Knowledge Bank

Noble gases, specifically 4He concentrations, can be a useful tool for determining the rates of groundwater recharge, subsurface residence times, and groundwater flowpaths, especially in difficult to model complex formations such as fractured aquifers. This thesis focused on developing an approach to determine the age of formation waters within coalbed methane (CBM) reservoirs in the Powder River Basin (PRB). Specifically, the work explored the difficulties and corrections required to use the 4He in-growth method to determine the residence time of groundwater in the fractured coal seams in this area. Coal seams are considered the primary aquifer of the PRB with groundwater being recharged by water from the west and flowing toward the basin center. Using the noble gas geochemistry of coal seam solids to conduct experimental work to determine the diffusional rates of 4He from coal seam solids using step-heating experiments and bulk releases I was able to determine the 4He accumulation rates in coal seams from the PRB. I then compared the data to produced formation waters and free gas samples, in combination with numerical modeling approaches, to develop an approach for determining the residence times of formation waters in this area. The results of this work provide important insights on the flowpaths along which groundwater recharges into the PRB.