1. SPE-176960-MS
The Influence of Cleat Demineralisation on the Compressibility of Coal
Reydick D. Balucan, Luc G. Turner and Karen M. Steel, The University of Queensland
Copyright 2015, Society of Petroleum Engineers
This paper was prepared for presentation at the SPE Asia Pacific Unconventional Resources Conference and Exhibition held in Brisbane, Australia, 9–11 November
2015.
This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents
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written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words;
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Abstract
The natural fracture system of coal is the primary conduit for gas flow. Low permeability is in many
cases attributed to low fracture porosity and/or connectivity. Mineral occluded coal cleats are known to
considerably reduce coal permeability. Whilst cleat demineralisation has been found to increase coal
permeability, its influence on compressibility is poorly understood.
In this study, we investigated the influence of mineral dissolution and secondary mineralisation on the
compressibility of coal (𝐶𝑓) following acidification with hydrochloric and hydrofluoric acid (HCl-HF). In-
situ HCl core flooding and core immersion in 3% and 15% HF yielded a less compressible core (𝐶𝑓 =
0.006 from 0.020 bar-1
) with sustained, enhanced permeability to brine (k = 0.40 from 0.10 mD). We
attribute improved stress resilience and better fluid flow characteristics to the dissolution and
subsequent secondary mineralisation of the core’s circumferential periphery.
Predictive geochemical speciation using OLI Analyzer 9.1, for surveying mineral solubilities and
precipitation tendencies, identified the formation of radio-dense neofluoride salts K2SiF6 and CaF2.
Structural modifications and mineralogical changes detected from scanning electron microscopy-
electron diffraction spectroscopy (SEM-EDS), confirmed the presence of these salts. Results suggest
that mineral alteration and subsequent secondary mineralisation of the core periphery following HCl-HF
acidisation yielded well-formed crystalline salts, apparently serving as in-situ generated proppants
buttressing newly created void spaces for enhanced fluid flow and improved resistance to increasing
confining stress.
Introduction
Core demineralisation using HCl and HF
Fracture stimulation via cleat demineralisation has been considered for enhancing coal permeability. In
acidising sandstone formations, sequential acid floods consists of: a pre-flush with HCl, a main flush
with HCl-HF mixture and an over-flush with HCl (Williams et al., 1979, Guo et al., 2007). The pre-flush
enables carbonate dissolution thereby preventing the re-precipitation of calcium fluoride or other
unwanted products during clay dissolution. The pre-flush also displaces water from the wellbore and
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2. 2 SPE-176960-MS
near-wellbore region, minimising direct contact between the HF solution and sodium or potassium ions
thereby reducing formation damage by the precipitation of insoluble sodium or potassium fluorosilicates
(Williams et al., 1979). The HF-HCl mixture allows clay dissolution, whilst the HCl component maintains
low pH to further prevent HF reaction products. The over-flush with HCl removes spent HF and
prevents re-precipitation of fluoride compounds.
A similar sequential acid treatment for coal cores, as illustrated in Figure 1, could possibly be employed
for cleat demineralisation and permeability enhancement. The HCl-HF tandem may serve the purpose
of demineralising the coal of its carbonate and clay components, respectively. In our prior study of coal
demineralisation in HF and HCl (Steel et al., 2001), it has been shown that HF proved effective in
reducing the ash content of coal. Interestingly, HF concentration also does influence the reaction and
precipitation chemistries, the latter being more prevalent at higher concentrations.
Figure 1: Schematic of sequential cleat de-mineralisation treatment, (a) Pre-flush with HCl to enable carbonate
dissolution; (b) main-flush with HF/HCl to enable clay dissolution; (c) over-flush with HCl to remove spent HF and
prevent re-precipitation of fluoride compounds.
