Iron sulfide scale inhibition: Limitations at sour conditions

Sour oil and gas production is commonly associated with sulfide scaling challenges originating from the produced aqueous phase. Iron sulfide (FeS) is one of the most common sulfide scales, and recent studies have shown promising dispersant chemicals are available to mitigate its deposition. In addition, successful applications have been reported in the literature, particularly from the North Sea. However, some of the limitations of these FeS chemical dispersants become evident under more severe (high H₂S) sour conditions, such as those found in the Middle East, Russia and Canada. The dispersant efficiency depends on the scale particle size, and larger particle sizes usually require higher dispersant dosages. Other factors that may influence the inhibitor dosage include reactant concentrations (cations and anions), pH, salinity and inhibition time. These factors were investigated using a newly developed anaerobic experimental setup that allows the careful addition and withdrawal of fluids from a closed anoxic system. Anaerobic vessels, such as vials and tubes, are deployed equipped with septa (thin membranes). Syringes were used to infiltrate the septum with minimal interference from sulfide retention while maintaining isolation from atmospheric oxygen. Testing was performed over a sulfide concentration range from 100 to 1,000 mg/L. Higher levels of sulfide required higher loadings of scale inhibitor, essentially as a result of particle size increase. In addition, varying the salinity also had a significantly influence on the required dispersant concentration to maintain FeS suspension in solution. At lower pH condition, smaller FeS particles were produced and often inhibition was somewhat obscured by solubility effects. Also, suspending the FeS for longer periods of time required higher dispersant concentrations. More severe sour conditions exceeding 1,000 mg/L of aqueous sulfide, have a detrimental effect on the both the efficiency and economics of the FeS inhibition treatments. In addition, the current high-performance dispersants cannot be squeezed into tight formations or shales, as their high molecular weight may cause severe formation damage. For such applications, alternative inhibition methodologies are required, and non-chemical inhibition may be considered.