Methane Desorption Research Articles

Pore accessibility and trapping of methane in Marcellus Shale

Accessibility of pore space in unconventional reservoirs is an important factor influencing both methane storage capacity and the kinetics of methane desorption. The determination of open (accessible) versus closed (inaccessible) porosity is therefore vital for the prediction of gas production potential. This study investigates accessibility of pores to methane in overmature middle Devonian Marcellus Shale samples (cut parallel and perpendicular to bedding) using small and ultra-small neutron scattering (SANS and USANS) with contrast matching (CM), supplemented by other complementary techniques, such as mercury injection capillary pressure (MICP) and low pressure gas (N2 and CO2) adsorption.Our results demonstrate that for the samples studied, only about 6% of pores with diameter 25–500 nm are accessible to methane. The accessible fraction for pores larger than 500 nm is 35%. For nanopores smaller than 25 nm, pore accessibility could not be quantitatively determined due to increased methane density and condensation effects in confinement. Our observations indicate that methane penetrates the accessible small mesopores and micropores down to at least 1 nm in diameter, and the density of confined deuterated methane (CD4) is 0.68 g/cm3 for pores of diameter 25 nm and gradually increases with the decreased pore size. Moreover, elevated gas pressure causes formation of additional high-density methane nano-clusters. These clusters have a form of slightly anisotropic polydisperse discs oriented along bedding plane, about 1–12 nm in diameter and with average thickness of 3.6 nm. Utilizing samples cut parallel and perpendicular to the bedding, this study also briefly addresses anisotropy of pores. Based on the isosize intensity ratio ℛ, our SANS and USANS results demonstrate anisotropy in the out-of-bedding direction and suggest that the degree of anisotropy depends on the pore size. Specifically, for pore diameters ~2.5 to 250 nm, the degree of anisotropy is larger than for pores ~500 nm to 6 μm in diameter. Finally, comparison of pore size distribution results calculated from SANS/USANS to those obtained using MICP shows good agreement at low pressures, but large difference at pressures above 1000 bar. This discrepancy requires further testing; it is possible that the high mercury pressure used in MICP alters the mesopore and micropore structure of shales.

Laws of Gas Diffusion in Coal Particles: A Study Based on Experiment and Numerical Simulation

In order to research on the law of methane released through the pore in coal particles, the methane desorption experiments were conducted, respectively, on four types of particle size of coal samples under three different initial adsorption pressures. The cumulative methane desorption quantity (CMDQ) with time increasing was obtained to show that the reciprocal of CMDQ was in linear relation with the reciprocal of the square root of time, and the correlation coefficients were all above 0.99, on basis of which an empirical formula of CMDQ was established. Then, according to Fick diffusion law and Darcy percolation law, the mathematical models of methane emission from the spherical coal particles were created, respectively, and the corresponding calculating software was programmed by the finite difference method to obtain the simulated CMDQ of each sample under different conditions. The methane emission rate functions (MERF) of the simulation and the experiment were also calculated, respectively. Comparative analysis between the numerically simulated outcomes and the assay results reveals that the simulation outcomes as per Darcy’s law match the experimental data better, while the simulated results by Fick’s law deviate greatly, which indicates that the methane flowing through coal particles is more in accordance with Darcy’s law.

