Kinetic modeling of individual gaseous component formed from coal in a confined system

Kinetic modeling of individual hydrocarbon formation is important for gas compositional and stable carbon isotopic prediction in a gas field. However, the maximum yields of individual gaseous hydrocarbons vary with heating rates in a confined system. This makes it impossible to kinetically model their formation directly on the basis of first-order reaction models due to the difficulty in assessment of their generation potentials. This study focuses on modeling the yields of individual gaseous hydrocarbons from coal in a confined system. On the basis of kinetic data obtained under open systems, methane conversion at 600 °C at a 2 K/h heating rate in our confined systems is estimated to be 97%, and the potential yield of methane from this coal is estimated to 148 ml/g kerogen. As an intermediate product in a series reaction, ethane has variable maximum yields with different heating rates, which makes it difficult to assess the ethane potential in coal. This is also the case for the other heavy hydrocarbon components (C3+). In the present study, the maximum yields of heavy gases (C2+) increase with decreasing heating rates, which can be modeled with kinetics of series (coupled) reactions, e.g., generation followed by cracking. The kinetic parameters vary with the assumed variable potentials (18 ml/g kerogen, 15 ml/g kerogen and 12 ml/g kerogen) but the same yield is obtained for the given heating rates, for example, at 2 K/min, 2 K/yr, 2 K/Myr, and 10 K/Myr, which suggests that the assumed variable potentials have no significant influence on modeling results. The ethane yields modeled from three different heating rates, namely at 1 K/h, 2 K/h, and 20 K/h, coincide well with the laboratory pyrolyzed data. Under geological conditions, the modeled yield of methane increases with maturation (Ro). For heavy gaseous components (C2–5), the quantity remains steady at 1.2–1.6% Ro, then decreases at the post-mature stage (1.8–2.2% Ro). Gas dryness decreases with maturity increasing in the range of 0.6–1.0% Ro, then increases to 0.95 in the higher maturity range (2.2% Ro). Those data generally fit with the occurrence of gases from the humic organic matter in nature, indicating the applicability of this kinetic model in sedimentary basins for prediction of gas composition.