Effect of Elemental-Sulfur Deposition on the Rock Petrophysical Properties in Sour-Gas Reservoirs

The deposition of elemental sulfur as a solid phase results in plugging the pore space available for gas flow and reduces the reservoir productivity. Elemental sulfur also can be deposited as a liquid phase if the reservoir temperature is greater than 115 degrees C. The liquid sulfur restricts the inflow of gas because of the big density difference between liquid sulfur and gas, which could be 20 times or more. For isothermal conditions in the reservoir, the reduction in reservoir pressure to lower than a critical value causes the elemental sulfur to deposit in the formation, and in turn the gas-well productivity is affected. Accurate prediction of sulfur deposition will help better manage sour-gas reservoirs with potential sulfur-deposition problems. In this paper, a new analytical model was developed to predict the effect of sulfur deposition on the damage of the near-wellbore region. The damage was quantified through the investigation of the effect of sulfur deposition on rock porosity, relative permeability of the gas phase, and the change in rock wettability. The main objective of this model is to investigate the effect of radial distance on formation damage. Different rock-and fluid-property correlations were used in the model. Previous studies did not consider the change in gas properties with pressure, and numerically modeled the sulfur deposition as a condensate in a gas/condensate reservoir. This paper will consider sulfur adsorption, and the actual physical properties of the sulfur will be used in the developed model. The optimum gas-flow velocity that will maximize the sulfur solubility in the gas will be determined. A coreflood experiment was performed to determine the effect of sulfur deposition on the carbonate-rock permeability, porosity, and wettability. The experiment was used to determine the effect of sulfur adsorption on the rock petrophysical properties. The contact angle was measured for a carbonate core saturated with sulfur by use of a pendant-drop tensiometer. Analytical and numerical solutions showed that the deposition of sulfur was affected by the radial distance from the wellbore and sulfur-solubility changes as a function of the pressure drop. Sulfur deposition was found to have a great effect on the rock wettability, and in turn the gas production will be affected. The model can be used to predict the critical flow velocity that the gas can flow without precipitating sulfur. The optimum gas-flow velocity was estimated to be a range that maximizes the sulfur solubility in the gas. It was confirmed experimentally that the sulfur deposition reduced the carbonate-core porosity and permeability, and changed the contact angle (rock wettability). The contact angle increased, which means sulfur adsorption on the rock surfaces changed the rock toward more-gas-wet rock.