Modeling sulfur plugging of fractured wells under non-Darcy and non-equilibrium sulfur deposition

Abstract

The issue of sulfur plugging has been a concern for engineers for a long time. Some conventional models for vertical wells have been presented by authors; however, analytical models of sulfur plugging for fractured wells have not been reported. Based on non-Darcy and non-equilibrium theory, this article presents a new sulfur deposition prediction model for fractured wells, and a new concept, named sulfur deposition equivalent wellbore radius (SDER), was induced into new equations to describe fractured well characteristics. In a conventional gas reservoir, the fracture conductivity is constant, but here it is changeable due to the presence of sulfur deposition. The SDER value will increase with the increasing fracture conductivity but will reach a stable point at a certain fracture conductivity value. The value of that stable point is equal to 0.5, which is just the value of the equivalent wellbore radius with infinite conductivity fractures. The influence of some important parameters on sulfur deposition was investigated, including gas production, relaxation time, water saturation, gas temperature, fracture conductivity, initial permeability, flow pressure, and radial distance. The results show that the plugging of the formation by deposited sulfur occurs later during the non-equilibrium process, occurs earlier when the flow rate is as high as expected, occurs earlier as the water saturation increases, occurs earlier under high temperature conditions, occurs earlier as the non-Darcy flow becomes stronger, and occurs earlier as the fracture conductivity becomes smaller.