Experimental study of the adsorption-induced coal matrix swelling and its impact on ECBM

Abstract

Carbon dioxide (CO₂) enhanced coalbed methane (ECBM) is an effective method to improve methane (CH₄) production and this technology has already been used to increase gas production in several field trials worldwide. One major problem is the injection drop in the later period due to permeability decrease caused by coal matrix swelling induced by CO₂ injection. In order to quantify the swelling effect, in this work, coal samples were collected from the Bulli coal seam, Sydney Basin and adsorption tests with simultaneous matrix swelling measurement were conducted. The adsorption and swelling characteristics were analyzed by measuring the adsorption mass simultaneously with the strain measurement. Then experiments were conducted to replicate the ECBM process using the indirect gravity method to obtain the swelling strain change with CO₂ injection. The results show that the coal adsorption capacity in CO₂ is almost two times greater than that in CH₄, and nitrogen adsorption is the least among these gases. A Langmuir-like model can be used to describe the strain with the gas pressure and the swelling strain induced by gas adsorption has a linear relationship with gas adsorption quantity. Moreover, swelling strain increase was observed when CO₂ was injected into the sample cell and the swelling strain was almost the sum of the strains induced by different gases at corresponding partial gas pressure.

Keywords

coal / sorption strain / ECBM / adsorption isotherm / swelling

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