CBM and CO 2-ECBM related sorption processes in coal

Unconventional gas, such as shale gas, tight gas or coalbed methane is able to offer a new and attractive perspective for low-carbon fuel supply. Furthermore, they may provide possibilities for CO2-storage and coevally for enhanced gas recovery. This is suggested by theoretical considerations of displacing the adsorbed methane by a more strongly adsorbing gas such as CO2. Optimizing the development of these unconventional resources, while minimizing their environmental impact, requires a more fundamental understanding of the coupled physico-chemical processes involved. In order to better understand the interaction of gas and water with coal of different maturity, a measuring plan has been developed and executed. The general observation is that CO2 and CH4 sorption capacity of dry coals shows a parabolic trend with rank with a minimum between 1.1 and 1.3% vitrinite reflectance. The CH4 and CO2 sorption capacity of moist coals, in contrast, shows a linear increase with increasing maturity. The sorption capacity ratio of CO2 and CH4 for dry coals has an approximately constant ratio of 2 over the entire maturity range. For moist coals this ratio is higher at low maturity values and decreases to values around 2 with increasing maturity. The highest CO2 to CH4 sorption capacity ratios are observed at low surface coverage. Based on the literature review an extended measuring concept was developed to investigate the competitive interaction between sorbed water and gases such as CH4 and CO2. The determination of the required high-precision CO2 sorption isotherms is challenging because in the temperature and pressure range of interest (T=308-350K) the density of supercritical CO2 increases rapidly with increase pressure. To improve the reliability of the sorption measurements, an inter-laboratory comparison between different European research institutes has been conducted (Chapters 2 and 3). During the first inter-laboratory comparison the comparability between the manometric and the gravimetric sorption methods was demonstrated. In the second inter-laboratory test the properties of natural coal sample and their influence on the accuracy of the measurements was investigated. The observed excess sorption maxima for coals are 1.77±0.07, 1.37±0.02 and 1.41±0.02 mol kg-1 in the order of increasing vitrinite reflectance. A decrease of the pressure at maximum excess sorption has been observed from lignite to bituminous coal to semi-anthracite. This shift varies from roughly 6.89±0.5 MPa, 6.68±0.4 MPa, down to 5.89±0.6 MPa in the order of increasing vitrinite reflectance. These could be related to an increase in micropore volume. Furthermore, these chapters contain recommendations and best practice rules for CO2 sorption isotherm measurements on coals. During the subsequent phase of the PhD project the competitive interaction between sorbed water and gases such as CH4 or CO2 was investigated (Chapter 4). A conceptual model was developed explaining CO2 and CH4 sorption on coal as a function of rank and the interaction mechanism of adsorbed water molecules. The adsorption behaviour of CH4 and CO2 was studied on a sequence of coal samples, covering the maturity range from 0.5 up to 3.3% vitrinite reflectance. A deeper understanding of the physical processes of the competitive sorption of CH4, CO2 and water has been achieved. Based on the thermodynamic description of the experimental observations presented in this thesis, the conceptual molecular model has been improved, by combining experimental data and published molecular dynamic simulations. The proposed molecular concept clarifies the thermodynamic process of CH4 and CO2 sorption on coals, the three fundamental kerogen types and the effects of adsorbed water. The main parameters controlling ultimate gas recovery from coalbed methane (CBM) deposits are the natural cleat permeability and the total gas content. Among the three gas storage mechanisms (compressed gas in the open porosity, dissolved gas in the formation water and sorbed gas), sorption is the dominant mechanism for CBM reservoirs. Sorption behaviour changes with coal rank, but also with temperature and pressure and hence with burial history. The fundamental understanding of the thermodynamics of CH4 sorption capacity as a function of temperature and pressure is a prerequisite for assessing methane saturation in CBM reservoirs (Chapter 6). The results of this study lead to the conclusion that the geothermal gradient is much more important than the hydrostatic pressure gradient for the CH4 sorption capacity as a function of depth. For CBM as well as CO2-ECBM activities a low geothermal gradient is favourable. Furthermore, a low-rank coal deposit requires a lower pressure draw-down than high rank coal deposits to desorb an equivalent amount of methane. The study of the thermodynamics of adsorption suggests that it is more likely to observe under-saturated CBM reservoirs in high rank than in low rank coal deposits