Species of sulfur in sour gas reservoir: Insights from in situ raman spectroscopy of s-h2s-ch4-h2o system and its subsystems from 20 to 250°C

Abstract

Thermochemical sulfate reduction (TSR) is the most important mechanism for the generation of high-concentration H2S in gas reservoirs. Sulfur speciation in sour gas is one of the key factors controlling the rate and extent of TSR in gas reservoirs. However, experimental studies on S species in sour gas are limited due to the toxicity and corrosion of S and H2S. Fused silica capillary capsules (FSCCs) are inert to S and H2S and, therefore, were employed in this study as microreactors containing the S-H2S-CH4-H2O system and its subsystems, representing the composition of sour gas. The in situ Raman spectra of each system were collected continuously during the process of heating from 20°C to 250°C. The results showed the following: (1) a Raman peak at 2500 cm-1 was detected in the liquid S phase of the S-H2S-CH4 -H2O system at 120-250°C, which was attributed to H2Sn. A Raman band at ∼533 cm-1 was detected in the aqueous phase of the S-H2S-H2O-CH4 system at 250°C and was assigned to S3 -, suggesting that S3 - and H2Sn are important S species in sour gas reservoirs at elevated temperatures. (2) The Raman peak at 2500 cm-1 disappeared at 20°C, indicating that H2Sn decomposes into S and H2S. During gas extraction, the decomposition of H2Sn will cause S deposition in pipelines. (3) In addition to S3 -, H2Sn could be the intermediate valence S species involved in the TSR reaction.