A major operator in Saudi Arabia is currently performing coiled tubing drilling in some gas wells. Fluid returning from well contains a high concentration of hydrogen sulfide (H2S), with an average of 3.0% to a maximum of 8.0% v/v of H2S reported. This gas is partially dissolved in the water produced from the well. The sour water is separated throughout two different vessels, then it is discharged into an atmospheric tank and ultimately into large, open bio-pits that a major operator designed to collect the effluents from the wells.
The composition of the sour water coming from the second separation vessel shows dissolved H2S content as high as 3,500 ppm. The current water treatment to scavenge H2S relies on a solution of iron-gluconate that is injected as a liquid after the second separator, before the water enters the atmospheric tank. However, the volume of iron-gluconate injected is excessive due to stoichiometric reactions.
A field-trial application was proposed utilizing a Triazine-based product as an alternative to the current treatment because Triazine can scavenge twice the amount of H2S as compared to iron-gluconate. In addition, the reaction products are water soluble, which means they will not precipitate into the sludge deposited at the bottom of the bio-pits, avoiding risks of later release of H2S when they are emptied.
The key performance indicators (KPIs) set by the major operator during the field-trial application included the treatment of H2S dissolved to a concentration less than 100 mg/L in the waste water. The sulfide concentration was monitored before and after chemical treatment utilizing the Garret Gas Train (GGT) for sulfides.
A total of 1,930 ft in 3 laterals were drilled, and H2S concentration in the waste water was successfully reduced to the desired levels utilizing the Triazine-based product with fewer volumes of chemical treatment.