Adsorption Factors in Enhanced Coal Bed Methane Recovery: A Review

Enhanced coal bed methane recovery using gas injection can provide increased methane extraction depending on the characteristics of the coal and the gas that is used. Accurate prediction of the extent of gas adsorption by coal are therefore important. Both experimental methods and modeling have been used to assess gas adsorption and its effects, including volumetric and gravimetric techniques, as well as the Ono–Kondo model and other numerical simulations. Thermodynamic parameters may be used to model adsorption on coal surfaces while adsorption isotherms can be used to predict adsorption on coal pores. In addition, density functional theory and grand canonical Monte Carlo methods may be employed. Complementary analytical techniques include Fourier transform infrared, Raman spectroscopy, XR diffraction, and 13C nuclear magnetic resonance spectroscopy. This review summarizes the cutting-edge research concerning the adsorption of CO2, N2, or mixture gas onto coal surfaces and into coal pores based on both experimental studies and simulations.

A new desorption-induced permeability dynamic model and its application in primary coalbed methane recovery

In the process of dewatering and recovery of coalbed methane, coal permeability exhibits a quite unique feature due to the interference of matrix shrinkage and stress effects. A new theoretical dynamic model was proposed for coal permeability based on the assumptions of matchstick geometry of the coal and uniaxial strain condition. Distinct from previous models such as P&M and S&D models, our model relates the gas-sorption-reduced strain to the change of surface energy of coal solids. One of the advantages of this model is that it does not require the sorption-reduced strain as an essential input, and therefore eliminates the related laborious and expensive laboratory measurement. The model was validated by fitting it to two sets of public data and shows an excellent match with the observed data. The results also indicate that our model has a better performance in predicting the permeability dynamics than P&M and S&D models. Additionally, a sensitivity analysis of the effect of input parameters on permeability dynamics was conducted by gray-relation theory, and the initial porosity and reservoir temperature are demonstrated to exert a most distinguished effect on the permeability dynamics. Finally, the proposed model was incorporated into a numerical simulator and successfully applied to conduct a history match of the gas and water production rate in a developed territory.

Upgrading Biogas from Small Agricultural Sources into Biomethane by Membrane Separation

The agriculture sector in Poland could provide 7.8 billion m3 of biogas per year, but this potential would be from dispersed plants of a low capacity. In the current study, a membrane process was investigated for the upgrading biogas to biomethane that conforms to the requirements for grid gas in Poland. It was assumed that such a process is based on membranes made from modified polysulfone or polyimide, available in the market in Air Products PRISM PA1020 and UBE UMS-A5 modules, respectively. The case study has served an agricultural biogas plant in southern Poland, which provides the stream of 5 m3 (STP) h−1 of biogas with a composition of CH4 (52 vol.%), CO2 (46.3 vol.%), N2 (1.6 vol.%) and O2 (0.1 vol.%), after a pretreatment. It was theoretically shown that this is possible to obtain the biomethane stream of at least 96 vol.% of CH4 purity, with the concentration of the other biogas components below their respective thresholds, as required in Poland for gas fuel “E”, with methane recovery of up to 87.5% and 71.6% for polyimide and polysulfone membranes, respectively. The energetic efficiency of the separation process is comparable for both membrane materials, as expressed by power excess index, which reaches up to 51.3 kWth kWel−1 (polyimide) and 40.7 kWth kWel−1 (polysulfone). In turn, the membrane productivity was significantly higher in the case of the polyimide membrane (up to 38.3 kWth m−2) than those based on the polysulfone one (up to 3.13 kWth m−2).

Numerical Simulation of Gas Hydrate Production Using the Cyclic Depressurization Method in the Ulleung Basin of the Korea East Sea

The depressurization method is known as the most productive and effective method for successful methane recovery from hydrate deposits. However, this method can cause considerable subsidence because of the increased effective stress. Maintenance of geomechanical stability is necessary for sustainable production of gas from gas hydrate deposits. In this study, the cyclic depressurization method, which uses changing the bottomhole pressure and production time during primary and secondary depressurization stage, was utilized in order to increase stability in the Ulleung Basin of the Korea East Sea. Various case studies were conducted with alternating bottomhole pressure and production time of the primary and secondary depressurization stages over 400 days. Geomechanical stability was significantly enhanced, while cumulative gas production was relatively less reduced or nearly maintained. Specially, the cumulative gas production of the 6 MPa case was more than three times higher than that of the 9 MPa case, while vertical displacement was similar between them. Therefore, it was found that the cyclic depressurization method should be applied for the sake of geomechanical stability.