VOLUMETRIC SWELLING/SHRINKAGE CHARACTERISTICS OF WESTERN CANADIAN COALS: IMPLICATIONS FOR SEQUESTRATION OF ACID GASES

Volumetric swelling and shrinkage experiments were carried out on moisture equilibrated Western Canadian Basin coals varying in rank from sub-bituminous ‘C’ (Rr 0.46%) to medium volatile (Rr 0.62%) using He, N2, CH4, CO2 and H2S. With increasing pressure helium results in progressive shrinkage and no measurable adsorption occurs for all coals. The amount of compressibility due to hydrostatic pressure in helium varies with coal rank and type. N2, CH4, CO2 and H2S adsorption on the other hand results in marked swelling of the coal matrix. The order of volumetric swelling in decreasing order is H2S> CO2> CH4> N2. The volumetric swelling with H2S is about 2-15 times higher than CO2, 5-20 times higher than CH4 and 40-130 times higher than N2 at their saturation adsorption pressures. The volumetric strains are strongly and positively correlated with adsorbed gas quantities and are proportional to the amount of gas adsorbed.

Reduction of helium hydrostatic pressure results in expansion of the coal and recovery of the coal matrix whereas desorption of N2, CH4 and CO2 results in shrinkage of the coal matrix. The amount of matrix expansion accompanying decreasing helium hydrostatic pressure ranges from 0.025% to 0.062% and varies with coal type and decreases with increasing coal rank. The overall volumetric shrinkage for N2 varied from 0.04% to 0.15% and for CH4 0.31% to 1.80%. Since the volumetric shrinkage of CH4 desorption is significantly greater than the volumetric swelling by N2 adsorption, N2 injection will enhance the CH4 production from coalbeds.

Since volumetric strain varies with volume and type of fluid sorbed, drilling and completion fluids may result in swelling or shrinkage of the coal matrix and thus enhance or reduce permeability. For coalbed methane reservoirs, completions utilizing carbon dioxide will promote desorption of methane from coal (selectivity of CO2 over CH4). The volumetric swelling associated with CO2 sorption is markedly greater than shrinkage due to CH4 desorption and hence there will be a net decline in permeability. Similarly attempts to sequester CO2 or H2S in coal seams will result in swelling of the coal to a greater extent than the initial shrinkage due to CH4 production and hence result in loss of permeability. Thus it may be necessary to blend nitrogen and carbon dioxide to inhibit permeability loss.