A Field Study on Simulation of CO₂ Injection and ECBM Production and Prediction of CO₂ Storage Capacity in Unmineable Coal Seam

Abstract

CO2 sequestration into a coal seam project was studied and a numerical model was developed in this paper to simulate the primary and secondary coal bed methane production (CBM/ECBM) and carbon dioxide (CO2) injection. The key geological and reservoir parameters, which are germane to driving enhanced coal bed methane (ECBM) and CO2 sequestration processes, including cleat permeability, cleat porosity, CH4 adsorption time, CO2 adsorption time, CH4 Langmuir isotherm, CO2 Langmuir isotherm, and Palmer and Mansoori parameters, have been analyzed within a reasonable range. The model simulation results showed good matches for both CBM/ECBM production and CO2 injection compared with the field data. The history-matched model was used to estimate the total CO2 sequestration capacity in the field. The model forecast showed that the total CO2 injection capacity in the coal seam could be 22,817 tons, which is in agreement with the initial estimations based on the Langmuir isotherm experiment. Total CO2 injected in the first three years was 2,600 tons, which according to the model has increased methane recovery (due to ECBM) by 6,700 scf/d.

Secondary coal seam as a barrier to CO2 leakage

“…A dual porosity system means that any flow can take place in both coal matrix and cleats. Flow through the coal matrix is a diffusional process (Rogers, 1994;He et al, 2013). In general, coal bulk matrix usually has low to mid porosity and a large surface area which contains a very high percentage of adsorbed methane.…”

“…The molecules of the gas become attached to the internal surface of the coal by adsorption, which makes it different from absorption where a substance becomes trapped inside another (Mazzotti Ripepi, 2009). The Langmuir adsorption isotherm assumes that the gas is adhered to the coal and covers the internal surface as a single layer (Mazzotti et al, 2009;Ripepi, 2009;He et al, 2013). It also assumes that the gas stored in coal by adsorption could be present in a condensed, near liquid state.…”

“…In order to simulate the primary and enhanced (using carbon dioxide injection) coalbed methane production, a real sequestration project was conducted in the Marshall County, West Virginia by He et al (2013). The model was developed by using the Computer Modeling Group's GEM (Generalized Equation-of-State Model) simulator (CMG, 2012).…”

“…In our current reservoir model, coal swelling/shrinkage and permeability changes during methane production were also modeled by using the Palmer and Mansoori model (Palmer and Mansoori, 1996;CMG, 2012). All the data that have been used in this study can be found elsewhere (He et al, 2013). In addition, history matching was conduct to…”

“…CO 2 injection into coal seams provides a dual-benefit of not only CO 2 storage, but also enhanced gas production. CO 2 provides a great affinity towards coal, which is about twice that of methane(Chhajed, 2011;He et al, 2013). For that reason, when carbon dioxide is injected into a coal reservoir, the micropores of the coal desorb the existing methane and adsorb the injected carbon dioxide.…”