiri et al., 1994 . Some of these mapping methods possibly could be

adapted for use in describing core from coal-gas reservoirs or from outcrop analogs.

A variety of factors have been cited as affecting cleat development. These factorsinclude coal rank, coal composition, and layer thickness. To the extent that the impact of

these factors on cleating has been explored quantitatively, average cleat spacing has

been used to characterize the cleats. Other factors, such as mineral fill, degree of

tectonic and compactional deformation, and coal age, have received little to no attention.

Researchers have observed that cleat spacing varies with coal rank, decreasing fromlignite through medium volatile bituminous coal Ammosov and Eremin, 1963; Ting,

.1977; Law, 1993 , and increasing through anthracite coals, forming a bell-shaped

distribution of cleat spacing. Based on outcrop and core data from North American coal, .Law 1993 found that face cleat spacing ranges from approximately 22 cm in lignites

. . R vitrinite reflectance values of 0.25 0.38% to 0.2 cm in anthracites R valueso o.more than 2.6% . The best fit to the data is an inverse exponential equation:

ss0.473P10 0.398rR o 3 .

.where s is cleat spacing in centimeters. Eq. 3 predicts a similar decrease in cleat

spacing with increasing rank from the lignite to medium volatile bituminous, but alsoindicates that spacing remains constant at vitrinite reflectance values above 1.5% Fig.

.8 . However, only two data points from low-volatile bituminous and higher-rank coals

define the trend, so rank-dependence of cleat spacing remains uncertain.Variation in cleat spacing with rank may reflect competing processes of fracture

.formation and annealing Levine, 1993 . Bedding-parallel compaction fabrics and

deformation or flow of coal around rigid grains shows that flattening predates fracturing


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Fig. 8. Cross-plot of vitrinite reflectance versus cleat spacing for coals ranging from lignite to anthracite . .vitrinite reflectance of 0.25 to 2.6q % . Mean face-cleat spacing. Adapted from Law 1993 . Data were

collected only from bright coal lithotype layers. Ash content varies among coals and affects cleat spacing,

however ash content was not reported.

in most coals. Highly strained coals at high rank deform predominantly by flow ratherthan fracture. Because tectonism commonly accompanies high thermal maturation, it is

reasonable that tectonism may obliterate previously formed cleats in many anthracites

apparently barren of cleats. It is an open question whether annealing processes are an

unrecognized but important process modifying fracture patterns in low-rank coal. If so,

regular variation in cleat spacing with rank within the lignite to high-rank bituminous

range might not be expected.

Many authors have noted that cleat spacing varies with coal type and ash content .Spears and Caswell, 1986; Tremain et al., 1991; Law, 1993 . Bright coal lithotypes

. .vitrain generally have smaller cleat spacings than do dull coal lithotypes durain .Kendall and Briggs, 1933; Stach et al., 1982 . Coals with low ash content tend to have

smaller cleat spacings than do coals with high ash content. Organic-rich shales also

commonly have closely spaced fractures that resemble cleats. This suggests that

geochemical processes such as shrinkage related to coal composition are key to intense

development of fractures.

The effect of compositional layering on fracture spacing has received much attention.

Examples where average spacing is linearly proportional to coal lithotype-layer thick-ness have been found Spears and Caswell, 1986; Grout, 1991; Tremain et al., 1991;

. .Close and Mavor, 1991; Law, 1993 . Daniels et al. 1996 , in contrast, found no patternof cleat spacing with bed thickness. If we interpret cleat height as equal to lithotype

.layer thickness t , and assume a linear aperture-spacing relation the result is a linear

proportionality between cleat spacing and lithotype layer thickness. Equating cleat


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. .height and lithotype layer thickness again, the combination of Eqs. 1 and 2 implies

that lithotype layer thicknesses follow a power law size distribution:

Nsqtyr 4 .

where Nis cumulative number of lithotype layers having a thickness of tor larger, q isa measure of the total sequence thickness, and r is fractal dimension. Coal lithotype

.layer thickness data collected by Smyth and Buckley 1993 follow a power law size

distribution. The empirical fit to these data has a fractal dimension significantly greater

than one, implying that thin layers account for more coal volume than thick layers.

Comparisons of cleat spacings versus bed thickness have typically not accounted for

the range of cleat sizes present. For example, Upper Cretaceous Fruitland coal beds have .numerous interbedded ash layers tonsteins that define coal bed thicknesses. A cleat

spacing-bed thickness relation is apparent if cleats that span the coal layer are used and

shorter cleats are neglected, and then only for beds that are thinner than 10 to 20 cm. For .these, cleat spacing is slightly less than layer thickness Tremain et al., 1991 . Other coal

beds in the western United States for example, Upper Cretaceous Adaville Formation.coal beds at the Elkol mine, Wyoming have a spectrum of cleat sizes and spacings that

resemble those of Fruitland coal beds, but these coal seams lack thin interbeds. Similarhierarchies of fractures in massive beds have been observed in non-coal rocks Nelson,

. 1985 and in cooling cracks in glass Nemat-Nasser and Oranratnachai, 1979; DeGraff.and Aydin, 1993 . This implies that the apparent fracture spacingrbed thickness

relationship in Fruitland coals may be an illusion.

Such observations suggest that, except perhaps for the thinnest beds, interbed-layerthickness is not a primary control on cleat intensity. However, a possible explanation for

hierarchical spacing without obvious layer boundaries may be found in the mechanical

similarity of cooling, contracting lava flows and dehydrating, contracting coals. DeGraff .and Aydin 1993 show that cooling rate strongly affects fracture propagation, resulting

in closer spaced fractures for higher cooling rates. Possibly variations in contraction rate .of coal controlled by chemical processes may govern the spacing of cleats of a given

size.

