Modeling Laboratory Permeability in Coal Using Sorption-Induced-Strain Data (original) (raw)
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Improvements in Measuring Sorption-Induced Strain and Permeability in Coal
All Days, 2008
Total worldwide CBM in-place reserves estimates are between 3500 Tcf and 9500 Tcf. Unminable coal beds have been recommended as good CO2 sequestration sites as the world prepares to sequester large amounts of greenhouse gases. In the U.S., these coal seams have the capacity to adsorb and sequester roughly 50 years of CO2 emissions from all the U.S. coal-fired power plants at today's output rates. The amount and type of gas adsorbed in coal has a strong impact on the permeability of the coal seam. An improved mixed gas adsorption isotherm model based on the extended-Langmuir theory is discussed and is applied to mixed gas sorption-induced strain based on pure gas strain data and a parameter accounting for gas-gas interactions that is independent of the coal substrate. Advantages and disadvantages of using freestanding versus constrained samples for sorption-induced strain measurements are also discussed. A permeability equation used to model laboratory was found to be very accura...
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 .
International Journal of Coal Geology, 2010
Although coal-gas interactions have been comprehensively investigated, most prior studies have focused on one or more component processes of effective stress or sorption-induced deformation and for resulting isotropic changes in coal permeability. In this study a permeability model is developed to define the evolution of gas sorption-induced permeability anisotropy under the full spectrum of mechanical conditions spanning prescribed in-situ stresses through constrained displacement. In the model, gas sorption-induced coal directional permeabilities are linked into directional strains through an elastic modulus reduction ratio, R m. It defines the ratio of coal bulk elastic modulus to coal matrix modulus (0 b R m b 1) and represents the partitioning of total strain for an equivalent porous coal medium between the fracture system and the matrix. Where bulk coal permeability is dominated by the cleat system, the portioned fracture strains may be used to define the evolution of the fracture permeability, and hence the response of the bulk aggregate. The coal modulus reduction ratio provides a straightforward index to link anisotropy in deformability characteristics to the evolution of directional permeabilities. Constitutive models incorporating this concept are implemented in a finite element model to represent the complex interactions of effective stress and sorption under in-situ conditions. The validity of the model is evaluated against benchmark cases for uniaxial swelling and for constant volume reservoirs then applied to match changes in permeability observed in a field production test within a coalbed reservoir.
2011
Porosity, methane sorption capacity, diffusivity and permeability of a suite of vitriniterich coals from the Horseshoe Canyon and Mannville formations of the Western Canada Sedimentary Basin were investigated. Coal rank ranges from subbituminous to medium volatile bituminous, equilibrium moisture is between 2.32%-23.75%, and ash is up to 72% although < 20% on average. Total coal porosity estimated using mercury porosimetry and helium pycnometry is between 4.4% and 18%. Helium pycnometry porosity is higher than mercury porosimetry porosity because the smaller molecular diameter of helium allows it to access coal pores which are inaccessible to mercury at test pressures. Greater vitrinite content is generally correlated with higher coal total pore area due to the abundant microporosity in vitrinite. Coal methane sorption capacity is up to 23.5 cc/g on a moisture equilibrated basis and is up to 40.4 cc/g for dry coals. Moisture equilibrated and dry coals sorb differently due to competition for adsorption sites in coal between methane and moisture. No relationship is observed between sorption capacity and coal rank or between maceral content and sorption capacity because of the narrow rank and maceral composition of the samples studied. Permeability was investigated on crushed coals and plugs with crushed permeability not exceeding 1.79•10-2 md while plug permeability is up to 0.9 md. Average diffusivity is estimated to be around 10-11 to 10-12 m 2 /s. Coal matrix properties influence crushed permeability. Inertinite-rich coals have higher matrix permeability and diffusivity because of the greater macro-and meso-porosity of iii inertinite. Plug permeability is dependent on coal matrix properties and the presence of fractures on tested plugs. Coals with better developed fractures are more permeable than coals with poorly developed fractures at the same effective stresses. Probe gas type influences plug permeability. Helium permeability measurements are higher than permeability measured with methane or nitrogen. Permeability difference with probe gas is attributed to a combination of different probe gas molecule size, relative swelling effects of probe gas on coal and associated changes at in-situ stress during tests. Understanding the reasons for permeability variations in coals will help in more focused coal bed methane exploration and development.
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.
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.
Insights, Trends and Challenges Associated with Measuring Coal Relative Permeability
E3S Web of Conferences
Due to the poroelasticity of coal, both porosity and permeability change over the life of the field as pore pressure decreases and effective stress increases. The relative permeability also changes as the effective stress regime shifts from one state to another. This paper examines coal relative permeability trends for changes in effective stress. The unsteady-state technique was used to determine experimental relativepermeability curves, which were then corrected for capillary-end effect through history matching. A modified Brooks-Corey correlation was sufficient for generating relative permeability curves and was successfully used to history match the laboratory data. Analysis of the corrected curves indicate that as effective stress increases, gas relative permeability increases, irreducible water saturation increases and the relative permeability cross-point shifts to the right.
A Permeability Model for Coal and Other Fractured, Sorptive-Elastic Media
SPE Journal, 2008
Summary This paper describes the derivation of a new equation that can be used to model the permeability behavior of a fractured, sorptive-elastic medium, such as coal, under variable stress conditions. The equation is applicable to confinement pressure schemes commonly used during the collection of permeability data in the laboratory. The model is derived for cubic geometry under biaxial or hydrostatic confining pressures. The model is designed to handle changes in permeability caused by adsorption and desorption of gases onto and from the matrix blocks in fractured media. The model equations can be used to calculate permeability changes caused by the production of methane (CH4) from coal as well as the injection of gases, such as carbon dioxide, for sequestration in coal. Sensitivity analysis of the model found that each of the input variables can have a significant impact on the outcome of the permeability forecast as a function of changing pore pressure; thus, accurate input dat...
Study of Permeability of Coal Samples Subjected to Confining Pressures
2015
Permeability is assessment of the ability of rock to transmit fluid flow through the rock body. It can be affected by rock structure due to the grain size, formation and the pressure or concentration gradient existing within and across it. Past studies focused on the relationship between permeability and axial stress on rock, and there has been limited research on the impact of circumferential stress and volumetric deformation on permeability. A programme of laboratory tests was conducted on coal samples to evaluate the permeability of coal under different confining pressures. A specialised permeability apparatus known as Multi-Functional Outburst Research Rig (MFORR), was used to study rock permeability under various confining pressures. Methane permeability tests on cylindrical coal samples were conducted at varying axial stress up to 3 MPa and confining CH4 gas pressures between 0.2 MPa and 3 MPa. It was found that by increasing the confining gas pressure the permeability value d...