7/27/2019 A Flow Assurance Study on Elemental Sulfur Deposition in Sour Gas Wells

1/1

SPE 147244

A Flow Assurance Study on Elemental Sulfur Deposition in Sour Gas WellsYula Tang, Joe Voelker, Chevron Asia South E&P; Cengizhan Keskin, Zhenggang Xu, Bin Hu, SPT Group;Changqing Jia, China National Petroleum Company

Copyright 2011, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in Denver, Colorado, USA, 30 October2 November 2011.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not beenreviewed by the Society of Petroleum Engineers and are subject to correction by t he author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission toreproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

AbstractThis 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. Anoffset 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 predictprecipitation 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 S 8 phasediagram.

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.

IntroductionElemental sulfur occurs in solid, liquid and gas phases, depending on pressure, temperature, and gas composition. Solid

sulfur exists in rhombic crystalline form when the temperature is less than 96C, in monoclinic crystalline form up to 118 C,

and in gas form above 120 C. Elemental sulfur in gas or liquid phases will have a ring structure with eight atoms linked

together (S8), that will break down above 157

C. As most reservoirs do not exceed this temperature, elemental sulfur iscommonly referred to as S8. Commonly the solid phase is also referred to as S8, as is the case in this paper.

S8 occurs as a dissolved species in virtually all deep sour gas reservoirs. Studies indicate the solubility of S8 is positivelycorrelated to H2S concentration.

4,5,7,10 Sulfur precipitation during gas production is induced by a reduction in S 8 gas phase

solubility, below the in-situ concentration, as a result of decreases in pressure and temperature.

Solid sulfur deposition can occur in the reservoir, tubing, and surface facilities. Prediction and mitigation of sulfur

deposition are crucial to the economic viability of sour gas fields. Sulfur deposition risk occurs in the tubing during theprocess of well start-up, stable production, and shut-in. It is critical to evaluate the risk and seek mitigation at the planning

stages of field development.

Flow assurance risk is assumed to develop from two possible S8 precipitation processes in the tubing, in this study:

Continual aggregation of S8 on the tubing wall during start-up and production: during well start-up or stable production,the tubing wall is cooler than the center of hot gas stream. S8 will tend to precipitate from the boundary layer that