A novel model to determine gas content in naturally fractured shale

A shale reservoir’s gas content is an indispensable, crucial parameter used in exploration, formation evaluation, block selection and exploitation. Canister gas desorption measurements are used to quantify lost gas volumes that are key to determine the gas content of the reservoir Usually, a lost gas volume is evaluated using the United States Bureau of Mines (USBM) direct method and/or a polynomial curve-fitting method. These methods extrapolate desorption data to time zero in the curve of a cumulative desorption volume versus the square root of time. Lost gas time is the time spent lifting the core out of a hole, handling and placing it inside a canister for evaluation., For cores with densely developed natural fractures, neither the USBM method nor the polynomial curve-fitting method can accurately estimate the lost gas volume if the lifting, handling and canister storage takes a long time. The lost gas period is a non-isothermal gas desorption process; the surrounding temperature keeps declining. Experiments were conducted to determine the methane desorption volumes of a Duvernay shale sample under various pressures and temperatures. Using Boyle’s law and the multilayer adsorption model, a novel mathematical model was built to calculate lost gas volume, including free gas and desorption gas. Because gas desorption is an endothermic procedure, equations for the inhaled energy were developed to overcome the van der Waals force between the molecules, deorbit the vibration equilibrium and the migration of molecules into the air. Finally, a shale sample with dense natural fractures from the Duvernay formation of Alberta, Canada, was used to validate the mathematical model. For comparison, the results from the USBM method, polynomial curve-fitting method and model’s calculations are illustrated in tables and figures. The outcomes exposed the results from the USBM method and polynomial curve-fitting method as deviating from the real gas content in the Duvernay formation without the consideration of the free gas loss. The model is reliable for performing gas content evaluations because it considers dual-pore structure, PVT behaviour and free gas escape simultaneously.

Evolution of methane ad-/desorption and diffusion in coal under in the presence of oxygen and nitrogen after heat treatment

When fire occurs in an underground goaf, with the high temperature and a gas environment of injected nitrogen, the coal structure is affected and the methane adsorption characteristics are changed. In this study, coal samples were heated in different gas atmospheres; isothermal adsorption, scanning electron microscopy, and proximate analysis were conducted to study the evolution of coal adsorption characteristics during heat treatment. It was found that an increase in the heat-treated temperature leads to the development of pores and cracks, providing more adsorption sites for methane. It was also found that in a high-temperature environment, oxygen reacts with coal to form oxygen-containing functional groups, leading to an increase in the methane adsorption capacity. The hysteresis of methane desorption first decreases and then increases with the increase of temperature. We think that the decrease of hysteresis may be due to the decrease of moisture and volatile content, which makes the pores more unblocked. And the increase of hysteresis may be caused by the increase of pores and the narrowing of pore throat. The diffusivity of methane first decreases and then increases with an increase in temperature. The thermal expansion of the coal caused by heat treatment leads to a change in pore structure and diffusivity. The SEM experiment shows that the pore and crack structure changes with an increase of heat-treated temperature. With oxidation, the coal surface melts and produces a “bark-like” structure. Through proximate analysis, it was found that heat treatment leads to a significant decrease in volatile and a significant increase in fixed carbon content, which is closely related to the methane adsorption capacity. The research results provides valuable information to the engineering of prevention of spontaneous combustion in goaf and the control of methane disasters.

Multiscale Lattice Boltzmann Simulation of the Kinetics Process of Methane Desorption-Diffusion in Coal.

The methane desorption and diffusion characteristics in coal are key factors affecting coalbed methane productivity. In this paper, we developed a lattice Boltzmann model for methane migration in the multiscale porous media of coal. In the simulation, the diffusion of methane in macropores/fractures is assumed to follow Fick’s law, and that in the coal matrix is treated as Knudsen diffusion. In addition, the Langmuir adsorption kinetics equation is employed to describe the dynamic process of methane adsorption and desorption. The results indicated the following: (1) The specific surface area and fracture proportion of the coal will increase with the employment of hydraulic fracturing, which may prompt the gas desorption-diffusion efficiency. (2) The flow and diffusion of methane are closely related to each other. When the gas diffusivity is poor, the desorption-diffusion can be effectively accelerated by increasing the drainage intensity, but when the gas diffusivity is fine, the flow velocity has little influence on the methane desorption. In practice, if the estimated methane diffusion coefficient is below the order of 10–5 m2/s, more attention should be paid to its accuracy; otherwise, the obtained results may have a large deviation from the real value. (3) In the typical range of average pore sizes of coal, gas desorption rate growth with the increase of pore size makes the low-rank coal more advantageous in exploitation due to its larger average pore size. With the decline of reservoir pressure, the low- and high-rank coals more easily desorb methane than medium-rank coal. (4) In the kinetic study of the coalbed methane desorption-diffusion process, the accuracy of the obtained results may depend on the adsorption and desorption rate constants if the desorption rate constant is less than 106 1/s.