Cleat compressibility
The influence of effective stress on coal permeability is dependent upon a number of variables
including loading type (radial, biaxial or triaxial), cleat characteristics (aperture, orientation, tortuosity),
fluid type (adsorbing or non-adsorbing), coal rank and wettability. Cleat compressibility (𝐶𝑓) can be
calculated by deriving a relationship between stress and permeability, as was originally presented by
Seidle et al. (1992):
𝑘 = 𝑘0 exp (−3𝐶𝑓(𝜎 − 𝜎0)) (1)
where 𝑘 is the permeability, 𝐶𝑓 is the cleat compressibility and 𝜎 is the effective stress. The 0 subscript
refers to the condition at a reference state. By fitting Eq. 1 to laboratory measurements of permeability
at varying effective stress, 𝐶𝑓 can be calculated.
The removal of mineral occlusions in coal fractures, could increase effective cleat aperture (𝑏) and
alter𝐶𝑓. Figure 2 shows three possible modes by which 𝐶𝑓 and 𝑏 will vary following cleat de-
mineralisation: (1) 𝐶𝑓 increases and 𝑏 decreases, (2) constant 𝐶𝑓 and 𝑏, (3) 𝐶𝑓 decreases and 𝑏
increases. Knowledge of the influence of cleat de-mineralisation on compressibility is necessary when
predicting permeability change during primary production i.e. a significant increase in 𝐶𝑓 will reduce the
observed permeability gain through de-mineralisation, conversely a decrease in 𝐶𝑓 can further enhance
permeability. Proppant injection may be required to maintain newly developed fracture aperture during
primary production (Keshavarz et al., 2014a, Keshavarz et al., 2014b, Khanna et al., 2013).
HCl
HF/
HCl HCl
Carbonate
Clay
Flow direction
a b c
MATRIX
MATRIX
MATRIX
MATRIX
MATRIX
MATRIX

3. SPE-176960-MS 3
Figure 2: Possible modes of cleat aperture (𝑏) change following de-mineralisation, (1) 𝐶𝑓 increase and 𝑏
reduction, (2) no change in either 𝐶𝑓 or 𝑏, (3) 𝐶𝑓 and 𝑏 increase.
Whilst dissolution and removal of primary mineralisation could lead to either of possibilities described in
Figure 2, complete demineralisation may not be practically achievable. Compounding reactions such as
incomplete mineral dissolution and secondary mineralisation make predictions extremely complex,
producing results beyond the expected modes. Fundamental studies on the influence of mineral
alteration and secondary mineralisation on compressibility is therefore necessary to improve current
understanding and developing appropriate approaches to sustain or enhance permeability and
decrease compressibility.
Experimental
Core sample description and preparation
A whole core sample retrieved from a vertical borehole in the Bandanna formation of the Bowen Basin
(Australia) was used in this study. Further core sample details are shown in Table 1. Bulk volume and
core porosity were determined by liquid displacement and vacuum saturation respectively. The core
was encapsulated in a 3 mm thick polyurethane resin to fill in surface fractures and prevent fluid
leakage during core flooding experiments.
Table 1: Core sample details.
Sample Depth (m) 𝑫 (mm) 𝑳 (mm) 𝝓 (%) MAD (g)
Core A 1249.74 76.20 88.09 4.47 520.91
MATRIX
MATRIX
MATRIX
MATRIX
Carbonate
Clay
Cleat aperture
Fluid flow
(1)
(2)
(3)
De-mineralised cleat
0.0
0.2
0.4
0.6
0.8
1.0
1 2 3 4 5 6
k/k0
σ (MPa-1)
Mineralised cleat
Confining stress (

4. 4 SPE-176960-MS
Core flooding rig setup
Steady state measurement of core permeability (𝑘) was performed using the core flooding rig shown in
Figure 3. A Hassler core holder (Core Laboratories, USA) allowed radial loading of the core up to a
maximum pressure 100 bar. Pore pressure and confining pressure were provided by two separate high
pressure dual cylinder Quizix pumps (Chandler Engineering, USA). A dome loaded back pressure
regulator (Equilibar, USA) was used to maintain pressure drop across the core. Wetted parts were
predominantly Hastelloy to provide corrosion resistance. Further details of the core flooding procedure
are described in the proceeding sections.
