SPE 149535 New Model to Predict Formation Damage due to Sulfur Deposition in Sour Gas Wells

Elemental sulfur (S8) is often present in considerable amounts in sour gas reservoirs at the reservoir conditions (pressure and temperature). For the isothermal conditions in the reservoir, the reduction in reservoir pressure below a critical value will cause the elemental sulfur to deposit in the formation. Sulfur deposition can cause severe loss in the pore space available for gas, and in turn it will affect the gas well productivity. Accurate prediction of sulfur deposition in the reservoir will help in better management of sour gas reservoirs with potential sulfur deposition problems. In this paper a new analytical model was developed to predict the formation damage due to sulfur deposition. This model can be used to study the effect of sulfur deposition on gas relative permeability, reservoir porosity, skin damage and reservoir rock wettability. The main objective of this model is to investigate the effect of radial distance on formation damage. Accurate correlations of different rock and fluid properties were used in this model for improved predictions. Accurate correlations of gas viscosity and gas compressibility were used, as the sulfur solubility is a strong function of gas viscosity and gas density. These correlations were used for the calculation of sulfur solubility at reservoir conditions. Model predictions showed that sulfur deposition depends on the radial distance from the well bore. The most damage occurred in the 3 ft around the wellbore. As the radial distance increases the effect of sulfur deposition becomes negligible. Unlike previous models, which neglected the effect of pressure on gas properties, accurate correlations were used in the new model. Also, various sulfur solubility correlations were tested using the new model. A reduction of 2000 psi in the reservoir pressure, causes a 40 % loss of reservoir porosity at a radial distance of 3 ft from the wellbore and almost 85 % loss in the gas relative permeability at the same distance. Introduction Elemental sulfur is often present in considerable quantities in sour gas at the reservoir pressure and temperature. The gas production decreases the reservoir pressure, and in turn the solubility of sulfur in the sour gas decreases (Kennedy and Wieland 1960). Elemental sulfur is present as a dissolved species in virtually all deep sour gas reservoirs. Sulfur precipitation is induced by a reduction in the solubility of the sulfur in the gas phase beyond its thermodynamic saturation point as a result of decrease in pressure and temperature. The change in pressure and temperature occur during production operations and can result in sulfur deposition in the reservoir, wellbore and surface facilities (Hands et al. 2002 and Shedid et al. 2006). Deposition of elemental sulfur in the near-wellbore region may significantly reduce the inflow performance of sour gas wells. Deposition of the liquid sulfur in the reservoir may impair both the reservoir porosity and permeability and results in the productivity impairment of the gas well. T he decline in the well productivity from a dry sour gas reservoir (south west Alberta) with a 16 vol. % H2S was attributed to sulfur deposition in the formation (Mei et al. 2006). Sulfur in the gas phase reacts to form a hydrogen polysulfide species at high temperatures and pressures. Deposition of elemental sulfur occurs when changes in pressure and temperature helps in the decomposition of polysulfide to elemental sulfur and H2S (Xiao et al. 2006). Sulfur deposition in the formation, especially, in the vicinity of the wellbore may significantly reduce the inflow performance of sour gas wells. Wells have become completely plugged with sulfur in certain sour gas reservoirs after several months of production. Accurate prediction and effective management of the sulfur deposition are, therefore, crucial to the economic viability of sour gas reservoirs. Many analytical and numerical models were developed to predict the effect of sulfur deposition on the inflow performance of gas wells. There are some shortcomings in previous models which may not allow for accurate prediction of