Emergency Chemical Scavenging of Hydrogen Sulfide During Gulf of Mexico Well Flowback

摘要

During an initial Drillship flowback of a newly completed subsea GoM well, the produced fluids were found to contain unexpected and significant concentrations of hydrogen sulfide (H_2S). Despite no detectable levels of H_2S in the native reservoir fluids produced from nearby wells in the same sand zone, the flowback separator gas-phase consistently showed PES concentrations in excess of 100 ppm. It is believed that the souring was caused by irregularities during the completion phase, and the flowback was suspended indefinitely until the serious HS&E concerns could be addressed. Collaboration between the operator and the chemical partner resulted in appropriate operating procedures and chemical selection to allow the well to flowback through existing infrastructure. To mitigate the presence of sour fluids in the carbon steel subsea flowline and topsides plant, the chemical partner was able to prove through laboratory testing that a liquid-phase subsea H2S Scavenger application would prove highly effective. To deliver the chemistry, the scavenger was injected topsides (25 ppm scavenger to 1 ppm gas-phase H_2S) into a continuous dead oil circulation that would be used mix with the sour fluids at the wellhead and provide a turbulent flow regime for mixing. This stage of the procedure was preceded by a 500-gallon oil-soluble corrosion inhibitor pill delivered between two pigs to protect the infield flowline from any acid gas induced corrosion. Once well flowback was re-initiated with the treated dead oil circulation, the chemistry was observed to be highly effective at scavenging EES in the subsea flowline. Draeger tubes and inline gas meters showed no detectable levels of gas-phase PES during continuous scavenger treatment. To confirm success of the chemical treatment, injection was temporarily ceased and H2S levels were once again detected and began to climb. The value delivered by the relatively small amount of scavenger chemical was tremendous, as the chemical partner was able to help the operator prevent multi-million dollars in workover expenses. Though not immediately quantifiable, the mitigation of topsides HS&E concerns was equally as valuable since the souring was neutralized prior to boarding.