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.