A Flow Assurance Study on Elemental Sulfur Deposition in Sour Gas Wells

Abstract This paper summarizes a flow assurance study for elemental sulfur deposition in the tubing for the Chuandongbei Gas Project (CDB), a greenfield sour dry gas development project in Sichuan, China. The project is a co-venture of Chevron and China National Petroleum Company (CNPC). The development contains several fields, all containing dry gas with 8-17% H2S, and 8-10% CO2 Elemental Sulfur (S8) dissolved in the gas may precipitate in both the near-well reservoir region, and tubing, within the pressure and temperature range predicted for gas well flow in the fields. The precipitate may form gas flow restrictions. An offset operator producing gas from the same reservoir interval and having similar composition has reported flow problems attributed to S8. This study focused on the prediction of S8 deposition in the tubing, during both production and shut-in periods. Numerical transient well pressure and temperature modeling with the OLGA wax deposition module was used to predict precipitation and deposition of S8 in the tubing. Presently, no other S8 deposition model is available. The model estimates the pressure and temperature gradient between the bulk fluid and tubing wall, and molecular diffusion rates through the laminar sub-layer of the fluid velocity profile. An S8 phase equilibruium model calibrated by measured phase behavior from a laboratory synthetic gas having the composition of CDB field measured surface gases was used to generate the S8 phase diagram. The study indicates that shut-in periods present the greatest S8 deposition risk: suspended sulfur precipitate accumulates at bottomhole during shut-in periods, possibly forming a flow restriction. Mitigation through solvent treatment applied after shut-in periods is therefore planned. S8 deposition on tubing walls during flow periods was found to present relatively low flow assurance risk. This study also provides operational design recommendations for well start up, sustained flow, and shut-in periods, as well as flow assurance mitigation design.