A New Approach to Combined Sulfide- and Carbonate-Scale Predictions Applied to Different Field Scenarios

Abstract

Summary Unlike other types of inorganic scales, carbonate and sulfide scales are directly correlated to the in-situ concentration of acid gases such as carbon dioxide (CO2) and hydrogen sulfide (H2S), which influence the local pH and availability of reactive species. The common approach to sulfide- and carbonate-scale prediction often does not account for three-phase CO2 and H2S partitioning at different temperatures and pressures throughout the system. This leads to the use of inaccurate compositions and pH values for the mineral-scaling calculations. In this paper, we apply a rigorous work flow (step-by-step procedure) derived from a compositional pressure/volume/temperature (PVT) model to calculate molecular CO2 and H2S distribution, three-phase relative volume changes, compositional changes, and scale-precipitation trends from the reservoir to the wellhead-separation stage using commonly available surface field data, a full PVT software package, and scale-prediction software of the user's choice. A simplified version of the work flow was previously applied to high-CO2 and -H2S gas/condensate wells with production of condensed water only (Verri et al. 2017b). This paper focuses on the field application of our “Rigorous General Work Flow” (Verri et al. 2017a) to North Sea oil wells with high water-cut and medium H2S levels (approximately 2,200 ppmv in the separator-gas phase) to provide sulfide- and carbonate-scale-prediction profiles from reservoir to separator. The combination of reservoir-, production-, and chemical-engineering models using specific iterative processes (within the work flow) has provided a new and rigorous step-by-step procedure for the prediction of combined sulfide and carbonate scales in oil and gas wells, which can be implemented by anyone using any PVT and scale-prediction software.