Even cleat spacing is apparent in many coal beds. Although they have been reported

by mine operators, few distinct cleat swarms zones of anomalously closely spacedfractures have been described. This may partly result from lack of coal exposures

.where intense localized fracture development can be seen Fig. 9 . Some swarms are .associated with faults Shepherd et al., 1981; Laubach et al., 1991 and locally these

features are evident in seismic records. This is an area where more information is

needed, since these structures potentially are a cause of highly productive areas within

coal beds.

Finally, indirect quantification of cleat spacing comes from data on sizes of mined .coal fragments Bennett, 1936; Turcotte, 1992 . Mass fractions of mined coal fragments

.as a function of fragment size determined by sieve analysis are consistent among theseams and collieries analyzed 19 data sets, from United Kingdom coals of unknown. .rank, composition, and age , and form lines on a loglog plot Fig. 10 . If we assume

that coal fragments in a data set have statistically similar shapes and mass densities, then


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Fig. 9. Cleat patterns and potential sources of anomalous cleat attributes: fracture swarms and differential

compaction zones. Diagram shows schematic fracture patterns in coal.

we can convert mass fraction data into cumulative numbers R. Marrett, 1994, unpubl.

.rept. and characterize the size distribution of fragments. The spacing of bounding cleatsprobably define the size of an individual fragment, so we can interpret the converted

.Fig. 10. Coal fragment size distribution from sieve analysis of coals in Colliery B Bennett, 1936 . Data .represent volumetric 3D sampling of mass fraction having linear dimension less than plotted value. For

clarity, 0.5 was added to log mass fractionrnumber values for coal seam 1 and 0.5 was subtracted from log

mass fractionrnumber values for coal seam 5.


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data as a volumetric sample of the cleat spacing size distribution. This interpretation

suggests that cleat spacing follows a power law of the form:

Nsmsyd 5 .

where Nis volumetric cumulative number of spacings of s or larger, m is a measure of

coal volume and cleat frequency, and d is the fractal dimension. Best fit values of d .range from 2.31 to 2.34. Eq. 5 plus a linear aperture-spacing relation suggests that

csd; however this is not true of the values determined. Although there are a variety of

possible explanations, the most likely is that the assumption involved in averaging

observed average spacings is incorrect.

2.3. Cleat network geometry and connectiity

If all fractures in a coal bed were isolated, then flow rates observed after draining.those fractures directly intersected by the wellbore would be limited by matrix

.permeability i.e., no fracture enhancement of permeability . Network geometry and

connectivity of fractures in a system clearly are important to the permeability enhance- .ment that fractures can provide Fig. 9 . For example, coal-bed permeability may be 3 to

10 times greater in the face cleat direction than in other directions McCulloch et al.,.1974 , reflecting the strong preferred orientation and greater length of interconnected

fractures in that direction. On a local scale, cleat connectivity results from cross cutting

and abutting relations. Maps of cleat patterns show that connections within the networkmay be accomplished by fractures of vastly different size Tremain et al., 1991; Chen

.and Harpalani, 1995 . Vertical connectivity of cleat networks is commonly limited by

the termination of small cleats at interfaces between coal types, and large cleats at

coal-non-coal bed interfaces.

One approach to cleat network characterization is to count the various types of . .fracture ends i.e., connected, constricted, or dead-end Laubach et al., 1991 . Con-

stricted and dead-end terminations render fractures insignificant to permeability en-

hancement, although they might assist flow from the matrix into the fracture network.

Analysis of relative frequencies of different fracture-end types was done for two .exposed bedding surfaces of coals in the San Juan Basin Laubach et al., 1991 . One

coal-bed surface showed the expected result that cleat systems are well connected, due

to systematic development of butt cleats extending from one face cleat to another. Theother bedding surface indicated poor connectivity among cleats, possibly because only

large cleats were considered. Uncertainty about timing and origin of butt cleats which.may in some case be restricted to near-surface locations clearly will impact interpreta-

tion of fracture connectivity in coal beds at depth. Cross-cutting relations and contrasts

in orientation suggest that not all cleats in a given bed formed simultaneously.

2.4. Cleat petrology

Cleat apertures may or may not contain authigenic minerals commonly clays, quartz,. and calcite or organic material and resin Spears and Caswell, 1986; Daniels and

.Altaner, 1990 . In coal mining, such minerals have an impact on coal quality, but for

coalbed methane, diagenetic alteration of the cleat network due to precipitation of


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authigenic cements can occlude or preserve fracture porosity, and thus the ability of

fractures to conduct fluid. This is an area that has been relatively little studied. Minerals

in coal cleats have been described by several authors, including Macrae and Lawson . . . .1954 , Hatch et al. 1976 , Cobb 1979 , Hanes and Shepherd 1981 , Spears and

. .Caswell 1986 , Daniels et al. 1996 .

In several coal beds, paragenesis of cleat-filling minerals indicates that the develop-ment of fractures took place during coalification and progressive. Spears and Caswell .1986 showed regional and stratigraphic variations in cleat-fill mineralogy and devel-

oped a paragenetic sequence of mineral precipitation for Westphalian A Canock coals inthe United Kingdom. The first minerals to precipitate were sulfides pyrite, sphalerite

. and galena followed by quartz and clay minerals, then carbonate minerals calcite and. .ankerite . Hatch et al. 1976 found a similar paragenetic sequence in Pennsylvanian

coals in the Illinois Basin. The presence of multiple mineral phases indicates that cleats .remained open permeable over extended periods of time or that they were episodically

reopened, and that the composition of fluid migrating through the cleats was variable.