OpenFOAM solver of the methane behaviour near the coal mine tunnelling face and its application

Abstract The methane near a tunnelling face seriously affects production safety in coal mines. A model considering methane seepage, adsorption, desorption and coal damage processes was established in this research. The open field operation and manipulation (OpenFOAM) solver was compiled to numerically solve the established model. The model is validated against data published in a previous theoretical study. The solver was used to investigate the effect of different parameters on methane emission regularity. This solver demonstrates that the effects of the original stress, coal cohesion and coal internal friction angle on the methane emission rate are limited, but their effects on the width of the fractured zone and effective stress are great. The effects of the initial methane pressure and coal adsorption parameters on the methane emission rate are also notable, but their effects on the width of the fractured zone and effective stress are limited.

Numerical Simulation Research on Well Pattern Optimization in High–Dip Angle Coal Seams: A Case of Baiyanghe Block

The Baiyanghe block in Fukang, Xinjiang, China, is rich in coalbed methane (CBM) resources, and several pilot experimental wells have yielded high production. Due to the high dip angle (35–55°) of the coal seam in this area, the lack of understanding of the geological characteristics, the physical properties of coal, and gas–water migration law lead to immature development techniques and poor overall development benefits. We first conducted desorption and adsorption tests on low-rank coal of this area and found residual gas in the coal. We established a coalbed methane desorption model suitable for this area by modifying the isotherm adsorption model. Next, by analyzing the influence of the gas–water gravity differentiation in the high–dip angle coal seam and the shallow fired coalbed methane characteristics in this area, we discovered the leakage of CBM from the shallow exposed area of the coal seam. Given the particular physical property of coal and gas–water migration characteristics in this area, we optimized the well pattern: (i) the U-shaped along-dip horizontal well group in coal seams is the main production well for gas production with a spacing distance of 312 m; (ii) a multistage fracturing well drilled in the floor of coal is for water production; and (iii) vertical wells with a spacing distance of 156 m in the shallow area is to capture CBM leakage. Using numerical simulation and net present value (NPV) economics models, we optimized the well pattern details. Applying our CBM desorption model, the numerical simulator can improve the accuracy of the low-rank coalbed methane productivity forecast. The optimization results demonstrated the following: 1) the cumulative gas production of single U-shaped well increased by 89% with the optimal well spacing, 2) the cumulative gas production of the well group increased by 87.54% after adding the floor staged horizontal well, and 3) the amount of CBM leakage decrease by 67.59%.

Influences of controlled microwave field irradiation on physicochemical property and methane adsorption and desorption capability of coals: Implications for coalbed methane (CBM) production

The influences of microwave irradiation on physicochemical properties and methane adsorption and desorption capability of four ranks of coals were investigated to explore the feasibility of enhancing coalbed methane (CBM) production by using controlled microwave field irradiation. The results indicate that the microwave irradiation changes coal pore structure characteristics. In particular, the micropore surface area and volume of low-rank coals decrease, whereas those of medium- and high-rank coals increase slightly. The mesopore surface area and volume of all the coals after microwave irradiation decrease. Overall, microwave irradiation decreases the adsorption pore volume with diameter less than 7.00 nm of all the coals for CH4. Furthermore, microwave irradiation enhances the connectivity of adsorption pores and seepage pores of low-rank coals and decreases the connectivity of adsorption pores of medium- and high-rank coals. With respect to surface chemistry property, microwave irradiation reduces the total amount of primary oxygenic groups including C–O, C = O and –COOH of all the coals. Finally, microwave irradiation decreases the maximum CH4 adsorption capability on all the coals by 2.50–18.18%, because of the decreasing number of adsorption pores for CH4. Moreover, microwave irradiation weakens CH4 adsorption and desorption hysteresis on coals, probably due to the decreasing CH4 adsorption-induced coal matrix swelling. The decreasing hysteresis implies that controlled microwave field irradiation is beneficial to enhancing CBM recovery.

