44815-AC2
Prediction of Coalbed Gas Production Using Comprehensive Petrographic, Geochemical, and Isotopic Monitoring
The occurrence of microbially produced coalbed gas has made sedimentary basins containing low-rank coals targets for coalbed methane (CBM) exploration. The Illinois Basin is one of such examples; it has significant volumes of CBM stored in high volatile bituminous coals. Our research project aims at reliable biogenic coalbed gas prediction via comprehensive geochemical and isotopic monitoring. In our second year of PRF-funding we advanced several related coalbed gas sub-projects. We further characterized microbial communities in Indiana coal beds by molecular and geochemical approaches. Our results suggest that coal organic matter is biodegraded to simple molecules, such as H2 and CO2, which fuel methanogenesis and the generation of CBM reserves. Our Ph.D. student, Dariusz Strąpoć, who graduated in October 2007, included these results in his Ph.D. dissertation. Our results have been published in the International Journal of Coal Geology (Strąpoć et al., in press) and presented at the 2008 AAPG Annual Convention & Exhibition in San Antonio, Texas in April 2008. We investigated the controls on CBM volumes in the eastern part of the Illinois Basin. The Seelyville Coal Member of the Linton Formation (Pennsylvanian) in Indiana was selected for this study because this coal is the major target for CBM production. Our study documents variations in pore characteristics within a coal seam at a single location and compares these variations with changes occurring between the same coal at different locations. The study also explores the influence of mineral matter and maceral composition on mesopore and micropore characteristics, and discusses implications of these variations for CBM content. We documented large variations in CBM content across a coal seam at a single location. Because of this variability, we conclude that the entire thickness of a coal seam must be desorbed in order to determine the CBM content reliably and to accurately calculate the level of gas saturation. Our results have been summarized in a paper by Mastalerz et al. (in press). We received the Best Poster Award of the Coal Division at GSA in Denver in 2007 for the presentation of our data. We repeatedly collected sets of CBM and production waters from several wells in Sullivan County, Indiana to analyze long term production-related compositional and isotopic trends. This added data to our previous 27-month monitoring cycle, resulting now in a 40-month long monitoring period. So far we have not observed any significant trends over time, which is in agreement with earlier observations (Strąpoć et al., 2007; Strąpoć et al., in press). We will continue this periodic sampling over the next years and expect that more time will be needed for consistent trends to develop. We continue and improve the geochemical and isotopic characterization of gases from two monitoring wells and one CO2 injection well. Close to 50,000 kg (i.e., 50 tons) of CO2 have been injected into the well so far (Midwest Geological Sequestration Consortium (MGSC) - Illinois Basin Regional Carbon Sequestration Partnership). Original coalbed gas does not contain molecular oxygen. During our analyses of desorbing gases from coal we discovered that atmospheric oxygen from air, which is a contamination when fresh coal is initially loaded into desorption canisters, is rapidly consumed microbially and chemically over about two weeks. Gas compositional data from a desorption experiment over 28 days are shown in Figure 1. The relative concentration of desorbing methane increases throughout the duration of the experiment. During the same time, the nitrogen concentration decreases due to increasing dilution by methane. The same is true for the oxygen content, but the oxygen concentration reaches zero rather than mirroring the asymptotic approach of nitrogen towards a finite level. A constant ratio of N2/O2 ≈ 4 would be expected if oxygen were a conservative tracer for air contamination. Previous strategies for calculating the yield of CBM from coal in desorption canisters had assumed that oxygen can be used as a proxy for air contamination, whereby a correction could be applied to subtract oxygen and the ~4-fold volume of atmospheric nitrogen from the bulk desorbing gas. However, it is now clear that such calculations of ‘air correction’ based on oxygen result in an under-estimation of atmospheric nitrogen contamination, and therefore over-estimate the nitrogen content in the desorbing coalbed gas mixture. Our graduate student Hui Jin is preparing a manuscript for the International Journal of Coal Geology. Our second year of PRF funding for research on coalbed gas partially supported the dissertation projects of two graduate students. Dariusz Strąpoć defended his Ph.D. dissertation in October 2007. Hui Jin is our current M.Sc. student. Figure 1. The relative concentrations of CH4, N2, and O2 in gas sampled from a canister of coal over 28 days. 