Fig. 11. Patterns of cleat minerals in Cretaceous Frontier Formation coals, western Wyoming. All diagrams . .show plan-view relations. a Blocky calcite and quartz on opposite fracture walls. b Two generations of .calcite in face and butt cleat, respectively. c Early-formed quartz along face cleat separated by later crossing

.butt cleat having calcite fill. d Intergrowths of early quartz and late calicte, recording progressive cleat

porosity reduction.


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.Crack-seal features in cleats Hatch et al., 1976 imply episodic fracture opening.

Locally, the mineralogy of coal cleats mimics that of nearby fractures in sandstones. For

example, calcite and quartz in face and butt cleats correspond to authigenic mineral .suites in adjacent Cretaceous sandstones in parts of Texas and Wyoming Fig. 11 .

.Levine 1993 demonstrated that cleat-filling calcite in Lower Pennsylvanian coals

precipitated early during coalification. Precipitation of calcite or other minerals in cleatsmay preserve the only evidence of early formed cleats because minerals inhibit

annealing during subsequent coalification. In the San Juan Basin, calcite precipitation

probably occurred after coalification and subsequent uplift and erosion along basin

margins. Bacteria introduced into Fruitland coal beds by meteoric water metabolized wet

gases and organic compounds in the coal to generate secondary biogenic methane and .carbon dioxide Scott et al., 1994 . Formation waters in the cleat system became

saturated to oversaturated with respect to calcite resulting in precipitation of isotopically .heavy calcite in cleats Scott et al., 1991 . Cleat development probably occurs at least in

part during early coalification, thus cleats in some coals may be filled with organic . .matter during late coalification Crelling et al., 1982 . Rice et al. 1989 reported that

.exsudatinite a hydrogen-rich material , generated during coalification, filled pores or

voids within the vitrain component of Fruitland coal beds. Patterns of regional and

stratigraphic variations in cleat-filling minerals could therefore help identify areas of

differing cleat openness and target the most conductive parts of fracture systems.

2.5. Local ariations in cleat style

In addition to size, intensity, and diagenetic variations, cleat permeability could beaffected by shifts in fracture style; that is, changes from opening-mode fractures to

faults. Faults are well recognized in coal beds, and some coal beds have pervasive small .faults Ammosov and Eremin, 1963 . Such faults may post-date cleat formation,

although faults that accompany early compaction are known Law, 1976; Tremain et al.,.1991 . Moreover, cleat intensity and size locally vary with proximity to faults and

.position on folds Schultz-Ela and Yeh, 1992 so abrupt variations in cleat style, as well

as intensity, could be anticipated in these settings.

Coal exposures in Upper Cretaceous Rock Springs Formation in southwest Wyoming

exhibit significant variations in cleat style over distances of a few to tens of meters . .Laubach et al., 1993 Fig. 12 . Closely spaced normal faults abruptly substitute for

opening-mode fractures in coal beneath some sandstone lenses having blocky lateral

terminations. Because these faults have little or no porosity, the coal that contains them

has low permeability compared to coal having generally porous cleats. These changes infracture style can be explained by differential, late-stage compaction Laubach et al.,

. .1993 Figs. 1214 . Most differential compaction occurs by ductile flow beforeconsolidationrcoalification advances enough for fracture to occur perhaps largely

during deposition of units immediately overlying peatrcoal beds. It is late-stage

compaction of a few percent that occurs from shallow burial to depths of;1000 m ormore that can influence cleat development.

.In floodbasin settings such as this example, compaction is greatest in peat 10:1 and .mud and least in channel-belt sandstones 1.5:1 . Sand deposition in any given vertical


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Fig. 12. Cross section and structural data showing shifts in cleat style, from opening-mode fractures to faults,beneath a sandstone lense in Mesaverde Group, Cretaceous, central Wyoming modified from Laubach et al.,

. .1993 . a Lower hemisphere equal-area projection of poles to face cleats and great-circle arc of bedding and

. .average cleat orientation. b Same projection, showing fault attitudes in area beneath lense. c Average

attitudes for pole to bedding, pb; trace cleat, tc; trace bedding, tb; pole to normal faults, nf; pole to cleats, fa.

sequence may be concentrated over compacting floodbasin andror thin channel margin

deposits, producing stacked, lenticular sandstonecoal sequences. During subsequent

burial loading-late stage compaction-relatively rigid sandstone lenses can distort adja-

cent coal beds, creating folds.

Changes in fracture style due to stresses caused by the shape of overlying andunderlying strong rocks, and perhaps even the evolving shape of the basin itself, has to

date not been thoroughly investigated. It is interesting to note that shifts in cleat

orientations in the San Juan Basin that have been ascribed to separate episodes of .tectonism see below; Laubach and Tremain, 1991 also correspond to changes in

sandstone trends.

Fig. 13. Geometry of coal compaction where coal is interbedded with lenticular, and more rigid, sandstone .modified from Laubach et al., 1993 . Strain during compaction can be resolved into pure shear and simple

. .shear components, determined by layer dip q and compaction ratio R . t, early stage thickness; trR,

late-stage thickness. Ellipses show strain at an early and late stages of compaction.


