HR: 16:30h
AN: H32G-03 INVITED     [PDF]
TI: Iodine-129 and Chlorine-36 as Tracers of Hydrodynamic Processes in Coalbed Methane Systems: Environmental Implications
AU: * Snyder, G T
EM: gsnyder@rice.edu
AF: Rice University Earth Science Department, MS-126 P.O. Box 1892, Houston, TX 77251-1892 United States
AU: Riese, W R
EM: Rriese1@bp.com
AF: BP America Production Co., MC 19.40 501 Westlake Park Blvd., Houston, TX 77079 United States
AU: Fehn, U
EM: fehn@earth.rochester.edu
AF: Department of Earth and Environmental Sciences, University of Rochester, 227 Hutchison Hall, Rochester, NY 14627 United States
AU: Moran, J E
EM: moran10@llnl.gov
AF: Lawrence Livermore National Laboratory, L-231 P.O. Box 808, Livermore, CA 94550 United States
AU: Pelzmann, W L
EM: pelzmawl@bp.com
AF: BP America Production Co., MC 19.40 501 Westlake Park Blvd., Houston, TX 77079 United States
AB: Models which predict the response of coalbed methane systems to commercial exploitation generally assume that the coals host groundwaters which belong to a dynamic through-flowing system. For this to occur, fluid migration must be focused along localized fractures and cleat structures. This situation can facilitate enhanced methane recovery through carbon dioxide reinjection, but may also lead to premature breakthrough of the injected gasses to production wells, thereby leaving stranded resources behind in the reservoir. Water encroachment from surrounding areas may also occur and create problems. The emerging alternative view of coal-hosted hydrodynamic systems is that they are essentially static systems which host connate formation waters. Fluid migration is precluded by the presence of gas and its influence on relative permeability in the fractures and cleats. While the removal of these waters will produce local drawdown and allow production of the adsorbed methane, it also represents extraction of a limited and non-renewable hydrologic resource. Understanding which of these hydrodynamic environments is present is critical if informed decisions regarding resource development and exploitation are to be made. We used the $^{129}$I and $^{36}$Cl systems as indicators of the migration history of fluids in the Fruitland Formation coals in the San Juan Basin of Colorado and New Mexico. Because of their long half-lives (15.7 Myr and 0.3 Myr respectively ) these cosmogenic isotopes provide useful constraints on the processes which have occurred since the peat deposits were buried 73 million years ago, and can provide direct evidence on the validity of hydrodynamic models. Our results indicate several isotopically distinct sets of waters are apparent within coals of the Fruitland Formation. The infiltration of recent surface waters is clearly limited to the uplifted basin margins, indicated by the similarity of $^{129}$I/I ratios, $^{36}$Cl/Cl ratios, $\delta$D, and $\delta^{18}$O signatures to those of local rivers and lakes. The rest of the basin is characterized by connate formation waters, some of which were modified by infiltration of groundwaters at the time of Laramide uplift. Isotopic characterization of the source and migration of waters within coalbed systems provides a promising avenue for determining if and how a resource should be developed. Further research is needed to determine the response of environmental isotopes to drilling and gas production.
DE: 1040 Isotopic composition/chemistry
DE: 1832 Groundwater transport
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
SC: Hydrology [H]
MN: 2003 Fall Meeting