CO2-ECBM and CO2 Sequestration in Polish Coal Seam – Experimental Study

Purpose Methane recovery is interesting not only because of its clean combustion; it is also beneficial for the environment because of the reduction of the amount of methane emitted into the atmosphere, which is important because of methane’s significant impact on the greenhouse effect. However, desorption of methane is a slow process, significantly dependent on the coalification of coal, its porosity and petrographic composition. Injection of carbon dioxide into the coal bed under sufficient pressure might be a factor in stimulating the efficiency of this process, as – because of preferential sorption – carbon dioxide displaces methane molecules previously absorbed in the coal matrix. Methods The measurements were made for Polish low-rank coal used for the analysis of methane recovery from Polish coal mines. Coal samples were collected from sites used for geological, sorption and petrographic research, as well as for the assessment of the reservoir’s genetic origin CH4 content. Experimental studies of sorption were performed with the use of the volumetric method at a lower and higher gas pressure. Results The methane isothermes show more than double the reduction of adsorption along with increasing temperature. The most significant changes of sorption capacity due to temperature variations can be seen when observing the difference in the course of the hysteresis of sorption/desorption of the gas as a function of temperature. In cases where there is a temperature of 323 K, a temperature hysteresis loop might indicate larger quantities of methane trapped in the porous structure of coal. In cases of carbon dioxide as sorbate, a similar shape of sorption isotherms occurred at both temperatures, while the temperature increase caused approximately double the reduction of sorption capacity. Also the isotherm’s shape is similar for both temperatures of measurement, indicating no effect of temperature on the amount of gas within the structure of the tested coal. High-pressure isotherms of CO2 and CH4 are confirmed in the literature, proving that carbon dioxide is the gas that allows the best penetration of the internal structure of bituminous coal. The critical temperature of CO2 (304.5 K) is so high, that sorption measurements can be performed at room temperatures (293, 298 K), where activated diffusion is relatively fast. Practical implications Understanding the sorption of gases is the primary issue, related to the exploitation of coal seams, when explaining the mechanism of gas deposition in coal seams and its relationship with outbursts of rocks and gases in mines. Originality/value The results indicate successful sorption of carbon dioxide in each experiment. This provides the rationale to study the application of the coal tested to obtain methane genetic origin genetic methane with the use of the CO2 injection.

Impact of cleanup additive on methane desorption on Longmaxi shale

After multistage hydraulic fracturing, it is essential to understand the effect of fracturing fluids trapped in shale gas reservoirs on the shale-well. To address the effect of cleanup additive as a major composition of slickwater fracking fluids on methane desorption after hydraulic fracturing, we performed a series of experiments, including X-ray to obtain the mineralogical composition of the gas shale sample, low-pressure nitrogen adsorption to obtain pore size distribution, and series of adsorption and desorption tests. We compared the effect of different methane densities of adsorbed phase on methane absolute adsorption and results demonstrate that the methane adsorption is significantly undervalued. The methane adsorption isotherm and desorption isotherms before and after cleanup additive treatment were well fit by Freundlich model. Furthermore, we compared the desorption isotherms, desorption efficiency, free gas content as well as cumulative gas production before and after treatment. These findings indicate that cleanup additive treatment results in the decrease of adsorption capacity and the increase of adsorption intensity. There exists an intersection between two curves of desorption efficiency before and after treatment. Cleanup additive significantly translates adsorbed phase into free phase to improve free gas content, and increases cumulative gas production. These results can be very helpful for reserves evaluation, knowledge of the role of cleanup additive both increasing slickwater flowback and improving gas recovery.

