The Effect of Water Occlusion on Gas Production in Coal

Mahoney, Shilo Anthony (2017). The Effect of Water Occlusion on Gas Production in Coal PhD Thesis, School of Chemical Engineering, The University of Queensland. doi:10.14264/uql.2017.416

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Author Mahoney, Shilo Anthony
Thesis Title The Effect of Water Occlusion on Gas Production in Coal
School, Centre or Institute School of Chemical Engineering
Institution The University of Queensland
DOI 10.14264/uql.2017.416
Publication date 2017-03-16
Thesis type PhD Thesis
Supervisor Victor Rudolph
Thomas Rufford
Karen Steel
Total pages 205
Language eng
Subjects 040309 Petroleum and Coal Geology
0914 Resources Engineering and Extractive Metallurgy
091406 Petroleum and Reservoir Engineering
Formatted abstract
Relative permeability is of fundamental importance to understand and model the flow of
hydrocarbons and water in porous rocks including coal, and thus relative permeability is critical in
prediction of commercial gas and water production rates from coal seam gas (CSG) reservoirs.
Despite relative permeability being a primary parameter for determining reservoir performance, the
fundamental physics of how or where water may occlude or block cleats in a coal seam, and thereby
interfere with gas production rates, is not fully understood. This project aims to improve the
understanding of water occlusion in CSG reservoirs through fluid experiments with model coal cleats
and to evaluate the potential impact of water occlusion of reservoir performance.

The aim of this thesis is to develop a new experimental methodology that can be used identify the
main control factors that affect wettability in coal. There are two key objectives to satisfy the thesis
aim 1) develop methods to make artificial channels in coal and 2) create a world first microfluidic
device that assess micro scale flow through coal cleats, known as the cleat flow cell (CFC). The initial
features to be evaluated will include: lithotype, surface roughness, surface composition, specifically
chemical functional groups and pressure.

The first experimental chapter (Chapter Four) evaluates five different experimental methodologies
developed to create artificial micro cleats (20-40 µm) to replicate in situ coal cleat characteristics,
such as: width, depth, chemical and physical properties. A single channel was made in polished coal
from five different open Bowen Basin coal mines, Isaac Plains North (IPN), Oaky Creek (OAK),
Moorvale (MVL), Coppabella South (COP.S), and Coppabella East (COP.E) with a rank indicator
Rmax% 0.98 - 1.91 %. The techniques to create the channel included: reactive ion etching (RIE) in
oxygen plasma with a photolithography process to make the desired pattern on coal surface, UV Laser
ablation, mechanical machining, a mechanical scratch technique, and a chemical etching technique
that used potassium permanganate (KMnO4). Characteristics of the artificial channels were assessed
using a surface step profiler, scanning electron microscopy, micro-Raman spectroscopy and light
microscopy. A sixth methodology to create an artificial channel using pressed coal powder was also
used as a means of evaluating the influence of the surface chemistry on the wetting behaviour, without
the interference of surface topography.

In Chapter Five, I report the effect of rank and lithotype on the wettability of coal in microfluidic
experiments in two types of artificial microchannels; (1) reactive ion etched (RIE) channels and (2)
die-cast channels prepared by pressing powdered lithotype concentrates. Contact angles and entry
pressures of air and water in the artificial cleats were measured in imbibition experiments performed
with a CFC. The relative contact angles measured in CFC imbibition experiments were in the range
110 -140° in the RIE channels and 85°-115° in the pressed discs, which are larger contact angles than
measured on the flat bulk surfaces of these samples by the conventional sessile drop technique (58°-
85°). The CFC observations show that the surface roughness of coal in inertinite-rich dull bands
effects contact angle and the entry pressure of the air-water interface differently to the vitrinite-rich
bright bands, with both lithotypes presenting unique wetting states. Drainage experiments revealed a
thin residual water film on the inertinite cleat wall, not observed on the smoother vitrinite channel.
The experimental observations are used to present a modified Cassie Equation model to predict coal
contact angles based on the fractions of dull and bright bands, surface mineral content, and surface

In Chapter Six, I report the effect of five surface treatments on pressed coal discs: three 1 wt %
nanoparticle solutions of MgO, SiO2, and Al2O3 and two chemical solutions, a 15 vol.% hydrogen
peroxide and 2 wt.% Silicad® solution. Contact angles were measured using conventional sessile
drop technique and drainage experiments were performed using the CFC instrument. Relative contact
angles on the treated samples varied based on treatment, compared to an untreated sample reference
coal. Analysis of sessile drop results demonstrated that the 2% Siliclad® solution displayed the largest
relative contact angle range (121 -136°) compared to the reference cell (104-84°) meaning that the
Siliclad treated coal surface had become more hydrophobic. Conversely, the 15% vol. hydrogen
peroxide treatment indicated a more hydrophilic surface was generated with contact angles between
30 -40o. The 1% vol. MgO and SiO2 treatments exhibited a decrease in contact angle (~30 -50o), yet
Al2O3 did not show any measurable change in angle. Subsequent CFC imbibition results
demonstrated that the nanoparticles treatments were not effective in the cleat as the contact angles
were inconsistent, yet the Siliclad and hydrogen peroxide treatments reflected similar contact angle
results to the sessile drop values. Drainage experiments clearly showed the Siliclad treated samples
were hydrophobic, with no residual film present, while hydrogen peroxide treated channel had a thick
residual water film. Based on these results H2O2 and Siliclad were selected to treat a packed bed coal
core. These cores were used to measure the influence that surface wettability has on the relative
permeability of the cores using the steady state method, with the hydrogen peroxide treatment shifting
the gas and water permeability curve crossover point to the right and the Siliclad treatment shifting
to the left (compared to the untreated sample).

This study has provided valuable insight into the wetting behaviours of coal lithotypes and the effect
this has on gas-liquid flow through microchannels in coal. The results of this thesis provide the basis
to consider an improved relative permeability model that explicitly accounts for the effect of coal
lithotype and the unique flow regimes that are generated based on surface wettability.
Keyword Relative permeability
Coal rank
Reactive ion etching
Two phase flow
Microfluidic flow
Contact angle

Document type: Thesis
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Created: Tue, 07 Mar 2017, 12:53:24 EST by Shilo Mahoney on behalf of Learning and Research Services (UQ Library)