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Fig. 14. Finite-element model results after differential compaction of sandstone and coal beds due to 1 km of

overburden. Shear stress and differential stress are augmented in coal beds below abruptly tapering edges ofsandstone lenses, favoring fault development, whereas under gradually tapering lenses shear stresses are not

.sufficiently enhanced to cause shifts in fracture style. a Rotation of principal stress axes beneath edge of lens . .during elastic deformation. Shear stress is also high everywhere beneath lens not shown . b Strain due to

.nonrecoverable plastic yield equivalent plastic strain . Left-dipping contours are consistent with failure on

left-dipping faults. Model by D. Schultz-Ela.

2.6. Regional cleat orientation patterns

In some respects, cleat patterns on a basin scale are better known than those of

fractures in non-coal rock types, mainly because the dominant cleat set in an outcrop can

generally be readily identified. Thus, regional maps of cleat orientation have sharply


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Fig. 15. Map showing face-cleat domains and domain boundaries in Pennsylvanian and Permian coals in West .Virginia. Modified from Kulander and Dean 1993 .

distinguished domains of uniform and variable cleat strike Shepherd et al., 1981;.Kulander and Dean, 1990, 1993, Laubach and Tremain, 1991; Laubach et al., 1992

.Figs. 1517 . The uniformity of cleat orientations over wide areas was one of thefeatures that caught the attention of observers as early as the 1800s see Raistrick and

.Marshall, 1939 . Regions hundreds of square kilometers in area have uniform cleat .orientations Ver Steeg, 1942; Kulander and Dean, 1990; Laubach et al., 1992 .

However, dominant cleat strike may also shift abruptly from one coal seam to the next .within an area as small as a single colliery Dron, 1925 . Domains of uniform strike

having a range of sizes, and zones of transition between them, can impede or channelize

fluid flow in a cleat system. Such orientation patterns played a prominent role in guiding .speculation on the origins of cleat Dron, 1925; McCulloch et al., 1974; Ting, 1977 and

remain a key attribute to be explained.

Conflicting interpretations of cleat domains have been proposed. They may representsimultaneous development of cleat in areas where stress orientations differ Kulander

.and Dean, 1993 or multiple, superposed episodes of cleat development associated with

rotation of stress directions, perhaps coupled with coals entering burial conditions that .favor cleat formation at different times Laubach et al., 1992 . Uniformity of cleat

strikes within domains indicates that fractures responded to regionally coherent stress

patterns, which could reflect plate-scale stresses or stresses related to uplift or basin


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Fig. 16. Structure map of San Juan Basin contoured on Huerfanito bentonite, face-cleat strikes in Fruitland

Formation coal beds, and cleat-strike domains. Domain boundaries are gradational and approximately located. .BArIA is BondadIgancio anticline. Modified from Laubach and Tremain 1991 .

geometry. On basin scales, transitions between domains of uniform cleat strike range

from gradual to abrupt. In some cases, domains are associated with specific folds or

faults. For example, alignment of cleats normal to fold hinges and fault traces was . . .pointed out by Freiser 1914 , Ver Steeg 1942 , Nickelsen and Hough 1967 , and

.Close 1993 . However, fractures that maintain uniform strike across changing fold .trends may predate folding Kulander et al., 1980; Kulander and Dean, 1993 .

The San Juan Basin provides an example of a transition zone between domains of .differing face-cleat strikes in Cretaceous Fruitland coals Fig. 16 . These are separated

by an east-trending transition zone and can be traced in the basin with data from

oriented coal cores. Interestingly, this zone is coincident with the so-called coalbed

methane high productivity fairway. In the northern part of the basin, northwest-striking

cleats predominate, but in the south, northnortheast- and northeast-striking cleats are

most common. The transitional boundary between domains is a wide zone having

inconsistent and variable face-cleat strike. Both northwest and northeast face cleatstrikes are evident in this area, and coal beds with mutual abutting and crossing cleat

patterns are prominent. Cleats with opposite abutting relations occur in adjacent beds,

and less commonly in adjacent parts of the same bed.


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Fig. 17. Map of cleat system in the Piceance Basin, Colorado.

Although convincing mechanical models for development of cleat domains have not

yet been tested against well defined patterns and well-dated cleat sets, the Piceance

Basin has evidence of progressive cleat domain formation. A domain of eastnortheast-

striking face cleat in the south in Cretaceous Mesaverde Group coals is replaced in the .northern part of the basin by westnorthwest-striking cleat Fig. 17 . Thermal history

analysis shows that the northern coals reached higher thermal maturity, and possibly the

threshold of initial cleat development at vitrinite reflectance values between 0.3 and .0.5%, later than coals in the south Scott et al., 1996 . Based on kinetic modeling,

.Mesaverde peats over most of the Piceance Basin reached the lignite stage R s0.3%o .at approximately the same time 61.7"1.4 my, Table 1 . However, it took longer for

organic matter to reach vitrinite reflectance values of 0.5% in the north than in the south.

Coalification in the southern basin occurred more rapidly and 0.5% vitrinite reflectance


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Table 1

Summary of geochemical processes and cleat development during coalification

.Volatile components Vitrinite reflectance %

aCompactional water loss 0.2 to 0.4bMaximum rate of structural water loss 0.3 to 0.5

b

Maximum rate of carbon dioxide loss 0.4 to 0.7bMaximum wet gasrcondensate generation 0.5 to 1.1

b,cMaximum methane generation 0.8 to 1.5

Initial cleat development 0.3 to 0.5

Cleat development 0.3 to 2.0q

Cleat annealing 0.3 to 2.0q

a .Law 1993 .b .Burnham and Sweeney 1989 .c .Tang et al. 1991 .

was reached approximately 50 Ma, or 22 million years after deposition. Coalification in

the north proceeded more slowly and 0.5% vitrinite reflectance was not attained until 41

Ma, or approximately 31 million years after deposition. .If cleat development occurred during the peat to lignite transition R s0.3% , theno

heterogeneous directions of paleo-maximum horizontal stress westnorthwest and.eastnortheast were active in the basin at this time. However, if cleat development

occurred at 0.5% vitrinite reflectance, the two separate cleat domains could represent a

temporal shift in homogeneous paleostress directions. Evidence for a temporal shift

includes different face-cleat orientations in Cretaceous and Tertiary coals in the samearea.

