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Documenting Past Oilfield Arsenic Corrosion Inhibitor Usage: A Pit Groundwater Example From Lake St. John Field, LA


Arsenic corrosion inhibitors were commonly used in mid-20th century oilfields to protect wellbores during acid jobs, to “pickle” wells while temporarily shut-in, and as field-wide inhibitors circulated in produced waters. Arsenic-bearing wastes were often placed into earthen pits into the 1970s, whether as spent acids or “pickle” fluids disposed into a well’s cleanout pit (hundreds to over 1000 mg/l dissolved arsenic), or as produced waters disposed of into larger pits near tank batteries or saltwater disposal wells (about 5 mg/l arsenic). Modern public documentation of past arsenic corrosion inhibitor usage and impact is known for two oilfields—the former Naval Petroleum Reserve no. 1 in Elk Hills Field, CA, and the Lake St. John Field (LSJF), LA. Both fields were historically operated by Standard Oil of California who used the W-41 arsenic corrosion inhibitor. Historic documents and 1950s patents describe this chemical’s usage and disposal. The U.S. DOE began Elk Hills study of arsenic-impacted soils (only) in 1986 (see paper by Blake, this meeting), while the LSJF study of soils and groundwater began in 2006 related to litigation. All shallow-sand groundwater (8 to 22 ft below surface) below LSJF 1950s produced water pits are impacted by dissolved arsenic above background. A proposed groundwater arsenic reduction-dissolution model failed to consider arsenic corrosion inhibitor usage. Lowest arsenic groundwater values are found below a pit whose bottom was partially in sand (up to 0.17 mg/l), while highest values are found below pits with a few feet of clay below them (up to 0.91 mg/l). Another 1950s pit with 16 ft of clay below it was not sampled for groundwater, but described oil-stained cores indicate that fluid movement out of the pit occurred through cracks. One pit’s shallow groundwater was sampled six months after excavation and closure—these were the lowest arsenic values found as compared to one pre-closure and several post-closure analyses over seven years. Pit geochemical patterns indicate that relative ion mobility in groundwater is chlorides > iron > arsenic. Elevated iron below pits is from iron that was present in corrosive produced waters. ORP values below a heavily-sampled pit are slightly reduced as compared to nearby monitoring wells, but elevated pit groundwater arsenic or iron does not vary with ORP. Highest arsenic and iron values are associated with chlorides, although chloride relative mobility has decreased its signature.