Figure 3: Core flooding rig setup.
Core permeability was calculated as follows:
𝑘 =
4𝑄0 𝜇𝐿
𝜋𝐷2(𝑃1 − 𝑃2)
(2)
where 𝑄0 is the flowrate (m3
s-1
), µ is the liquid viscosity (Pa.s), 𝐿 is the core length (m), 𝐷 is the core
diameter (m) and 𝑃1 − 𝑃2 is the difference between the inlet and outlet pressure (Pa) respectively.
Experimental stages and timeline
The experimental stages are as described below and an experimental timeline with test conditions are
illustrated in Figure 4. In all tests, including acidification, 4% KCl is used to minimise clay swelling.
 Stage A – core evacuation to ~ -0.5 bar. Saturation in 4% KCl for >12 h, Ppore =2 bar, Pc =20 bar.
 Stage B – absolute permeability to 4% KCl (𝑘0) and initial cleat compressibility (𝐶𝑓0) measurements.
 Stage C – HCl injection flow through (1% HCl/4% KCl).
 Stage D – absolute permeability to 4% KCl (𝑘1) and post HCl compressibility (𝐶𝑓1) measurements.
Stage E – removal of core from core holder whole core immersed in 3% HF for 5 days.
PT4
DAQ
COMPUTER
OIL
RESERVOIR
MASS
BALANCE
PT2
PT1
QX6000 DUAL
CYLINDER PUMP
RV1
BV9
BV6
OIL BLEED
BV5 BV4
REVERSE FLOW LINE
BV1
BACK PRESSURE
REGULATOR
MASS
BALANCE
`
BV3
PT3
DISTRIBUTION
PLUG
RUBBER JACKET
CONFINING FLUID
CORE SAMPLE
BALL VALVE
PRESSURE
TRANSDUCER
BV2
FLUID
BLEED
RELIEF VALVE
N2
N2 PRESSURE
RELIEF
BV4
NV1
PR1
QX6000 DUAL
CYLINDER PUMP

5. SPE-176960-MS 5
 Stage F – absolute permeability to 4% KCl (𝑘2) and post HF1 compressibility (𝐶𝑓2) measurements.
 Stage G – removal of core from core holder whole core immersed in 15% HF for 5 days.
 Stage H – absolute permeability to 4% KCl (𝑘3) and post HF2 compressibility (𝐶𝑓3) measurements.
In as much as in-situ flow through injection was desired, only HCl could be flowed through the core and
HF stimulation had to be performed off the core holder via core immersion. This is due to operational
(i.e. PEEK components in the pump valves) and significant safety risks involved with pumping >20 mL
of low to medium concentration HF at elevated pressures. The fully immersed core (front end facing
up), was suspended by two polyurethane blocks on its encapsulation for continuous stirring (PTFE-
coated magnetic bar) at 500 rpm.
Figure 4: Simplified experimental timeline
Scanning electron microscopy
Scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) was used to determine
the relative abundance and textural context of mineral occurrence for core samples i.e. major,
moderate and minor cleat mineralisation. Cleat minerals were carefully flaked out from core samples
using a scalpel and mounted on 12.6 mm diameter pin type mounts with carbon tabs. A low-vacuum
JEOL6460LA environmental SEM with EDS detector at UQ CMM was utilised for all analyses.
Backscatter electron (BSE) images were taken from regions of interest, with grayscale intensity
variation indicating different elemental composition. Minerals were then identified according to their
EDS spectra and atomic percentage. SEM-EDS was also used for the identification of precipitants
formed following core sample saturation in 3% and 15% HF.
