Page 271 - Fundamentals of Enhanced Oil and Gas Recovery

Enhanced Gas Recovery Techniques From Coalbed Methane Reservoirs 259

greater than its affinity toward CH 4 or N 2 , making coalbeds a suitable host for carbon
sequestration. The higher CO 2 affinity than methane can be explained by the geo-
metrical, electrical, and physical chemical properties of gas molecules and also the
functional groups on the coal surface [101,102].CO 2 molecules have a linear shape
and are smaller than CH 4 molecules. These features facilitate CO 2 entrance to more
restricted pore spaces and dislocate competitively preadsorbed CH 4 molecules out of
micropore surfaces [101]. Moreover, CO 2 has a more favorable interaction enthalpy
than methane, which increases CO 2 solubility in coals and may increase the CO 2 dif-
fusivity and sorption capacity [102].
Due to this higher affinity, a larger amount of CO 2 is adsorbed, relative to the
amount of desorbed methane. This leads to matrix swelling and may create opera-
tional problems in a CO 2 -ECBM project. As the matrix is swollen, the cleat openings
decrease, hence permeability decreases [103,104]. The injectivity decline was observed
in the pilot study in Allison unit. A numerical study showed that the permeability
might decrease by two orders of magnitude [103]. Mazzotti et al. conducted an exper-
imental study on the coal matrix swelling effect for different types of gases [105].In
the study, the changes of coal volume subsequent to exposure to different gases (CO 2 ,
N 2 ,CH 4 , and He) were measured. The coal was swollen more severely by CO 2 in
comparison with the other gases.
Nitrogen injection, known as N 2 -ECBM, is another well-studied technique for
methane recovery enhancement. In N 2 -ECBM, the deterioration of well injectivity is
not a concern, because N 2 is less adsorptive on coal than methane and CO 2 . One of
the first studies on N 2 -ECBM is the paper written by Puri and Yee in which they
investigated the impact of N 2 injection on methane recovery, through laboratory
experiments and modeling [3]. They concluded that introducing N 2 to coal enhances
methane recovery. N 2 -ECBM was piloted for the first time in Tiffany unit of
San Juan Basin, operated by BP America. N 2 was injected for 4 years (1998 2002)
and resulted in methane recovery increment of one volume per 0.4 volume of
injected N 2 . In other studies, it was observed that the N 2 injection also boosts the
production rate, rapidly and notably [106,107]. The main issue of N 2 -ECBM is the
early breakthrough of nitrogen. The low adsorbability of N 2 on the coal induces
matrix shrinkage that increases the well injectivity. But most of the injected N 2 travels
toward the producer(s), instead of being adsorbed on the coal matrix, which in turn
causes an early breakthrough. The experimental study conducted by Zhou et al. con-
firmed the early breakthrough problem [108].
A more promising result can be obtained by injecting a mixture of different gases.
Shi and Durucan carried out a micropilot study in the Fenn Big Valley. They argued
that by injecting flue gas, more desirable results can be obtained [26]. Durucan and
Shi investigated the overall performance of different N 2 /CO 2 mixtures on production
gas rate and the quality of produced gas [103]. It was seen that production rate