Promotion effects of microwave heating on coalbed methane desorption compared with conductive heating

As a clean energy resource, coalbed methane (CBM) has drawn worldwide attention. However, the CBM reservoir has strong adsorption capacity and low permeability and thus requires stimulation. As a means to stimulate coalbed methane recovery, thermal injection faces geological and economic challenges because it uses conventional conductive heating (CH) to transfer heat. Realized by the conversion of the electromagnetic energy into the thermal energy, microwave heating (MH) may be a sound stimulation method. Although previous research suggested that MH had potential as a stimulation method for coalbed methane recovery, it is not clear if MH is superior to CH for enhancing coalbed methane recovery. This paper compares the effect of MH and CH on methane desorption from coal using purpose-built experimental equipment. To compare the MH and CH experimental results, the desorption temperature for each CH desorption test was set to the maximum temperature reached in the correlative MH desorption test. The results show that although the cumulative desorbed volume (CDV) of methane under MH was less than that desorbed by CH in the initial desorption stage, the final total CDV under MH for the three different power settings was ~ 12% to ~ 21% more than that desorbed by CH at the same temperatures. CH and MH both change the sample’s microstructure but MH enlarges the pores, decreases methane adsorption, promotes methane diffusion, and improves permeability more effectively than CH. Rapid temperature rise and the changes in the coal’s microstructure caused by MH were the main reasons for its superior performance. These findings may provide reference for selecting the most appropriate type of heating for thermal injection assisted coalbed methane recovery.

Experimental study of slickwater volume effect on methane desorption on Longmaxi shale

A large portion of slickwater is trapped in gas shale formations after the hydraulic fracking process. To address the effect of slickwater retention on methane desorption, the adsorption and desorption isotherms for methane before and after slickwater treatment were measured based on the volumetric method using shale samples from the Longmaxi formation, China. We found the adsorption and desorption data for the methane before and after treatment fit well with the Freundlich model. We compared the effect of slickwater volume on desorption isotherms, desorption efficiency, adsorbed methane content and cumulative gas production. Slickwater treatment significantly reduces methane adsorption. The Freundlich constant k reduction decreases nearly linearly with the slickwater volume, and constant n increases nearly linearly first and then becomes slower with slickwater volume. Test results also reflected that slickwater treatment could improve methane desorption efficiency. The curve of desorption efficiency after treatment is significantly lower than before. Under 30 MPa pressure, the desorption efficiency increases by 50.93% when slickwater volume increases to 10.40 ml from 2.02 ml. A closed loop exists between two curves of adsorbed gas content before and after treatment, and its area increases with slickwater volume. Slickwater treatment improves cumulative gas production. The cumulative production of methane before treatment is smaller than after. 8.16 ml of slickwater is the best option for 130 g of shale powders to obtain the most cumulative gas production in the lab-scale condition. These results can be used to optimize the slickwater volume during hydraulic fracturing.

Tests of methane desorption and emission from samples of hard coal in the context of mine closures through flooding

Tests of methane desorption and emission from samples of hard coal in the context of mine closures through flooding

Experimental insights into the adsorption-desorption of CH4/N2 and induced strain for medium-rank coals

To support the understanding of the effect of N2-enhanced coalbed methane (N2-ECBM) application on medium-rank coals, four coal samples from Shanxi Province were selected to test the porosity, mechanical properties and strain induced by binary gas adsorption-desorption. The results show that the desorption hysteresis is weakened with increasing N2 fraction and is strengthened with increasing CH4 fraction, indicating that increasing the N2 fraction can reduce the selectivity of coal to CH4. The critical desorption pressure of a mixed gas increases in a high N2 fraction environment, promoting the desorption of CH4 and enhancing its recovery. This research clarifies the anisotropic strain characteristics induced by the adsorption-desorption of CH4/N2 and provides reliable data for anisotropic permeability numerical simulations in the N2-ECBM process. Swelling and shrinkage volume strain are negative-exponentially related to the N2 fraction, and the volume shrinkage strain is close to the volume swelling strain given a N2 fraction of 50%. There exists a polynomial relationship between the volume swelling/shrinking strain and adsorption capacity, which has a potential predictive effect on the volume strain variation in coal during N2-ECBM. The analysis of the adsorption volume swelling strain rate and desorption volume shrinkage strain rate indicates that the N2 fraction and micropore volume are the main factors controlling volume deformation. The volume strain decreases with increasing elastic modulus, but nitrogen injection weakens the effect of the elastic modulus on the strain. In this study, the desorption percentage increased and the volume swelling strain decreased with increasing nitrogen fraction. Nitrogen injection and sealing have a positive effect on methane desorption and the optimal N2 fraction should be greater than 50%. This work provides a reference for improving coalbed methane production.

