Distribution of sulfur deposition near a wellbore in a sour gas reservoir

Elemental sulfur precipitates from sour gas when reservoir pressure and temperature decrease. Sulfur deposition in a formation may significantly reduce the inflow performance of sour gas wells. This paper develops a micro-mechanical migration model and experiments which describe the law of sulfur precipitation, plugging and distribution near a wellbore. Based on the analysis of the sulfur deposition law and micro-mechanical migration theory, elemental sulfur mechanical models in pores are presented. The critical velocity of sulfur is calculated and the rule of precipitated sulfur distribution near a wellbore is deduced. Reservoir cores and supersaturated sour gas are utilized to observe sulfur precipitation and plugging in sulfur damage experiments, and the main influential factor is analysed. According to the models and experimental results, precipitated sulfur can decrease reservoir permeability. The liquid bridge force is the most important factor to affect reservoir permeability due to sulfur deposition. Precipitated sulfur cannot be carried away from pores if the liquid bridge force is considered; the critical velocity increases as the diameter of the sulfur particles increases, which may cause serious formation damage. Moreover, it can be seen from the results that the biggest volume of sulfur deposition does not occur at the bottom but near the bottom of a borehole. These results can be used to describe the profile of dynamic sulfur deposition and help a reservoir engineer to develop a plan for removing the sulfur near a wellbore. (paper)