3. Cleat origins

Cleats without any observable shear offset are opening-mode fractures, allowing

allows us to relate cleat orientation to past stress fields. However, driving mechanisms

responsible for controlling stress fields and for causing cleat formation are debatable. A

central question is whether cleats form during burial, at depth after lithification, or

during uplift. More specifically, we could ask the following questions: Are cleats theresult of contraction of coal beds during dewatering or chemical metamorphosis? Are

they caused by thermoelastic response of coal to regional tectonic extension andror

uplift? Are cleats merely a response to local stress perturbation caused by folding and

faulting? Is elevated pore pressure the key parameter, due to either water expelled from

coal structure or perhaps hydrocarbon gas generation? We cannot answer all of these

questions, but we can shed some light on the issues and point to observations that may

help resolve the problem of the origin of cleats.

3.1. Opening-mode fracture mechanics

At the turn of the century, the origin of cleats and joints was the subject of muchcontention Dron, 1925; Kendall and Briggs, 1933; McCulloch et al., 1974; Ting, 1977;

.Pollard and Aydin, 1988 . One aspect of this debate was whether these fractures were of


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shear origin or not. Although it was recognized that fractures could form in rock due to

tension, this type of fracture was considered to be limited to the near surface as a result

of folding, cooling, and contraction of igneous rocks, dehydration, or shear zone .formation Engelder, 1987 . Such fractures were not expected to show the planar,

sharp-sided aspect, and regionally persistent orientations that typified fractures in coals

.Kendall and Briggs, 1933 . It was thought that since tensile stress was unlikely to existat depth, all fractures at depth had to be of shear origin.

.Secor 1965 helped resolve this controversy by showing that the concept of effective .stress as presented by Hubbert and Rubey 1959 could account for opening-mode

fracture development in a compressive stress state if the pore pressure was sufficiently

high. However, debate persisted as some continued to interpret many joints as shear .fractures due to their conjugate geometry in relation to other fractures Hancock, 1985 .

Later work showed that plumose structure on fracture surfaces and lack of shear offsetprecluded a shear origin for many conjugate fractures Barton, 1983; Dyer, 1983;

.Nickelsen and Hough, 1967; Pollard and Aydin, 1988 .Cleats primarily accommodating opening displacement propagate along a plane of

zero shear stress, specifically the plane perpendicular to the least compressive principal .stress Lawn and Wilshaw, 1975; Pollard and Aydin, 1988 . This makes such fractures

indicators of past stress orientations, where vertical cleats include the maximum

horizontal stress direction at the time of their formation. Conditions under which cleats .or other opening-mode fractures form can best be described using fracture mechanics.

.K is called the opening mode or mode I stress intensity factor and measures theImagnitude of the stress concentration at the crack tip. For a vertical, uniformly loaded,

.planar crack, whose length is much greater than its height Fig. 18 , it takes on a simplerelationship:

1r2KsDs pPhr2 6 . .I

where Ds is termed the driving stress and h is the height of the crack Lawn and.Wilshaw, 1975 . When K exceeds a critical value, K , the fracture will propagate. ForI Ic

Fig. 18. Pore pressure effects promoting a fracture. See text for explanation.


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stress concentration to occur, there must be crack-opening displacement, and opening

displacement requires a positive driving stress. Driving stress is defined as:

Dss pys 7 . .Hmin .where s is the minimum in situ stress compression is positive and p is magnitudeHmin

of pore pressure acting inside the fracture. Driving stress can be positive under twoconditions-the local minimum stress acting on the fracture is tensile or the pore pressure

in the fracture exceeds the minimum stress. Absolute tension may be possible at or near

the surface, but for fracturing at depth, where even the minimum stress is compressive,

there must be some contribution from pore pressure.

Most subsurface rocks experience a negative driving stress most of the time. The

special situation of a positive driving stress at depth can come about under three basic

conditions pore pressure increases to exceed minimum stress, minimum stress

decreases to fall below the magnitude of the pore pressure, or a combination of both.

This is a limited set of conditions, but there are numerous processes that can beresponsible for the occurrence of these conditions. Some processes that might cause pore

pressure increase in coals are compaction, dehydration, and devolatilization Ting,.1977 . Mechanisms that cause a decrease in minimum stress are uplift, folding, cooling,

and either dehydration or devolatilization. All of these factors could potentially induce

vertical cleats.

Another result of analyzing cleats with fracture mechanics is that a mechanical

maximum for cleat aperture can be estimated. Assuming a coal behaves approximately

elastically when fracturing occurs, maximum aperture, e, for cleat should depend on

fracture height, driving stress, and coal elastic properties according to this relationship .Pollard and Segall, 1987 :

e s4 1yn2 Ds hr2 rE 8 . . .max

where E is Youngs modulus. Thus aperture should be linearly proportional to driving

stress and fracture height, and inversely proportional to rock stiffness as measured by E.

