In this study we examine the adsorption of He, N2, CH4 and CO2 on the mechanical and flow properties of sub-bituminous coal from the Powder River Basin, Wyoming. Lab measurements were conducted on one-inch diameter core samples of coal under hydrostatic conditions. The coal samples were vacuum dried before each test, then saturated by a test gas until steady state was reached. Measurements of adsorption, swelling strain, elastic stiffness, creep strain and permeability of both intact and crushed samples were carried out at a series of either increasing pore pressure or increasing effective stress. Our results show that the adsorption of CO2 is much larger than CH4, which is larger than N2. Hysteresis is observed among pure component adsorption and desorption isotherms which are Langmuir-type adsorption isotherms. Permeability shows a moderate decrease with increasing effective stress for He, CH4 and CO2. At constant effective stress, permeability decreases when the saturating gas changes from He to CH4 and CO2. Hysteresis of permeability with increasing and decreasing effective stress is not observed in crushed samples. The coal swells when CH4 displaces He and swells more when CO2 displaces He. The same is true of viscoplastic creep. Viscoplastic creep is greater in the presence of CH4 than He and more with CO2 than with CH4.
1. INTRODUCTION Unmineable coal is an important resource because of its potential to produce coalbed methane (CBM). CBM has grown to supply approximately 10% of US natural gas production and is becoming important worldwide as an energy source [1]. Furthermore, when CO2 is injected, it has the potential to enhance the amount of methane produced (ECBM) and to geologically sequestrate CO2 as an adsorbed phase. Thus, coalbeds also present a potential sink for greenhouse gas storage. Despite this great potential, the feasibility of ECBM and sequestration of CO2 at a given site is still largely dependent on predictions from numerical modeling [2]. On the other hand, injection of CO2 will change coal properties [3]. These fundamental issues need to be investigated before simulation or field pilot test. Lab measurements were conducted to investigate the property responses when coal is saturated with gas. Adsorption isotherm, mechanical and transport property changes are of the main interest, since these properties determine the behavior of CH4 production and CO2 sequestration. Understanding the adsorption properties of CH4 and CO2 is vital for the optimum development of techniques to recover CH4 while sequestering CO2 [4,5], as the shape of the adsorption isotherm can provide information on the adsorption process, the porosity and the surface area of the adsorbent [6], While most of the study show that gas adsorption on coal is monolayer adsorption (Langmuir type), Lin [7] shows that CO2 adsorption on intact coal is described by multilayer model (BET type). Also, factors including temperature, pressure, coal rank, moisture, and gas composition control the exact sorption behavior of a certain coal [3, 8, 9, 10, 11, 12]. Moreover, hysteresis was observed between adsorption and desorption isotherm with respect to gas composition [12].
