Experimental and analytical studies on permeability reduction of coal seam by CO₂ injection

Permeability of coal seam have a strong effect on CO₂ Enhanced Coal Bed Methane Recovery(CO₂-ECBMR) project. In particular, qualification of permeability reduction due to coal matrix swelling is a key parameter to control CO₂ injection and the project sustainability. Swelling experiments using visualization method were conducted. Crushed samples from Indonesia Low Rank Coal (LRC) were used and the experiments were carried out up to 10 MPa at 48° C temperature, resulting CO₂ in the supercritical conditions that more appropriate for CO₂ injection in the field. Coal swelling which was represent by upward surface movement of column was assumed as total expanded volume for each particle. With this assumption, crushed samples were modeled like block coal samples. For three coal samples, the maximum expanded volume due to the swelling by CO₂ adsorption has been evaluated as 0.03 at 10 MPa pressure. Since the swelling results were very comparable to others, this method has more advantage in term of sample preparation and experiment work compare to block coal. Based on the present swelling data, two analytical models proposed by Palmer and Mansoori (P-M) and Shi and Durucan (S-D) have been applied to Yubari field test. Permeability reduction (k/k₀) ratio by CO₂ swelling in the test has been estimated as 0.021 to 0.056 for the coal seam condition of 10 and 15 MPa of initial formation and injection pressures, respectively. Sasaki et al.¹⁵ proposed analytical radial flow model to evaluate CO₂ swelling ratio (β). The swelling ratio on permeability has been evaluated as β = 0.020 to 0.044 and original coal permeability k₀ ≈ 3 md by matching with monitoring data measured in the field. Both values of k/k₀ and β have shown extremely good agreements. Furthermore, initial permeability of the coal seam was also proved by fall-off data. Thus, it is concluded that the CO₂ injection rate can be expected by the models showing relationship between porosity, permeability and swelling of coal.