Study of Permeability of Coal Samples Subjected to Confining Pressures (original) (raw)
Related papers
Permeability Changes of Coal Cores and Briquettes under Tri-Axial Stress Conditions
Archives of Mining Sciences, 2014
The paper is dealing with the permeability of coal in triaxial state of stress. The permeability of coal, besides coal’s methane capacity, is the main parameter determining the quantity of methane inflow into underground excavations. The stress in a coal seam is one of the most important factors influencing coal permeability therefore the permeability measurements were performed in tri-axial state of stress. The hydrostatic three-axial state of stress was gradually increased from 5 MPa with steps of 5 MPa up to a maximum of 30 MPa. Nitrogen was applied as a gas medium in all experiments. The results of the permeability measurements of coal cores from the “Zofiówka” mine, Poland, and three mines from the Czech Republic are presented in this paper. As a “reference”, permeability measurements were also taken for coal briquettes prepared from coal dust with defined porosity. It was confirmed that the decreasing porosity of coal briquettes affects the decreasing permeability. The advanta...
International Journal of Greenhouse Gas Control, 2011
Permeability is one of the most important parameters for CO 2 injection in coal to enhance coalbed methane recovery. Laboratory characterization of coal permeability provides useful information for in situ permeability behavior of coal seams when adsorbing gases such as CO 2 are injected. In this study, a series of experiments have been conducted for coal samples using both non-adsorbing and adsorbing gases at various confining stresses and pore pressures. Our observations have showed that even under controlled stress conditions, coal permeability decreases with respect to pore pressure during the injection of adsorbing gases. In order to find out the causes of permeability decrease for adsorbing gases, a nonadsorbing gas (helium) is used to determine the effective stress coefficient. In these experiments using helium, the impact of gas sorption can be neglected and any permeability reduction is considered as due to the variation in the effective stress, which is controlled by the effective stress coefficient. The results show that the effective stress coefficient is pore pressure dependent and less than unity for the coal samples studied. The permeability reduction from helium experiments is then used to calibrate the subsequent flow-through experiments using adsorbing gases, CH 4 and CO 2 . Through this calibration, the sole effect of sorption-induced strain on permeability change is obtained for these adsorbing gas flow-through experiments. In this paper, experimental results and analyses are reported including how the impact of effective stress coefficient is separated from that of the sorption-induced strain on the evolution of coal permeability. (Z. Pan). methane recovered as an energy source, while providing the additional benefit of reducing greenhouse gas emissions by storing the CO 2 underground .
Permeability and volumetric changes in coal under different test environment
Acta Geodynamica et Geomaterialia, 2013
Permeability refers to the ability of coal to transmit gas when a pressure or concentration gradient exists across it. The permeability of coal is dependent upon factors that include effective stress, gas pressure, water content, disturbance associated with drilling and matrix swelling/shrinkage due to adsorption/desorption. A programme of laboratory tests were conducted on coal samples from the Bulli seam for evaluating the permeability and drainability of coal. Two different types of permeability apparatus were used in this study. The methods of permeability testing of coal under different triaxial conditions are discussed. Permeability testing of the Bulli seam coal with N 2 is described. The laboratory test results were found to be in agreement with the calculated permeability values.
Permeability testing of coal under different triaxial conditions
2012
Permeability refers to the ability of coal to transmit gas when a pressure or concentration gradient exists across it. The permeability of coal is dependent upon factors that include effective stress, gas pressure, water content, disturbance associated with drilling and matrix swelling/shrinkage due to adsorption/desorption. A programme of laboratory tests were conducted on coal samples from the Bulli seam for evaluating the permeability and drainability of coal. The study was conducted using two different types of permeability apparatus. The methods of permeability testing of coal under different triaxial conditions are discussed. Permeability testing of the Bulli seam coal sample with N 2 is described as an example in this study. Both the tests results and the values of calculated permeability were in agreement.
Experimental Study on Permeability of Coal Sample Subjected to Triaxial Stresses
Procedia Engineering, 2011
The permeability of coal subjected to triaxial compression was investigated to relate it to the change of stresses state, analogous to mining-induced redistribution of stress in the surrounding rock. Results show that the permeability of coal sample under triaxial compression tends to decrease with the increase in stress in each loading direction, which indicates that permeability of the coal is actually controlled by the evolution of cracks in the coals. It also means that the orientation of cleats and maximum principle stress could significantly controls the permeability of intact coal. In other words, loading stress vertical to face cleats can be much easier to narrow the aperture of face cleats of the intact coal sample. Besides, the broken coal sample under higher stress level shows higher permeability than that of the intact coal under lower stress environment. That is partly because that the higher stress level is applied, the more likely the broken coal is to crush, which can contribute to the formation of the connected crack and in turns increase the permeability of coal.
