N2 Injection into Coal beds and Its Effects on Coal Permeability and Enhanced CH4 Production: Insights from Laboratory Analyses

Chikatamarla, Laxmi¹, R. M. Bustin¹ (1) University of British Columbia, Vancouver, BC

The success of enhanced coalbed methane (ECBM) projects where carbon dioxide, nitrogen or flue gas are injected to promote methane production is strongly dependent on the fabric and mechanical properties of the coal, the relative adsorptive affinity of the coal to different gases and associated strain- all of which are coal composition and rank dependent. In order to establish the key variables that determine the relative success of ECBM, a series of adsorption, desorption and flow through experiments on coals of varying composition, rank and fabrics were carried out. Our N2 flow through experiments on CH4 saturated coal suggest that for optimum displacement of CH4 by N2, the coal needs to be saturated with methane and to be on the flat part of the adsorption isotherm such that injection of N2 at high pressure is not simply accompanied by further adsorption, swelling and loss of permeability. If the coal has an initial relatively high permeability such that reservoir pressure is lowered during production and injection of N2, the N2 will adsorb on to the coal, displace CH4 and shrink the coal matrix, resulting in an increase in permeability. Additionally, it was observed that change in permeability occurs during methane production due to change in effective confining pressure which depends on coal elastic properties in turn depends on coal mineral matter composition, gas saturation, pore volume compressibility etc. Our preliminary results are encouraging and have wide applicability in designing field N2 injection projects.