1. Introduction
Australia abounds with coal and coalbed methane (CBM) resources, with coal accounting for 32.2% of the total energy consumption in 2014–2015, which ranks as the second largest fuel supply, while CBM makes up 18% of the total gas production on an energy content basis [
1]. The mechanical properties of coal mass have been shown to have great influences on coal mining as well as coal seam gas extraction [
2,
3,
4,
5]. Many kinds of accidents can happen during coal mining process, among which coal mine outburst is one of the most catastrophic phenomena. Large volumes of CO
2 and/or CH
4 burst out suddenly and violently into coal workings due to strong stress relief, frequently resulting in heavy casualties and serious economic losses. In general, three main factors contribute to coal mine outburst, including stress condition, gas content and physic-mechanical properties of the coal mass [
2,
6,
7,
8], of which the mechanical properties of the coal mass is the key factor affecting outburst accidents. One of the most vital factors that determine coal mass strength is the adsorbate type and content in coal as coal may become weaker with the existence of a more reactive potential adsorbate [
4].
The mechanical properties of coal mass also affect the recovery process of coalbed methane. The relatively clean-burning nature of CBM has drawn much attention for its potential to provide energy source to a world in need of clean energy supplies. Enhanced coalbed methane (ECBM) technology, which implemented by injecting CO
2 (CO
2-ECBM) or N
2 (N
2-ECBM) or flue gas into the coal seam to replace the adsorbed CH
4, has become a promising technique to recover CBM, with less environmental pollution and higher recovery rates and CO
2-ECBM has the additional benefit of immobilizing large amounts of CO
2, which is the main greenhouse gas responsible for global warming [
9,
10]. However, during this process, the adsorption/desorption of gas/fluids create significant changes in the mechanical properties of coal mass. Changes in coal strength properties may occur due to gas and water sorption during the ECBM process and this leads to the reduction of coal seam permeability and recovery rates [
9]. Therefore, an investigation of the changes of coal mechanical property associated with adsorbate type and content is of great significance, not only for a better understanding of the outburst phenomenon during coal mining but also for a full appreciation of coal seam permeability changes, the key factor controlling CBM recovery during the ECBM process.
To date, the effects of adsorbates on coal strength changes have received considerable attention in the research literature [
3,
4,
5,
11,
12,
13]. Coal mass mechanical properties can be weakened by the adsorption of a more chemically reactive adsorbate. This can be explained by the theories of Gibbs [
14] and Griffith [
15]. For a continuous material with a unit thickness crack, the Griffith fracture criterion gives the tensile strength
σt which allows the crack to grow as shown in Equation (1):
where
is the Young’s modulus of the material,
is the surface energy per unit crack length and
is one-half the length of the crack.
The relation between surface energy and the amount of adsorption of the adsorbate in a given phase can be expressed with Gibbs’ adsorption equation, as indicated in Equation (2):
where
is the surface energy change due to change of adsorbate sorption,
is the surface concentration and
is the change of chemical potential of the
th adsorbate component.
Equation (2) suggests the following circumstances can lead to the reduction of surface energy: increased concentration of the adsorbate, chemical potential increment due to any change in the sorption environment and change of adsorbate from an inactive one to a more active one with higher chemical potential. Equation (1) shows that a reduced surface energy is correlated with a reduction of the tensile stress required to form or grow a new crack. Although Equation (1) is used for tensile stress circumstances, uniaxial compression strength is influenced by tensile brittle cracking and energy exchanges, according to a number of experimental results [
16]. Therefore, the Griffith criterion can be adopted for uniaxial compressive situations to examine strength alterations due to sorption effects.
A number of studies to date have concerned coal mechanical property changes as the effect of sorption, however, most studies focus on CO
2, CH
4 and N
2 sorption-induced coal strength property alteration [
3,
4,
5,
11,
17]. However, unlike conventional gas reservoirs which generally have little moisture content, coal seams are usually saturated with ground water in situ, especially for the more hydrophilic low-rank coals such as Victorian brown coal which can hold up to 70% of water by weight [
18]. It is therefore important to conduct investigations on the mechanical property changes of coal subjected to water saturation. However, few studies have been done on this subject, with the exception of Perera et al. [
3], Poulsen et al. [
13] and Perera et al. [
4] The unconfined compressive strength and Young’s modulus of coals are reduced by 14.6% and 16.2% due to water saturation, respectively, according to Perera et al. [
4], who used Victorian brown coal, while black coal exhibits a much higher strength reduction with reduction in UCS of 36.3% and Young’s modulus of 39.5%. However, their work only considered the complete water saturation condition, while the effect of different moisture contents in coal was not included. Furthermore, as seam water always contains a certain amount of salts, the related research on the effect of saline water saturation on coal mechanical properties is required, however, no study to date has conducted the relevant investigations on coals although a number of studies have confirmed that the existence of saline solutions can significantly affect the mechanical properties of other rocks [
19,
20,
21].
Therefore, this study intents to fill this gap by performing the relevant experimental studies on the effect of moisture content and brine saturation on the mechanical properties of coals. The salt used in the present study for brine solution saturation is sodium chloride as the chemical compositions of seam water in various parts of world share similar ingredients, with sodium and bicarbonates taking the major ion roles [
22], which suggests that the same seam water type can be expected, regardless of the formation’s lithology or age. In addition, chloride is the major component of seam water where the coal seam is in marine or marine-transitional formations [
23]. The coal samples used in this study is brown coal because of its highly hydrophilic character and its importance to Australian energy supplies, besides, despite the fact that most current ECBM projects are implemented in deep buried coal seams where high rank coals are generally expected, studies have shown that some deep lignite formations, for example the Calver Bluff formations of the Texas Wlicox Group with a buried depth deeper than 1067 m, have the potential for undertaking the ECBM projects [
24]. However, relevant studies on brown coal are scarce and the main purpose of the present study is therefore to distinguish the effects of water and brine saturation on the mechanical properties of brown coals.