Results and discussion
Backscatter SEM images and relevant spectrum obtained from spot analysis of cleat minerals extracted
OFF THE CORE HOLDER,
IMMERSION, NO FLOW
Core Immersion in 3% HF
V=0.5 L , T = 20 oC, P = 1 bar
t = 0 - 168 h , stir = 500 rpm
[HF] = 3% ; [KCl] = 4%
OFF THE CORE HOLDER,
IMMERSION, NO FLOW
Core Immersion in 15% HF
V=0.5 L , T = 20 oC, P = 1 bar
t = 0 - 168 h , stir = 500 rpm
[HF] = 10-15% ; [KCl] = 4%
IN CORE HOLDER, FLOW
Initial Compressibility, Cfo
σeff=10-70bar, Pc=22-52 bar;
Pp =12 bar (P1 = 14, P2 = 10)
HCl Flow Injection
σeff = 10 bar, Pc = 22 bar;
Pp =12 bar (P1 = 14, P2 = 10)
Post HCl Compressibility, Cf1
σeff=10-70bar, Pc=22-52 bar;
Pp =12 bar (P1 = 14, P2 = 10)
IN CORE HOLDER, FLOW
Post HF1 Compressibility,Cf2
σeff=10-70 bar, Pc=22-52bar;
Pp =12 bar (P1 = 14, P2 = 10)
IN CORE HOLDER, FLOW
Post HF2 Compressibility,Cf3
σeff=10-70bar, Pc=22-52bar;
Pp =12 bar (P1 = 14, P2 = 10)
t

6. 6 SPE-176960-MS
from the core are shown in Figure 5. This particular sample contained mixed layer kaolinite and
ankerite. Ankerite was observed as a clearly defined layer (Figure 5a) or as veinlets over the surface of
kaolinite platelets (Figure 5b). The dominant cation (Fe-Ca-Mg) within ankerite carbonates varied
considerably, though an example spectrum is shown in e. Minor anatase spherules that had grown over
the surface of kaolinite platelets were also observed (Figure 5c).
Figure 5: (a) – (c) SEM images of core cleat mineralisation; (d) – (f) Examples of EDS spectra obtained from spot
analysis (labelled 1 – 3). Ka = kaolinite, An = ankerite and Ti = anatase.
Following the 3% HF acidification test, a white gel was noticed at the two ends of the core. The
formation of silica gel (Si(OH)4) is expected to occur when there is insufficient HF to react with all of the
quartz and clay as Al(III) competes successfully over Si(IV) for fluoride ions. Following the 15% HF
acidification test, visible signs of core degradation were evident, with the circumferential periphery
having most pronounced structural alteration. Figure 6 shows these changes as seen from the front end
of the core. Besides fragmentation of cracked sections, widening of the core-polyurethane
encapsulation gap is noticeable. It is apparent that core immersion had attacked both front and back
ends of the core as well as its circumferential periphery.
Figure 6: Images of the core front (a) prior to acidification, and (b) post acidification with 15% HF.
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
keV
003
0
100
200
300
400
500
600
700
800
900
1000
Counts
C
O Al
Si
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
keV
001
0
100
200
300
400
500
600
700
800
900
1000
Counts
C
O
Mg
Al
Si
Ca
Ca
Fe
Fe
Fe
Fe
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
keV
006
0
100
200
300
400
500
600
700
800
900
1000
Counts
C
O Al
SiTi
Ti
Ti
Ti
V
a b c
d e f
1 - Kaolinite 2 - Ankerite 3 - Anatase
a b

7. SPE-176960-MS 7
Geochemical speciation using OLI Analyzer 9.1 simulating the dissolution of < 250 g of kaolinite in 3%
and 15% HF at 25 °C and at 1 bar in the presence of 20 g KCl, 10 g CaCO3 , 5 g SiO2 and 2 g TiO2
predicted the formation of various fluoride-containing precipitates and K-Al containing secondary
mineralization. Table 2 summarises these solids and indicates whether we have observed and
confirmed the presence of either the metastable or its stable phase.
Table 2: Predicted solids and the presence of their stable or metastable phases in the acidified core.