The process by which fractures propagate under the influence of pore pressure has .been termed natural hydraulic fracturing Engelder and Lacazette, 1990 , and there has

been controversy over how applicable this mechanism really is. Pore pressure and in situ

stress are not independent variables, due to poroelastic effects in rock. So care must be

taken in determining stress and pore pressure conditions under which opening-mode

fracture propagation occurs. However, by properly combining the driving stress equation

with an expression for minimum in situ stress, we can make generalizations about

conditions that might be necessary for cleat propagation. If we follow the approach of .Engelder and Lacazette 1990 , we can show that natural hydraulic fracturing is possible

for low pore pressure relative to overburden stress.

For illustrative purposes, and in recognition that the critical stress intensity factor canbe small under geologic conditions for saturated rocks Atkinson and Meredith, 1987;

.Segall, 1984; Olson, 1993 , we will assume a positive driving stress is sufficient for

crack growth. A more complete treatment of the problem can be found in Engelder and .Lacazette 1990 . For a positive driving stress, the following condition is necessary:

p)s 9 .min


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The expression for minimum stress due to gravitational loading only is:

s snr 1yn s yap qap 10 . . .min overburden

where n is Poissons Ratio, a is Biots poroelastic constant, and s is verticaloverburden

stress from overburden. Combining these two equations gives an expression for porepressure required for fracturing in terms of material properties and overburden stress

required for positive driving stress:

p) ns r 1ynq2n aya 11 . . .overburden

.Plotting this expression Fig. 18 shows that fracturing may require pore pressure

from as small as 0.1 times the overburden for ns0.1 and as0 to equal the overburden

stress for any Poisson Ratio and as1. Typical numbers for rock are ns0.2 and

as0.6, which would predict a pore pressure of 0.5 times overburden for fracturing to

occur, which is just slightly over hydrostatic in most basins, without reducing minimum

stress due to tectonics or coal shrinkage during compaction. If we include the other

effects that can reduce minimum stress, fracture mechanics relations predict fracturing at

sub-hydrostatic pore pressures, which imply that all coals should be fractured to some

extent.

As with fractures in non-coal rocks, cleats may vary in orientation in the vicinity of

other fractures or structures. Local fracture hooking or curving may occur due to .mechanical interaction of overlapping, en echelon cleat segments Pollard et al., 1982 .

Crack-path hooking is expected to be stronger when in situ stress is nearly isotropic and

inhibited when there is a strong differential compressive stress in the plane of the .fracture Olson and Pollard, 1989 . Patterns should exhibit less curving in thin rock

layers due to a diminished stress perturbation caused by three-dimensional effects .Olson, 1993 . Surface roughness of the fracture might also inhibit crack path hooking.

The overlapping segments of face-cleat traces are commonly straight and parallel rather

than hooked and curved even in thick coal beds, suggesting that significant crack-paral-

lel differential stress was present during fracture propagation.

3.2. Cleat formation

The ubiquity of cleats in a coal suggests that they result from processes common to

all seams. Regularly invoked causes for the formation of cleats are the extremely large .compaction and desiccation of peat coalification during burial relative to these

.processes in other sediments Ting, 1977 . Dehydration and associated shrinkage of

carbonaceous material during coalification create stresses that allow fractures to form.

.Fig. 19. Numerical model GEOSIM-2D of maximum and minimum horizontal stress trajectories in western .United States for late Tertiary Laramide deformation. Shaded boxes show basement uplifts that are eight

times stiffer than surrounding elastic medium. Model assumes 1 km northeast-directed shortening applied to . . .left overthrust side of model. a Pattern for entire region. b Part of model near Wyoming salient,

.comparing stress trajectories to measured face-cleat strikes solid dark lines in Cretaceous and Tertiary coals.


25/33



26/33


However, contraction alone is insufficient to account for the strong preferred orientation

of cleats over wide areas, since shrinkage tends to produce polygonal patterns like .mud-cracks or cooling cracks in lava flows Kendall and Briggs, 1933 . Preferred

fracture orientations suggest the existence of consistent differences in principal stresses

over wide areas. This suggests that the coal has been buried below the near surface and

has acquired strength in order to sustain the differential stress required for the regular, .planar geometry to the cleats Olson and Pollard, 1989 .

Tectonics can impart a significantly anisotropic stress field, and many workers have

attributed uniform patterns of cleat strike over large areas to the effect of tectonicallycontrolled stress orientation on fracture growth direction Kaiser, 1908; Freiser, 1914;

McCulloch et al., 1974; Price, 1966; Ting, 1977; Laubach and Tremain, 1991; Laubach.et al., 1992; Kulander and Dean, 1993; Close, 1993 . Cleats can then be used as

indicators of the kinematics of past tectonic events, or conversely, knowledge of tectonic

history in a basin or region can be used to predict cleat orientation. Recent examples of

. .this approach include Laubach et al. 1992 and Kulander and Dean 1993 . Fig. 19shows a comparison of face-cleat strikes in Cretaceous coal beds in the western United

.States to stress trajectories predicted by a numerical model Laubach et al., 1994 .

Agreement between prediction and observation is exact in some areas, but highly

divergent elsewhere.

Such models are obviously simplifications of the tectonic loading history of a region.

Plate-scale anisotropic stress fields exist for long periods, and do not require tectonic

events, as conventionally understood, for their formation. Yet a principal cause of

discrepancy between such models and measured cleat orientations is uncertainty in

.timing of cleat formation and thus the identity of relevant tectonic stress field s towhich cleat patterns should be compared. Timing of cleat formation is challenging to

specify precisely, and thus strong tests of this mechanism of cleat formation remain

elusive. If cleat patterns reflected a simple scenario of compaction and dewatering

without fracture as burial increases, followed by an abrupt change in brittleness and

fracture toughness properties at maximum burial and maturity, a plausible model would

be fracture due to a combination of pore pressure and minimum stress reduction caused .by exhumation Price, 1966 . Stress trajectories recorded by cleat patterns would be

those existing at uplift through some critical threshold.

