Elemental Sulfur Management in Acid Gas Field Development: A Case Study from Uzbekistan

This paper examines development challenges related to elemental sulfur content in reservoir fluid from an acid gas field in the Afghan-Tajik Basin of Uzbekistan. The productive reservoirs, located within Jurassic subsalt carbonate formations, contain natural gas saturated with gaseous sulfur. Our study evaluates the impact of sulfur condensation and solidification on well and reservoir performance and presents an integrated approach for incorporating these effects into field development planning. We developed an empirical model of sulfur deposition within well tubing that accounts for multiphase flow regimes and pressure-temperature profiles along the wellbore. The model builds upon laboratory experiments on sulfur solubility in sour gases (Brunner et al. 1980, 1988) and utilizes sulfur phase diagram. Field-observed solid deposition rates on tubing walls were used to calibrate the model. To investigate reservoir effects, we incorporated sulfur solubility and relative permeability functions for a three-phase reservoir system (gas – liquid sulfur – water) into a reservoir simulation model. Local grid refinements enabled simulation of liquid sulfur banking in near-wellbore zones (NWZ) and analysis of this phenomenon’s impact on well productivity. Our research identifies two primary consequences of sulfur phase transforms on production well performance: Reduced well productivity due to sulfur banking in near-wellbore zoneRestricted tubing flow from solid sulfur deposition Simulation results indicate that productivity index reduction due to sulfur banking is dependent on pressure drawdown and may reach 30% during the first year of production – a significant effect that must be addressed in field development design. Solid sulfur deposition on tubing walls predominantly occurs during annular multiphase flow regime. Progressive deposition gradually constricts flow, potentially resulting in complete blockage within several days to a month, depending on gas rate and pressure-temperature profiles. Notably, deposition progresses sufficiently slower at mist flow regime. Therefore, to maximize tubing efficiency, either mist multiphase flow or annular flow with minimal pressure/temperature drawdown should be maintained. Both consequences of sulfur phase transforms - reduced well productivity and restricted tubing flow – significantly affect field production and are sensitive to well operational parameters. By integrating our empirical tubing deposition model with reservoir simulation model, we developed a multiparameter optimization framework that minimizes the combined negative effect of sulfur phase transforms on field production. This approach enables selection of optimal well count, well construction specifications, well operational constraints, and solvent treatment schedule. The substantial negative impact of sulfur condensation and solidification on well productivity and operational efficiency represents a critical challenge for acid gas field development. This paper presents a practical approach implemented in an Uzbekistan field that addresses sulfur phase behavior in both the reservoir and wellbore tubing through the integration of reservoir simulation and empirical deposition modeling.