Predicted Solids Formula
Metastable/stable phase in acidified core
3% HF 15% HF
Quartz SiO2 Yes (gel) No
K-Feldspar KAlSi3O8 Unconfirmed Unconfirmed
Muscovite (KF)2(Al2O3)3(SiO2)6(H2O) Unconfirmed Unconfirmed
Potassium hexafluorosilicate K2SiF6 Unconfirmed Yes, crystalline
Fluorite CaF2 Yes, amorphous Yes, amorphous
Aluminium trifluoride trihydrate AlF3.H2O Unconfirmed Unconfirmed
Kaolinite
Rutile
Al2Si2O5(OH)4
TiO2
Confirmed
Unconfirmed
Confirmed
Unconfirmed
A closer inspection of the newly formed solids via SEM-EDS identified these as neofluoride salts, most
likely K2SiF6 and CaF2. Whilst it is unclear when these had formed, either from the 3% HF or from the
15% HF immersion, it is however very likely that these had formed during the latter due to better
availability of mobile F-
ions. Figure 7 shows the secondary electron-generated images of the
neofluorides along with the electron dispersive spectrum of the regions of interest.
Figure 7: Electron micrographs of the neoformed solids (a) potassium hexafluorosilicate salt, K2SiF6 and (b)
fluorite, CaF2 and their corresponding EDS spectrum of the regions of interest.
a

8. 8 SPE-176960-MS
Results from this study suggests that mineral dissolution and the subsequent formation of fluoride salts
had increased coal permeability from 0.10 to 0.40 mD and made coal more stress resilient. Neither of
the HCl or low concentration HF was able to achieve both effects. Figure 8 shows the normalised
compressibility trends. As can be seen, acidification with 1% HCl and 3% HF had virtually no effect on
the coal core compressibility. This was because the amount of HF was insufficient to react with all
quartz/clay in the core. During acidification with 15% HF, where fluoride salts had formed, the coal core
has higher permeability and also a lower compressibility, suggesting that it is more resilient to
increasing stress. This does suggest that by replacement of primary minerals (i.e., kaolinite and
calcite) with neofluoride salts, fluid pathways in the core remains supported and resists collapse under
increased confining stresses.
Figure 8: Normalised permeability vs. effective stress and calculated cleat compressibility following 4% KCl (𝐶𝑓0),
1% HCl/4% KCl (𝐶𝑓1), 3% HF (𝐶𝑓2) and 15% HF (𝐶𝑓3) treatments.
Conclusions
The influence of mineral dissolution and secondary mineralisation on coal compressibility (𝐶𝑓) following
acidification with hydrochloric and hydrofluoric acid (HCl-HF) was studied. In-situ HCl injection and
core immersion in low to medium concentration HF yielded a less compressible core (𝐶𝑓 = 0.006 from
0.020 bar-1
) with sustained, enhanced permeability to brine (k = 0.40 from 0.10 mD). The improved
stress resilience and better fluid flow characteristics we attributed to the dissolution and subsequent
secondary mineralisation of the core’s circumferential periphery. In particular, the formation of radio-
dense neofluoride salts K2SiF6 and CaF2 has identified and verified. The results from this study suggest
that mineral alteration and subsequent secondary mineralisation of the core periphery following HCl-HF
acidisation yielded well-formed crystalline salts that apparently served as in-situ generated proppants.
These neoformed salts appears to buttress newly created void spaces for enhanced fluid flow and
improved resistance to increasing confining stress.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
10 15 20 25 30 35 40
k/ko
σe (bar)
As received
Post HCl
Post HF low conc
Post HF med conc
𝑪 𝒇𝟎 = 0.020 bar
-1
𝑪 𝒇𝟏 = 0.020 bar
-1
𝑪 𝒇𝟐 = 0.020 bar
-1
𝑪 𝒇𝟑 = 0.006 bar
-1

9. SPE-176960-MS 9
Acknowledgements
This research was financially supported by industry funding provided via The University of Queensland
Centre for Coal Seam Gas (www.ccsg.uq.edu.au).
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