Study of Mechanisms for Naturally Occurring Radioactive Materials and Souring in Gas Production Facilities in the Antrim Shale of the Michigan Basin

ABSTRACT Naturally occurring radioactive materials (NORM) and souring have been observed in certain gas production facilities associated with wells producing from the Antrim shale in northern Michigan. The results of a study managed by the Gas Research Institute have shown that only 1 of 99 wells surveyed had detectable radioactivity while the separators had considerable radioactivity. Only four wells had high sulfate levels. Most of the other wells had low to moderate amounts of barium. The data suggest that barium sulfate scaling (with co-precipitation of NORM) does not occur until produced waters containing sulfate are co-mingled with waters containing barium. Above ground this occurs principally in the common piping, separators and tanks. High sulfate wells were not correlated with wells having evidence of corrosion nor with activities of acid producing bacteria. High sulfate levels were found in wells along the steeply folded and possibly faulted flanks of anticlinal structures. These wells also had aerobic bacteria. It can be inferred from these observations that sulfate may be associated with waters migrating into Antrim wells from underlying formations (e.g., Traverse where anhydrite (CaSO4) layers may lie close to the Antrim) possibly through faults, natural fractures or up wellbores drilled to the deeper formations. The role(s) of the aerobic bacteria are presently unclear. Their presence may, however, support the water migration model. More work is needed to confirm or deny these models. Souring was not positively correlated with high sulfate wells, but was positively correlated with sulfate reducing bacteria levels. Possible mechanisms for souring are: sour wells are fractured (naturally or by completion) into portions of the deeper formations which contain sour gas but not sulfate or that much of the migrated sulfate is converted to sulfide either by microbial activities in the wellbores, or reactions with organic material in the black shales.