The problem of degasification of coal beds is important because it is desirable to reduce the inflammable gas in coal mines as well as to recover economically the gas for use as a fuel. Permeability of coal is defined as a measure of the ease with which a gas flows through coal under a given pressure gradient, and it is of practical importance in the problem of degasification, The factors that affect the permeability of coal are also important in order to develop a more effective way of degasification. This study is concerned with the factors that affect the permeability of coal. Some of the major factors which affect the permeability of coal are the property of coal, properties of the natural coal gas, physical and chemical correlations between the coal and the gas, overburden pressure, and direction of the gas flow and other factors. In the case of these factors it is usually necessary to assess their effects in a qualitative way rather than in a quantitative one. For this reason, it would be impossible in the time allotted to this paper to present anything more than a brief discussion of their interrelationships. By experiment conducted for this thesis, it has been determined that (1) the permeability of coal differs from place to place, (2) the permeability of coal may or may not differ from bench to bench in the bed of a given location, (3) the permeability of dry coal does not differ significantly from that of wet coal, and also (4) in general, no significant difference in permeability was found when the gas flowed parallel to the coal bedding and when it flowed perpendicular to the coal bedding. / Master of Science

M.S.

This dissertation presents contributions to the understanding of the dynamic nature of permeability of Indonesian coal. It is the first-of-its-kind study, first presenting a comparison of experimental results with those obtained using existing analytical permeability models, and then modifying the existing anisotropic model for application to the unique physical structure of Indonesian coal. The first problem addressed in this dissertation was establishing the pressure-dependentpermeability of coal in a laboratory environment replicating in situ conditions for two coal types from the Sanga Sanga basin of Kalimantan, Indonesia. The change in permeability with depletion and the corresponding volumetric strain of coal were measured in the laboratory under uniaxial strain condition (zero lateral strain). Two gases, helium and methane, were used as the flowing fluids during experimental work. The results showed that, decreasing pore pressure resulted in significant decrease in horizontal stress and increased permeability. The permeability increase at low reservoir pressure was significant, a positive finding for Indonesian coals. Using the measured volumetric changes with variations in pressure, the cleat compressibility for the two coal types was estimated. In a separate effort, volumetric strain as a result of desorption of gases was measured using sister samples under unconstrained condition, in absence of the stress effect. Sorptioninduced strain processes were modeled using the Langmuir-type model to acquire the two important shrinkage parameters. All parameters calculated using the experimental data were used for the modeling exercise. The second component of this dissertation is the permeability variation modeling to enable projecting long-term gas production in the Sanga Sanga basin. For this, two commonly used isotropic permeability models were selected. These models, developed primarily for the San Juan coal, were unable to match the measured permeability data. This was believed to be due to the inappropriate geometry used to represent Indonesian coal, where butt cleats are believed to be absent. This was followed by application of the most recent model, incorporating partial anisotropy in coal. This consideration improved the modeling results although there clearly was room for improvement. The final challenge addressed in this dissertation was to consider the coal geometry appropriate for Indonesian coal, stack of sheets as opposed to a bundle of matchsticks. In order to incorporate the structural anisotropy for the stack of sheets geometry, two input parameters were modified, based on geo-mechanical anisotropy. After applying these to the modified model, the permeability modeling results were compared with the experimental data. The matches improved significantly. Finally, the effect of maximum horizontal stress on permeability of coal was estimated by using high and low maximum horizontal stress values and constant vertical and minimum horizontal stresses. The effect of maximum horizontal stress on permeability was found to be significant under uniaxial strain condition for both coals.

The steady flow method (SFM), most commonly used for permeability measurement in the laboratory, is not applicable for tight rocks, higher rank coals and coals under highly stressed condition because of the difficulty in measuring steady-state gas flowrates resulting from the tight rock structure of. However, accurate estimation of permeability of highly stressed coals is pivotal in coalbed methane (CBM) operations in order to precisely and effectively model and project long-term gas production. A fast and accurate permeability measurement technique is, therefore, required to investigate gas flow behavior of CBM reservoirs. The pulse-decay method (PDM) of permeability measurement is believed to be better suited for low-permeability rocks. In this study, application of the currently used pulse-decay laboratory permeability measurement techniques for highly stressed coals were evaluated. Considering the limitations of these techniques in permeability measurement of unconventional gas reservoirs, such as coal and gas shales, the conventional PDM was optimized by adjusting the experimental apparatus and procedures. Furthermore, the applicability of an optimized PDM was verified numerically and experimentally. This dissertation is composed of five chapters. To complete the research objectives as discussed above, it is necessary to have a profound understanding of the basic theories, such as, gas storage mechanism, gas migration, and permeability evolution during gas depletion in coalbed reservoirs. In Chapter 1, a brief discussion regarding the basic knowledge of reservoir properties and transport mechanisms is presented. The chapter also provides the appropriate background and rationale for the theoretical and experimental work conducted in this study. Chapter 2 presents the transient pressure-decay technique in permeability measurement of highly stressed coals and verifies the validity of Brace et al.’s solution (1968) by comparing it with Dicker and Smits’s solution (1988) and Cui et al.’s solution. The differences between these three solutions are discussed in detail. Based on the established permeability trends from these different solutions, a persuasive suggestion is presented for selection of the best alternative when testing coal permeability. Furthermore, permeability is regarded as a coupled parameter, resulting from the combined effects of mechanical compression and “matrix shrinkage” caused by desorption of gas. To isolate the role of gas desorption from the coupled result, a series of experiments were carried out under constant effective stress condition and a stress-dependent permeability trend was established. Chapter 3 proposes an optimized experimental design in order to improve the accuracy of the calculated permeability for sorptive rocks. In order to verify the optimized design theoretically, a modified mathematical model is presented and describes the one-dimensional fluid flow in porous media by a partial differential equation. The numerical solutions of the model are presented graphically to evaluate the fluid flow behavior in porous media. Finally, the validity of Brace et al.’s solution when testing sorptive rocks, without the need of consideration on the compressive storage and sorption effect, is elucidated. Chapter 4 demonstrates the efficiency and applicability of the optimized PDM through its direct application to experimental work designed to establish the permeability trend under best replicated in situ conditions. In this chapter, CO2 was used as the test fluid to profile and characterize the pulse decay plots due to its higher affinity towards coal than methane, and then establish the stress-dependent-permeability trend for highly-stressed CBM reservoirs. In this chapter, Brace et al.’s solution was also verified by comparing the laboratory data and computer simulated results obtained from the optimized mathematical model proposed in Chapter 3. The experimental work demonstrates that the optimized technique can be used for permeability tests of sorptive rocks without the need to carry out additional experimental work required to measure rock porosities and sorption isotherms. Finally, a summary and future research perspectives are presented in Chapter 5.