Laboratory investigation of zinc and lead sulphide inhibition

The formation of zinc sulphide (ZnS) and/or lead sulphide (PbS) has been a persistent problem, particularly in high temperature high pressure HT/HP fields. ZnS and PbS deposition can pose safety hazards and have serious economic consequences including reduction in well productivity and may require the implementation of an effective scale mitigation and removal strategy. HT/HP fields are prone to critical changes in temperature and pressure and, in addition, they usually have high salinity brines; indeed they are often referred to as HP/HT/HS systems. When these factors (pressue/temperature/salinity) vary together, they tend to trigger the formation of inorganic scales including sulphides. Apart from the role of temperature and salinity in scale formation, these (HT/HS) conditions often negatively impact scale inhibitor performance due to chemical degradation or incompatibility. The objective of this study was to investigate ZnS and PbS formation (as single or combined scales) and inhibition over a range of parameters including pH, temperature, salinity, time and initial Zn, Pb and H2S concentrations. Polymeric and phosphonate scale inhibitors (SIs) were tested using static scale formation experiments, with samples being analysed by inductively coupled plasma (ICP) analysis, Environmental Scanning Electron Microscopy (ESEM), pH and particle size distribution measurements. Of the seven scale inhibitors tested, only two demonstrated inhibitory capacity at active concentrations of 100 ppm or below. SI-2, a high-molecular weight polymer, was remarkably effective in preventing both zinc and lead sulphide deposition regardless of the final supernatant pH. SI-3 showed more limited efficacy compared with SI-2 with its highest inhibition being achieved at low pH values.This information is important to consider when designing scale inhibitor treatments; as carbon dioxide liberates from produced water due to decreasing pressureit causes the pH to increase, which may cause a drop in the inhibition efficiency of some scale inhibitors. Increasing the brine salinity had a detrimental impact on the performance of the tested scale inhibitors. Neither SI-2 nor SI-3 were able to prevent PbS deposition by ionic displacement of Zn from ZnS by Pb²⁺ despite the fact that both scale inhibitors were effective against PbS under the same conditions using the conventional scale inhibition experiments. The particle size distribution of the partially inhibited ZnS and PbS particulates was found to be dependent on the type and concentration of the scale inhibitor, the final pH and salinity. The difference in particle size could have significant effects on in-line filter blocking tests and produced water quality issues.