Shuheng Tang

Effects of pressure and temperature on gas diffusion and flow for primary and enhanced coalbed methane recovery

Due to the rapid increase of coalbed methane (CBM) exploration and development activities in China, gas adsorption and flow behavior for Chinese coals are of great interest for the industry and research community. How pressure and temperature affect the gas adsorption and flow on different rank coals are not only important for CBM recovery but also important for CO2 or N2 enhanced CBM recovery, since gases are often injected at a temperature different to the reservoir temperature. In this work, gas adsorption and permeability of three different rank Chinese coals are measured using CH4, N2 and CO2 at three temperatures, 20°C, 35°C and 50°C. Gas diffusivity and permeability with respect to gas species, pore pressure, effective stress and temperature are studied. The three coals are SQB-1 from Southern Qinshui Basin, JB-1 from Junggar Basin and OB-1 from Ordos Basin. Gas adsorption results show that both pressure and temperature have significant impact on adsorption behavior for SQB-1 and JB-1 using CH4. For higher rank coal SQB-1, adsorption isotherm tends to reach adsorption capacity quicker with respect to pressure. However, the maximum adsorption capacity is higher for the lower rank coal JB-1. Moreover, temperature has a stronger effect on reducing adsorption capacity for lower rank coal. Gas diffusivity results for OB-1 and JB-1 show that CO2 diffusivity is generally higher than that of CH4 and then N2. This could be related with their different kinetic diameters and their interaction with the coal. Both pressure and temperature have impact on gas diffusivity. In general, gas diffusivities increase with pressure and temperature. Permeability results show that it varies greatly with respect to coal rank with highest rank coal having the lowest permeability. Permeability is also strongly sensitive to effective stress and pore pressure. Temperature has a noticeable impact on permeability change. Permeability changes differently with temperature increase for the different rank coal samples studied. This may be attributed to the combined effect of coal strain change due to gas adsorption and thermal expansion. These results have significant implications for the design of enhanced CBM recovery and CO2 storage for different rank coals as injecting gas at different temperature and pressure would affect the CO2 injectivity and the CBM production rate.

Methane Sorption Capacity of Organics and Clays in High-Over Matured Shale-Gas Systems

Methane adsorption experiments were conducted on a series of organic-rich shales, isolated kerogens, and pure clay minerals at 60 °C and up to 20 MPa pressure. The maximum adsorption capacities of the two isolated kerogens (type I) (17.45 cm3/g and 12.41 cm3/g) were much higher than those of the clay minerals and shale samples. In the high-over mature stage, the affinity of methane for type I kerogens gradually increased, while the amounts of methane adsorbed decreased with increasing thermal maturity. Among the pure clay minerals, the methane adsorption capacity decreased in the following order: montmorillonite (4.02 cm3/g) > kaolinite (3.48 cm3/g) > illite (3.46 cm3/g) > illite/smectite mixed layer (3.1 cm3/g) > chlorite (0.88 cm3/g); the methane adsorption capacities were controlled by the effective surface areas available for adsorption. These clay minerals with higher Langmuir pressures exhibited weaker affinities for methane than the isolated kerogens. Moreover, the adsorption results of kerogen, shale, and illite at different temperatures (30 °C, 60 °C, and 90 °C) show that the VL values of kerogen decreased linearly with increasing temperature, while the amount of adsorbed water on clay minerals decreased with increasing temperature, which may have affected the methane adsorption capacity. The results show that the contributions of kerogens to the adsorption capacities of the two bulk shale samples were ∼ 43.08% and 56.58%, and the methane adsorption of clay minerals accounted for ∼ 44.12% and 16.74%.

Evaluation of the Coalbed Methane Potential by a GIS-Based Fuzzy AHP Model

Coalbed methane (CBM) potential evaluation is important for choosing the prospective target area for CBM exploration and production. Many unwise evaluations may have induced the high failure ratio of CBM exploration and development in China. Developing a comprehensive model for CBM potential evaluation is also complicated by the diverse geologic and engineering factors. In this paper, a Fuzzy analytic hierarchy (AHP) model for CBM evaluation is proposed, capable of tracking the uncertainty and imprecision and solving the multiple-criteria decision-making problems. The model is applied to evaluate the CBM potential of the eastern parts of Ordos basin. The results indicated that the Fuzzy AHP evaluation system is suggested to be a good tool for evaluating CBM potential.

The component fractionation effect in binary-component gas adsorption isotherm experiments on coal

Different gas adsorption isotherm experiments were carried out on the coal samples from Jincheng district, including N2, CH4, CO2, as well as the binary-component gas of CH4-N2 and CH4-CO2 of three different concentrations. In the binary-component gas adsorption isotherm experiments, the gas component with higher adsorption ability is adsorbed firstly. Thus the result in its relative concentration in the free phase shows a trend of decrease first and then increase, whereas the relative concentration of the gas component with lower adsorption ability shows a trend of increase first and then decrease. In the adsorption phrase, the relative concentration of the gas component with higher adsorption ability increases gradually, and the relative concentration of the gas component with lower adsorption ability decreases gradually. In the adsorption competition of the binary-component gas, the adsorption rate of the gas component with higher adsorption ability shows a trend of rapidity at first then slowness, but the adsorption rate of the gas component with lower adsorption ability shows a trend of slowness at first then rapidity. The component fractionation effect in the binary-component gas adsorption isotherm experiments is caused by the difference of the adsorption ability of coal of different gas components.