Exhumation probably accounts at least in some instances for formation of buttcleats, which are typically at right angles to face cleats, and are analogous to cross-joints

.as described by Engelder 1987 . Uplift and erosion increases driving stress for fracture

propagation due to a decrease in all of stress components. Pre-existing face cleats tend to

relieve induced stresses perpendicular to them and inhibit growth of additional parallel

fractures. However, stresses parallel to face cleats are not relieved, and these stresses

promote growth of secondary fractures perpendicular to pre-existing face cleats. Face

cleats that lack mineral fill will be open in response to stress changes caused by uplift

and will act as free surfaces against which younger butt cleats end.

However, in addition to unloading effects, coal experiences significant shrinkage dueto progressive devolatilization reactions, so uplift is not required to drive cleat forma-

tion. Moreover, core studies show that face and butt cleats exist at depth and are thus not

entirely due to uplift and unloading. Mineral suites in some cleats suggest formation


27/33


under conditions of increasing temperature and progressive burial Spears and Caswell,.1986 . Many cleat sets in outcrops of folded rocks evidently formed prior to folding

.Tremain et al., 1991 . Confirming that cleats can form in basins that have not had

episodes of uplift would require drilling in a suitable location such as the deeply buried

Tertiary coals of the ancestral Mississippi delta.

Time-temperature modeling, based on burial and thermal history, can be used toestimate the time required to reach the thermal maturity at which cleat genesis is most

likely if desiccationrdevolatilization occurs over a given vitrinite reflectance or rank .range Scott et al., 1996 , because coals undergo systematic chemical changes during

coalification that are marked by peaks in shrinkage and expulsion of volatiles. Although

moisture andror volatile matter loss could contribute to coal shrinkage, it is the

rearrangement of coal structure that is responsible for most shrinkage that could

contribute to cleat development. Loss of inherent moisture, which is free moisture

associated with micropores and capillaries andror physically sorbed to organic com-

pounds, results in significant volume shrinkage during the peat to lignite transition.However, these early formed shrinkage cracks close through annealing or repolymeriza-

.tion Levine, 1993 . During progressive coalification, cleavage of cross-linked, oxygen-

bearing functional groups in the coal structure results in additional shrinkage and cleat .development. Based on kinetic data from Burnham and Sweeney 1989 , the rate of

moisture loss increases significantly between vitrinite reflectance values of 0.3 and 0.5%

and reaches a maximum rate at 0.5%.

Based on when maximum shrinkage occurs, early cleat development probably occurs

between vitrinite reflectance values between approximately 0.3 and 0.5% or the peat

through subbituminous B coal. This is consistent with field observation of cleats in .lignites Law, 1993 . Although initially formed cleats at vitrinite reflectance values

around 0.3% may become annealed, progressive desiccation at vitrinite reflectance

values between 0.4 and 0.5% in the presence of either local or regional stresses

represents potential for fracture formation due to shrinkage. Thus, using burial history

and thermal modeling to identify when peat reaches vitrinite reflectance values of 0.3 to

0.5% gives an estimate of the time of cleat development. Petrologic study and

radiometric dating of minerals within cleats is the most reliable approach to testing such .interpretations Daniels and Altaner, 1990; Daniels et al., 1996 .

Compaction takes place during progressive coalification, but fracturing may only beassociated with latter stages of coalification, perhaps after nearly all compaction has

occurred. Few, if any, fractures are formed during peat compaction and associated loss

of non-structural adherent moisture. Shrinkage during the peat to lignite transition

involves loss of inherent moisture from micropores and capillaries and that physically

sorbed to organic compounds. Potentially, this could lead to fracture formation. Later

stages of coalification involve cleavage of cross-linked, oxygen-bearing functional

groups and additional shrinkage.

3.3. Annealing

.The possibility that cleats, once formed, could be destroyed annealed has not been

widely investigated, although annealing has been called on to explain the lack of


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fractures in some anthracites. Yet in the structural evolution of a complex organic .compound coal under the influence of burial and thermal histories, annealing poten-

tially plays a role in governing resultant fracture patterns. The evidence for annealing

comes from considering the interplay of compaction and fracture formation. The large

amount of vertical shortening associated with the early stages of peat consolidation

.imply that early-formed structures such as early cleats become distorted with progres-sive compaction. Processes that promote fracturing, such as dehydration, devolatiliza-

tion, and regional and local tectonics, affect organic deposits throughout the lignite to

anthracite rank. Yet similar-appearing cleats are in coals of various ranks, including .lignite and low-rank bituminous coal Law, 1993 . In a typical coal bed, cleats appear

nearly perpendicular to bed boundaries and compaction fabric, and are themselves not

deformed. Although at this time we are aware of little or no direct maceral or

mineral-matter petrographic evidence for cleat annealing, if compaction overlaps to a

significant degree with fracturing, the planar character of cleats in lignites implies

.operation of processes that destroy anneal these early formed cleats in higher rank andmore compacted coals. The observation that face and butt cleats are nearly always planar

and are not visibly rotated or deformed indicates that they must have formed relatively

late in the compaction history of the coal bed.

