Pore scale study on permeability stimulation and hydrogen geostorage in coal seams
Date of Award
Doctor of Philosophy
School of Engineering
Coalbed methane (CBM) is basically naturally fractured, and the cleat network plays the main role in providing the fluid flow path, hence the permeability measure in CBM relies principally on the characteristics of this network. However, the permeability of the cleat network in coals is typically low. Moreover, fractures in coal seams might be partially to fully filled with minerals, a process called mineralization, as the result of which the permeability measure would decrease, sometimes leading to the total blockage of fractures. Therefore, to provide a promising cleat network permeability, reservoir stimulation is an important part in development of low permeability CBM to induce new fractures and demineralize the original cleat network. The current study, in general, focuses on two stimulation techniques aiming at enhancing the permeability and connectivity of coal cleat network, namely liquid nitrogen (LN2) fracturing, and acid stimulation. Several techniques and equipment were used to conduct this research, such as Micro Computed Tomography (μ-CT) scanning, medical CT scanning, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), flooding set-ups, etc.
Although LN2 fracturing has been studied previously, there are some gaps in the research in this field. The first matter to consider in LN2 fracturing is to investigate the mechanism of this technique in fracturing coals. Therefore, the research presented in the third chapter of the current study, examines and quantifies the pore structure and connectivity evolution of a bituminous coal frozen in LN2, based on the in-situ morphological analysis through μ-CT scanning. This helps us to investigate the associated in-situ fracturing mechanisms, and measure the extent of induced fractures and damage to the rock in 3D at micrometre-scale. The results of this study shows the considerable potential of LN2 freezing in coal cleat network permeability enhancement. The μ-CT results clearly demonstrate that the cleat network is boosted following the treatment, where thoroughgoing fractures with a maximum opening of 13 μm appear, some of which are rooted in the original cleat network. While this application increases the porosity measure of the bituminous coal by 11%, core flooding tests and Lattice Boltzmann simulations show over double increase in the coal’s permeability value after liquid nitrogen exposure. This noticeable permeability enhancement is not only due to generation of new fractures, but also is attributed to the connection establishment of the cleat network with originally isolated pores and micro-cleats following the freezing, thereby increasing pore network connectivity.
Following conducting the above study, which revealed encouraging results in permeability and connectivity enhancement of the coal and illustrated the discovering mechanisms of LN2 fracturing approach, another research in this work focuses on examining the performance of this stimulation technique in different coal ranks. This research is presented in chapter 4 of the current study. This research thus is aimed at exploring the potential of this thermal shock in fracturing three main coal ranks, namely sub-bituminous, bituminous and anthracite. The 3D X-ray computed tomography results in μ-CT and macroscale (medical-CT) reveal a poor performance of LN2 fracturing in anthracite. On the contrary, the in-situ 3D visualization of the other two coal ranks suggests a promising fracturing performance. The porosity evolutions of bituminous and sub-bituminous coals through the treatment are 14% and 119% in microscale, respectively. Thus, this work suggests that LN2 fracturing approach may not be a reliable stimulation technique for high rank coals, although performs encouragingly in medium to low rank coals.
In addition to the above studies on LN2 fracturing potential and mechanism as well as its performance in different coal ranks, another research completed in this study focuses on the potential of several cycles of LN2 freezing/unfreezing, a process called freeze-thaw, which is presented in chapter 5 of the current study. This research is aimed at comparing the performance of this fracturing technique in one freezing cycle with up to three freeze-thaw cycles, and quantifies the efficiency of each cycle in cleat network evolution. μ-CT images revealed a promising efficiency in cleat network evolution after three freezing cycles, where the number of pores increased by 50% (92715 142650), and the number of interconnected pores almost doubled (42060 78905). SEM along with μ-CT images highlight more encouraging efficiency of second and third freezing cycles, particularly in terms of enhancing fractures interconnection. Mechanical properties analysis reveals more significant damage in the coal in the latter freezing cycles, where the indentation modulus was initially 3.49 GPa and decreased to 2.81, 2.11 and 1.52 GPa through three freezing cycles. Finally, the permeability of the coal under 1000 kPa confining pressure increased from 0.035 mD to 0.18 mD, with larger increments in later cycles. Therefore, this research highlights the superiority of LN2 freeze-thaw cycling over single freezing of coals with LN2 through conducting a quantitative comparison on the efficiency of each freezing cycle.
In addition to LN2 fracturing technique, this study works on acid stimulation of coals, and evaluates the potential of a combination of these two recovery enhancement approaches in coal’s permeability evolution, the results of which is presented in the form of a research article in chapter 6. This work suggests that a combination of these two techniques have a considerable potential for recovery enhancement in coals. In case where acidizing is performed first, in addition to the permeability enhancement because of cleat demineralization, a more promising LN2 fracturing application is expected, because the coal’s strength decreases following acidizing, and the coal is more prone to freezing. In the other sequence of applications, where LN2 freezing is accomplished first, not only the cleat network is extended and boosted due to the fracturing application, but also the following acidizing process benefits because of a better accessibility of the acid to remote mineralised fractures.
The findings in the above research articles show how a coalbed methane could be treated to provide an enhanced cleat network, where fluid flow is facilitated, and the coalbed methane is extracted. Such coalbed methane reserve would be depleted from gas at this stage. Generally, underground reservoirs might have a potential for gas storage following depletion from gas/oil. This could be beneficial from environmental aspect, for example unwanted gases such as carbon dioxide could be injected into these reservoirs. Additionally, in the recent years, depleted underground reservoirs have attracted the attention of energy sector as a possible storage site for generated hydrogen, as it is not a safe practice to store massive amount of this gas in surface reservoirs. Indeed, hydrogen has become a significant topic recently due to its dramatic potential as an energy carrier as well as environmental friendliness, while its storage is a challenge, thus the viability of storing this gas into a depleted coal seam was investigated in this study. In this application, coal permeability is a key parameter which determines how fast H2 can be injected and withdrawn again. However, it is expected to observe coal swelling when subjected to several gases, as a result of which coal permeability reduces significantly. Therefore, chapter 7 presents a research article, in which H2 gas is injected into a coal core and dynamic permeability is measured, while imaging the core via x-ray micro-tomography at reservoir conditions. Importantly, no changes in coal cleat morphology or permeability were observed in this study. This research thus suggests that H2 geo-storage in deep coal seams is feasible from a fundamental petro-physical perspective.
Access to this thesis is embargoed until 28 June 2025.
Akhondzadeh, H. (2022). Pore scale study on permeability stimulation and hydrogen geostorage in coal seams. https://ro.ecu.edu.au/theses/2551