Status of Hydrogen Sulfide in Bayou Corne Area
Prepared by Gary R. Hecox, Ph.D., Shaw Environmental & Infrastructure, Inc.
Shaw Environmental & Infrastructure, Inc. (Shaw) has reviewed existing data and is currently collecting new data to evaluate the conditions related to the hydrogen sulfide observed in the Texas Brine Relief Well #2 completed in the cap rock. At all times, protection of local residents and project staff is and will remain the top priority when making decisions and recommendations for response activities related to site conditions.
Currently there are five vent wells completed in the alluvial aquifer and one, Relief Well #2, completed in the cap rock. Four of the vent wells have positive wellhead pressure indicating gas accumulation; the fifth well has no wellhead pressure.
Acting out of an abundance of caution to the potentially changing conditions when notified about the detection of hydrogen sulfide gas on Tuesday November 20, 2012, Shaw shut down flaring operations on the two ORW vent wells currently being flared. This was done to allow for collection of quantitative hydrogen sulfide data and develop the appropriate plan and monitoring measures to safely go forward with flaring operations. ORW-2 has since resumed flaring and Texas Brine Relief Well #1 remained flaring. Flaring operations are being monitored for hydrogen sulfide by the local ambient air monitors and site personnel.
DNR and Shaw have discussed with Texas Brine what occurred at the Relief Well #2. The geophysical log on Relief Well #2 showed gas accumulation from 142 to 150 feet in the alluvial aquifer immediately below the base of the overlying clay and from 424 to 432 feet in the cap rock on top of the salt of the Napoleonville dome. On November 15, 2012 this well was perforated in the cap rock at depths of 424 to 427 feet and 455 to 457 feet. The gas accumulation zone in the alluvial aquifer was not perforated. On Monday November 19, 2012, Texas Brine set up the flare on this well and started flaring gas from the cap rock. Venting and flaring continued until late in the day when water started to be produced from the well. Texas Brine set up a gas/water separator and restarted flaring operations. After a very short period of flaring, site personnel noticed hydrogen sulfide odors from the water being discharged from the separator. At this time, one of the AreaRae monitors was used to check for hydrogen sulfide gas. When hydrogen sulfide was confirmed, Texas Brine stopped flaring operations, shut-in and locked the wells, and set a safety perimeter. The well currently remains shut-in and locked.
Shaw has checked the ORW vent wells for hydrogen sulfide. In the two wells being flared, no detectable hydrogen sulfide concentrations were measured in the gas. Low concentrations of hydrogen sulfide, consistent with a swamp environment, were measured in the two ORW wells that have not been flared. Shaw is checked all 18 of the shallow Geoprobe wells for hydrogen sulfide. All of these shallow wells were nondetect for hydrogen sulfide.
At this point in time, it appears that the hydrogen sulfide produced by Relief Well #2 was only from the cap rock and not the alluvial aquifer vent wells. While low concentrations of hydrogen sulfide have been detected at some of the bubble sites, the concentrations are well below those that would pose a human health risk. Other than Relief Well #2, all hydrogen sulfide concentrations measured are consistent with the production of hydrogen sulfide caused by decay of trees and other vegetation in a swamp environment.