An annealing process such as repolymerization can efficiently remove early formed .cleats Levine, 1993 . Quantitative work has been sparse on geochemical parameters that

govern both coal shrinkage, which can drive fracture formation, and ductile coal flow

and repolymerization, which might lead to fracture annealing. Cleat annealing occurs

when the coal structure is rearranged, and results in increased density, structural

anisotropy, and hardness. High-rank coals have undergone devolatilization so that coalshrinkage is a less likely driving force for fracture formation.

4. Cleat and coalbed methane

.Over 95% of the gas in coal is stored in micropores Gray, 1987 that are estimated to .have diameters ranging from 0.5 to 1 nm Van Krevelen, 1981 , values so small that the

matrix may have no effective permeability. Cleat-fracture porosity in coal is estimated to

be between 0.5 and as much as 2.5% Puri et al., 1991; Gash, 1992; Chen and Harpalani,.1995 . Although small amounts of free gas may exist in coal fracture systems, coalbed

methane is mainly adsorbed on the large internal surface area of the impermeable coal

matrix and fracture surfaces, and most produced methane may be adsorbed onto fracture

surfaces. Releasing adsorbed methane is accomplished by lowering reservoir pressure,

generally through removal of water and reducing hydrostatic pressure on the coal bed.

Therefore, coal beds generally must be partly dewatered to initiate gas production. After

desorption from coal, gases must diffuse through the coal matrix until encountering open

fractures. Interconnected, open fractures are conduits for gas and water flow to the

wellbore, and fracture surface arearcoal volume is critical to producible gas volume.For development of coalbed methane, important natural fracture attributes contribute

to permeability pathways for gas and water flow to wells. Also significant are fracture

properties that enhance or detract from coal stability and success of well completion and


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stimulation techniques such as horizontal drilling, open-hole cavitation, and hydraulic

fracture treatment. Horizontal wells and hydraulic fractures induced to enhance produc-

tion ideally should be oriented to maximize contact with the permeable natural fracture

system. For example, in United States Bureau of Mines degassification experiments in

shallow Appalachian Pennsylvanian coal beds, horizontal boreholes drilled perpendicu-

lar to face cleats yield 2 to 10 times the production rate of gas as holes drilled parallel toface cleat.

Changes in the fracture system during production are also important. Unlike more

conventional fractured reservoirs, during gas drainage from coal beds, absolute perme-

ability may vary by several orders of magnitude with changing effective stress, gas .pressure, and matrix shrinkage Sparks et al., 1995 . Key fracture attributes include size,

spacing, connectedness, aperture and degree of mineral fill, and patterns of preferred

orientation on local and regional scales. Progress in understanding cleat origins from

recent developments in fracture mechanics and chemical changes in the molecular

structure of coal during burial could lead to better predictions of these attributes inadvance of drilling.

Cleat intensity can affect coal stability and thus success of cavity stimulations.

Dynamic open-hole cavity stimulations involve high pressure build up by natural

accumulation of methane or air injection followed by sudden release of pressure,

inducing coal to cavitate into the wellbore. This procedure is successful in parts of the

San Juan Basin, but elsewhere in the basin conventional stimulation is more effective.

To date, cavity wells have not been highly effective in other basins worldwide. It isbeyond the scope of this paper to review cavity stimulation see Mavor and Logan,

.1994; Palmer et al., 1996 , but it is clear that strength of coal significantly influences the .spatial extent of permeability enhancement Choi and Wold, 1996 and that closely

spaced low-strength natural fractures can cause failure and deformation of coal over

long distances.

Areas of the San Juan Basin where open-hole cavity stimulations are most successful

coincide with a domain where two cleat sets overlap and interfere, and cleat is closely .spaced due to relatively high coal rank Laubach and Tremain, 1991 . This area of the

basin probably has higher overall fracture connectivity as a consequence of variable

cleat strike, changes in dominant cleat strike over short lateral distances and between

nearby beds, and greater abundance of small faults. Compared to other parts of thebasin, prevalence of two directions of strongly developed cleats in this transition zone

results in qualitatively greater coal friability. This intense fracture development may .enhance cavity formation Laubach and Tremain, 1991 . This example shows that,

together with other information, new cleat observations can help development of coalbed

methane resources.

5. Conclusions

Much quantitative observational work remains to be done on characterizing cleat size

patterns, network geometries, and the response of cleat systems to changing effective

stress conditions. Measurements of size distributions of apertures, in the context of


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rigorous scaling studies, can provide the basis for improved cleat porosity, permeability,

and elastic shear wave anisotropy calculations. A challenge will be obtaining reliable

data on these parameters at in situ conditions. Few data are available on apertures,

heights, lengths, connectivity, and the relation of cleat formation to diagenesis, charac-

teristics that probably are critical to permeability. Recent studies of cleat orientation

patterns and fracture style suggest that new investigations of even these well-studiedparameters can yield insight into coal permeability patterns.

More effective predictions of cleat patterns will come from advances in understand-

ing cleat origins. Although cleat formation has been attributed to a vague combination of

diagenetic and tectonic processes, a viable mechanical process for creating cleats has yet

to be demonstrated. Progress in this area may come from recent developments in

fracture mechanics and in appreciation of chemical changes in the molecular structure of

coal during burial and coalification.

Acknowledgements

We are grateful to Romeo Flores for inviting us to prepare this paper, to J. Close, R.

Flores, and B. Law for reviews for the International Journal of Coal Geology, and to W.

Ambrose for comments. S.E.L. and A.R.S. acknowledge collaborative studies with W.B.

Ayers, Jr., W.A. Ambrose, W.R. Kaiser, D.D. Schultz-Ela, C.M. Tremain, and R. Tyler.

R.A.M. thanks Amoco Production Co. for permission to publish work begun while

under their employment.

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