Investigation of CO2 Injection Induced Coal-Gas Interactions Available to Purchase

Understanding the role of the co- and counter-diffusive properties of CO₂ and CH₄ in coal has important implications for enhanced coalbed methane recovery (ECBM) and CO₂ sequestration in coals and for related issues of gas outbursts during mining. This study addresses how coal-gas interactions affect CO₂ injectivity, in particular the roles of coal deformation, gas flow, CH₄-CO₂ counter-diffusion and gas absorption/desorption on the evolution of transport and mechanical properties of fractured coals, and therefore on ECBM recovery.

A fully coupled coal deformation, gas flow, CH₄-CO₂ counter-diffusion and gas absorption/desorption finite element (FE) model was developed to investigate the combined net effects on evolutions of CO₂ injection related parameters. The FE model was successfully applied to match the experimental data; and a field scale model was constructed to quantify CO₂ injection rate and other transport parameters for ECBM under in-situ conditions. Model results indicated that (1) Coal rank has a converse influence on the CO₂ injection performance, lower coal rank reservoir could be more suitable to carry out CO₂-ECBM technology; (2) Initial permeability has positive impact on the performance of CO₂ replacing methane. This finding also has an important indication for the implementation of this technology in shale and other low permeability media; (3) CO₂ injection is very sensitive to changes in the injection gas Langmuir strain constants, so trying to reduce the resultant strain constant, such as using mixed gases like N₂ and CO₂ or flue gas, is an efficient solution to improve the methane recovery efficiency.

Using carbon dioxide to displace CH₄ in unmineable coalbeds has been paid more attention due to the global warming and the resource shortage. This technique has been proposed as one of the geologic strategies to mitigate increasing concentrations of CO₂ in the atmosphere and it is particularly attractive in those cases where the coal medium contains large amounts of natural gas (CH₄) [1-3].

Carbon dioxide is known to have a greater affinity to coal than methane. Early laboratory isotherm measurements for pure gases have demonstrated that coal can absorb approximately twice as much CO₂ by volume as methane. Recent research on CO₂ sorption capacity of different ranks of United States coal has shown that this ratio may be as high as 10:1 in some low rank coals [4]. This observation suggests that stronger affinity of CO₂ to the coal material could initiate a mechanism of displacement of the originally in-place CH₄, when CO₂ is introduced to the coalbed environment. This is extremely important not only for sequestration but also for coalbed methane production. Since the concept of coal seam sequestration was first proposed by Macdonald of Alberta Energy during discussions with Gunter in 1997 [1], a number of field CO₂-ECBM storage pilot projects have been carried out in North America, Europe (Poland), China, Japan. The Allison Unit pilot, which is located in the Northern New Mexico part of the San Juan Basin, represents the world’s first field trial of CO₂-ECBM in 1996 [5-6].