Impacts of gas pressure on carbon isotope fractionation during methane degassing—An experimental study on shales from Wufeng and Longmaxi Formations in southeast Sichuan, China

Methane carbon isotope fractionation (CIF) during shale gas desorption provides a new method to evaluating in-situ gas content and the ratios of adsorbed and free gas as well as predicting the production status of shale gas wells. Considering anisotropic pressures in shale gas in-situ reservoirs, five gas pressure values were selected for this study to perform methane isotherm adsorption-desorption experiments for four shale samples from the Wufeng and Longmaxi Formations in the southeastern Sichuan Basin, and the characteristics of carbon isotope fractionation during methane desorption processes were investigated. Results show that: (1) under different gas pressures, CIF of the in-methane transport process shows four stages: stable stage (I), lighter stage (II), heavier stage (III), and another lighter stage (IV); (2) CIF during methane desorption is impacted by multiple factors including the total organic carbon (TOC) content, pore structure, gas pressure, and adsorbed gas ratio, while material composition and pore structure largely determine the characteristics of isotopic fractionation in methane desorption, which are extremely important control factors; and (3) gas pressure has an obvious effect on CIF characteristics by changing the proportion of adsorbed gas, especially in stages II and IV, and the sample with developed organic matter (OM) had a stronger response. The lower the gas pressure, the earlier the desorption process corresponding to the start of significant isotope fractionation. The higher the ratio of adsorbed gas, the larger the range of isotope fractionation. This demonstrates the significance of adsorption-desorption for CIF in shale gas degassing. Furthermore, we used a technique integrating the Amoco curve fit (ACF) method and CIF to calculate in-situ gas content and the dynamic change in adsorbed and free gas ratios during gas degassing.

Effects of pore morphology and moisture on CBM‐related sorption‐induced coal deformation: An experimental investigation

AbstractCoalbed methane (CBM) is an important resource of energy. For CBM recovery, sorption‐induced coal deformation can cause significant reservoir permeability change. Moisture content and coal pore morphology affect the gas adsorption capacity and can alter the coal deformation of the coal seam. Therefore, it is crucial to establish a coal gas moisture‐coupled model for CBM production prediction. However, there are currently insufficient data available for quantitative analyses. In this paper, a series of typical sorption‐induced strain experiments were carried out during methane adsorption and desorption on coal samples with different metamorphic degrees and moisture content. The pore morphology and adsorption capacity of coals were measured to analyze the reason for different deformation of coals with various pore structures and moisture. Results show that the adsorption capacity and deformation is corresponding to the specific surface area of micropore, which first decreases and then increases with coal ranks. The deformation and adsorption gas content of dried coals is greater than that of natural moisture coals, which means that moisture can reduce the sorption capacity of coals, resulting in the decrease of gas adsorption‐induced coal deformation. There is also residual deformation of coal samples after gas desorption caused by the residual adsorbed gas in coals. This paper quantitatively investigates the effects of pore characteristics and moisture on coal deformation and the internal mechanism. This work will provide essential information for building out a fully cross‐coupled model of coal gas‐moisture relationships for CBM production prediction.

Pore structure of different macroscopically distinguished components within low-rank coals and its methane desorption characteristics