Evolution of coal permeability from stress-controlled to displacement-controlled swelling conditions
Fuel, 2011
When a coal sample is constrained either by displacements or by a confining stress, additional force and resulting stress develop within the coal. A simple ''free expansion + push back'' approach is developed in this work to determine the magnitude of this stress and its effect on permeability evolution. In this approach, the coal is allowed to expand freely due to gas sorption, and then it is pushed back by the applied effective stress to the original constrained conditions. The total ''push-back'' strains are used to calculate the change in coal permeability. This free expansion plus push back approach is applied to examine the variety of permeability responses observed in the laboratory and the veracity of their representation by theoretical models linking this behavior to gas sorption-induced swelling/shrinkage. These cases include (1) coal swelling tests under the uniaxial strain condition; (2) coal swelling tests under the displacement controlled condition; (3) coal swelling tests under the stress controlled condition. These responses are verified against other coal permeability models available in the literature and against experimental data and field data where few analytical solutions are currently available. In particular, this approach has led to a new coal permeability model that can be used to explain stress-controlled experimental observations. Stress-controlled swelling tests are normally conducted in the laboratory to characterize the evolution of coal permeability under the influence of gas sorption. Typically reductions in permeability are observed from gas-sorption-induced swelling even where effective stresses remain constant. This behavior remains enigmatic as the permeability of the porous coal is determined by the effective stress only. Our model is capable of replicating this apparently anomalous behavior.
Dynamic 4D coal permeability – the benefits of a constant volume reservoir
Much research and many field trials have shown that the permeability of coal seams varies inversely with the effective stress acting on coal. It was theorized by Gray (1992) that desorption-induced matrix shrinkage may counteract the effect of external stresses, allowing permeability to increase, depending on the relative strength of the two phenomena. This paper explores the geomechanical theory of reservoir stiffness and the arch-bridging effect, and uses geotechnical engineering principles and operating coal mine evidence to support the development of the important principle of a "constant volume" (CV) boundary condition applying to most deep CBM reservoirs, as opposed to the traditional assumption of a constant external stress condition, whether horizontal or verticaal. This CV principle allows for an initial external stress state to exist in a coal seam and allows the acting stress state to be dynamic, generally trending lower with matrix shrinkage, thus allowing permeability to increase as depletion of the CBM reservoir proceeds. Laboratory data on measured permeability under both constant external stress conditions and constant volume conditions shows the importance of this principle; permeability increases of between 100% and 1200% can be experienced, depending on the stage of depletion and thus desorption. Field performance from producing CBM reservoirs will be referenced to confirm the reality and significance of permeability enhancement due to matrix shrinkage in the context of constant volume behaving deep CBM reservoirs. the degree of adsorption or desorption of the fluid and the presence of any competitive sorption with other fluids; the relative permeabilities when multi-fluid/multi-phase flow is involved; the internal pore/gas pressure and the location of the flow (e.g. in the near-wellbore area with lower pressure and stresses); the initial 3D and dynamic stress state acting on the coal. The interplay of these factors, for any given rank of coal, will define its effective permeability, from which gas flow rates and gas reserves are predicted The relevant categories of permeability can then be summed up as shown in Table 1. As affected by Permeability category Flowing Fluid Absolute Effective-static-dynamic Relative Spatial orientation Horizontal-max. Horizontalmin. Vertical External state Unstressed Stressed-hydrostatic-triaxial-true triaxial Virgin in situ (pressured and 3D stressed) Permeability & net stress Many authors have reported on coal permeability measurements under external stress conditions, both at the laboratory scale and with field level tests:
International Journal of Greenhouse Gas Control, 2012
A series of coal permeability experiments was conducted for coal samples infiltrated both with nonadsorbing and adsorbing gases-all under conditions of constant pressure difference between the confining stress and the pore pressure. The experimental results show that even under controlled stress conditions, coal permeability decreases with respect to pore pressure during the injection of adsorbing gases. This conclusion is apparently not congruent with our conceptual understanding: when coal samples are free to swell/shrink then no effect of swelling/shrinkage strain should be apparent on the permeability under controlled stress conditions. In this study, we developed a phenomenological permeability model to explain this enigmatic behavior of coal permeability evolution under the influence of gas sorption by combining the effect of swelling strain with that of the mechanical effective stress. For the mechanical effective stress effect, we use the concept of natural strain to define its impact on the change in fracture aperture; for the swelling strain effect, we introduce a partition ratio to define the contribution of swelling strain to the fracture aperture reduction. The resulting coal permeability model is defined as a function of both the effective stress and the swelling strain. Compared to other commonly used models under specific boundary conditions, such as Palmer-Mansoori (P-M), Shi-Durucan (S-D) and Cui-Bustin (C-B) models, our model results match the experimental measurements quite well. We match the experimental data with the model results for the correct reason, i.e. the model conditions are consistent with the experimental conditions (both are stress-controlled), while other models only match the data for a different reason (the model condition is uniaxial strain but the experimental condition is stress-controlled). We have also implemented our permeability model into a fully coupled coal deformation and gas transport finite element model to recover the important non-linear responses due to the effective stress effects where mechanical influences are rigorously coupled with the gas transport system.