CO2 N2 Injection in Deep Coals and its Impact on Coal Permeability

This paper presents the results of a laboratory investigation carried out to determine the permeability of coal to different gases related to enhanced coalbed methane recovery (ECBM) operations, along with simultaneous measurement of horizontal strain. The work concentrated on replicating the conditions in situ by stressing cores of coal obtained from potential ECBM sites triaxially, and saturating them with methane. The horizontal strain and permeability under these conditions was considered the base value. The gas was then switched to nitrogen, replicating the _N2-ECBM alternative, monitoring the induced horizontal strain until equilibrium was attained, at which time, permeability was measured. The procedure was then repeated using _CO2. Permeability of coal to nitrogen was found to be the highest, followed by that to methane, and lowest for _CO2. Of the three gases, nitrogen is the least sorptive while _CO2 is the most sorptive in a coal-gas environment. However, the ratio of the measured methane/nitrogen and methane/ _CO2 strain coefficients did not match the corresponding permeability ratios. Hence, it was not possible to explain the permeability variation using sorption-induced horizontal strains alone. The explanation of permeability variation requires an approach involving changes in the stress regime as a result of horizontal strain, and stress-dependent permeability.

1. INTRODUCTION

Enhanced coalbed methane recovery (ECBM) has gained importance in the recent years as a means to recover a larger fraction of gas-in-place (GIP), and earlier in the life of coal-gas reservoirs, than possible by primary recovery using the pressure depletion technique. Typically, an ECBM technique involves injecting a second gas into the coal reservoir. Depending on the gas injected, the process can either be CO₂-ECBM, where CO₂ displaces the adsorbed methane within coal [1, 2], or N₂-ECBM, where nitrogen strips it from the coal [1]. The former is based on displacement desorption, where adsorbed methane is displaced by the competitive adsorption of CO₂, whereas in the latter case, injected nitrogen reduces the partial pressure of methane in coalbed, flushing it out from the cleat system and resulting in desorption of methane from the sorption sites. However, the former option is more attractive since it not only enhances the recovery of methane, but also offers the added benefit of storing large amounts of CO₂, sequestering it permanently. With injection of a second gas in coal, there is either an increase, or decrease, in the volume of gas sorbed within coal, depending on the sorptive affinity between coal and the injected gas. Consequently, there is a positive or negative sorption-induced strain in the volume of solid coal, resulting in closure, or opening up, of fractures in coal, and hence, significant changes in the coal permeability. At ECBM/CO₂ sequestration pilots around the world, there is evidence of changes in injectivity and permeability with continued injection [3]. At the Allison unit, operated by Burlington Resources, permeability decrease of two orders of magnitude was observed with CO₂ injection [4]. With nitrogen injection at the Tiffany unit, operated by BP, there was a five-fold increase in gas production after fourteen months of injection [4].