The pore structure of the macroscopically distinguished components is complex, which directly affected methane adsorption/desorption. In this paper, mercury intrusion porosimetry (MIP), liquid nitrogen adsorption (LNA), scanning electron microscope (SEM), nuclear magnetic resonance (NMR) were used to describe the pore structure of vitrain and durain within low-rank coals. The results indicate that pores mainly were classified as micropores and transition pores, in which micropores are dominant, and transition pores in durain are more developed than vitrain. The total specific surface area of durain is 1.13 times that of vitrain, but the BET specific surface area of vitrain is 1.32 times that of durain. Comparison of T2 spectra between saturation and centrifugation, the connectivity between and within different types of pores in durain is better than that of vitrain. The adsorption capacity of durain is higher than that of vitrain. The contribution of specific surface area (SSA) to methane adsorption is less than pore connectivity. The desorption ratio of durain is higher than that of vitrain, it can be considered that pore connectivity is the key factor to control the methane adsorption/desorption performance of different macroscopically distinguished components. The desorption ratio of the moisture equilibrium sample is relatively higher than that of the air-dried based sample, the isosteric adsorption heat of air-dried based sample is higher than that of the moisture equilibrium sample, showing that water plays a positive role in the process of methane desorption. The interface between vitrain and durain is the high-frequency position of methane production.

Molecular modeling on the pressure-driven methane desorption in illite nanoslits.

Understanding to the pressure-driven desorption of methane in shale formations is crucial for the establishment of predictive models used in shale gas development. Based on the grand canonical Monte-Carlo simulations of methane adsorption in illite slits of 1-5-nm wide, the pressure-driven desorption processes of methane in the nanoslits are studied with non-equilibrium molecular dynamics simulations. External forces are applied to the methane molecules to mimic a pressure drop that releases the adsorbed molecules and pushes them flowing directionally. Effect of pressure drop and slit aperture on the interchange between adsorbed and free phases of methane is investigated by a statistic analysis on the velocity and density distributions of methane molecules in the nanoslits under various conditions. A minimum pressure drop that initiates the methane desorption in the illite slit exists and varies with slit aperture. Our simulations reveal the microscopic mechanism of pressure-driven methane desorption, which would be useful for subsequent studies on the prediction of mineable yields for shale formations. Under pressure drop, adsorbed methane moleculesare desorbed to free phase and then transported to wellbore. The criterion of pressure drop for desorption increases with decreasing slit aperture.

Experimental investigation of the effects of ultrasonic stimulation on adsorption, desorption and seepage characteristics of shale gas

To verify the feasibility of applying ultrasonics to enhancing gas production from shale gas reservoirs, experimental study on the effects of ultrasonics on adsorption, desorption and seepage characteristics of shale gas using four samples from Sichuan Basin has been carried out. Adsorption and desorption of methane, were tested by volume method under different conditions, while the permeabilities of shale samples were tested by steady state method under different conditions. Results show that ultrasonics has a negative effect on the adsorption of shale gas, while a positive effect on desorption, resulting from the fact that ultrasonics treatment can enhance the vibration of gas molecules to break down the adsorption equilibrium. Further, the permeability dropped significantly with the effective confining pressure and pore pressure increasing, resulting from that high confining pressure decreases the area of seepage channels. The results also illustrate that permeability increases with the ultrasonics power increasing, presumbly by forming microfractures in shale which results in significant increase in the area of seepage channels.

Influence of coalification on methane diffusion dynamics in middle-high rank coals

ABSTRACT Understanding the effect of coalification on the diffusion of methane in middle-high rank coals (MHRC) is fundamental for optimizing the coalbed methane (CBM) drainage strategies. Safe coal mining relies critically on this approach. Hence, CH4 adsorption/desorption experiments and a new methane diffusion model were used to study methane diffusion behavior in five different metamorphic degree coals from China. The results indicate that, as the vitrinite reflectance (Ro) increase, the Langmuir volume (VL) shows a downward trend at first, then increases. Coalification especially for the third coalification jump (Ro = 1.26%) shows a turning effect on diffusion dynamics of MHRC. The desorption volume and initial diffusion coefficient (D0) both show a slight decreasing, then rapid increasing trend. The D0 of five coals jumps at Ro = 1.26%. VL increases alongside that of the micropores volume. With the decrease of the Raman parameter ID1/IG, the VL, methane desorption volume within 7200 s and D0 all increase. Results show that coalification changes the microporous structure and the macromolecular structure of the coal, which finally affects the diffusion capacity of coal. Coalification has a positive effect for the gas extraction and the development of CBM for the semianthracites.