Experiments on Methane Displacement by Carbon Dioxide in Large Coal Specimens: Fid-Bau Portal

Experiments on Methane Displacement by Carbon Dioxide in Large Coal Specimens

Liang, Weiguo / Zhao, Yangsheng / Wu, Di / Dusseault, Maurice B.

Abstract Carbon dioxide ($ CO_{2} $) is considered to be the most important greenhouse gas in terms of overall effect. $ CO_{2} $ geological storage in coal beds is of academic and industrial interest because of economic synergies between greenhouse gas sequestration and coal bed methane ($ CH_{4} $) recovery by displacement/adsorption. Previously, most work focused on either theoretical analyses and mathematical simulations or gas adsorption–desorption experiments using coal particles of millimeter size or smaller. Those studies provided basic understanding of $ CH_{4} $ recovery by $ CO_{2} $ displacement in coal fragments, but more relevant and realistic investigations are still rare. To study the processes more realistically, we conducted experimental $ CH_{4} $ displacement by $ CO_{2} $ and $ CO_{2} $ sequestration with intact 100 × 100 × 200 mm coal specimens. The coal specimen permeability was measured first, and results show that the permeability of the specimen is different for $ CH_{4} $ and $ CO_{2} $; the $ CO_{2} $ permeability was found to be at least two orders of magnitude greater than that for $ CH_{4} $. Simultaneously, a negative exponential relationship between the permeability and the applied mean stress on the specimen was found. Under the experimental stress conditions, 17.5–28.0 volumes $ CO_{2} $ can be stored in one volume of coal, and the displacement ratio $ CO_{2} $–$ CH_{4} $ is as much as 7.0–13.9. The process of injection, adsorption and desorption, displacement, and output of gases proceeds smoothly under an applied constant pressure differential, and the $ CH_{4} $ content in the output gas amounted to 20–50% at early stages, persisting to 10–16% during the last stage of the experiments. Production rate and $ CH_{4} $ fraction are governed by complex factors including initial $ CH_{4} $ content, the pore and fissure fabric of the coal, the changes in this fabric as the result of differential adsorption of $ CO_{2} $, the applied stress, and so on. During $ CO_{2} $ injection and $ CH_{4} $ displacement, the coal can swell from effects of gas adsorption and desorption, leading to changes in the microstructure of the coal itself. Artificial stimulation (e.g. hydraulic fracturing) to improve coalbed transport properties for either $ CO_{2} $ sequestration or enhanced coal bed methane recovery will be necessary. The interactions of large-scale induced fractures with the fabric at the scale of observable fissures and fractures in the laboratory specimens, as well as to the pore scale processes associated with adsorption and desorption, remain of profound interest and a great challenge.