2 replies to this topic

[stu]

Dear Sir,
How do we analyse the gas from oil well & to find whether it is sour or sweet. Will you please mention how much H2S or Co2 in the gas will be consider as sour or sweet.
Please mention the percentage level of H2S or Co2 in the gas to say it as sour.
Please clear my doubts.
Thanks.
With Regards,
Stu

[Zauberberg]

Hello Stu,

As far as I know, there is no general statement or standard that defines in which cases a gas is to be considered as sour or sweet, based on H2S/CO2 concentrations. However, there are some other standards, procedures and engineering practices related to H2S concentration issues:

1. According to IP15, when H2S concentration in process stream is above 500pm, system has to be considered as highly toxic and for equipment isolation purposes double block and bleed arrangement has to be provided.

2. According to ASME codes, process streams are cathegorized in a following way:

• Category A: Non-flammable, stable and non-toxic fluids which are liquid at prevailing ambient temperature and atmospheric pressure plus 0.5 bar, i.e. the vapour pressure is lower than 1.5 bar (abs) at ambient temperature. Example: water, slurries.
• Category B: Flammable, unstable or toxic fluids which are liquid at prevailing ambient temperature and atmospheric pressure plus 0.5 bar, i.e. the vapour pressure is lower than 1.5 bar (abs) at ambient temperature. Example: stabilised crude, gasoil.
• Category C: Flammable unstable and non-toxic liquids with a TGLR<250, OR Non-flammable, stable and non-toxic fluids which are gases or a mixture of gas and liquid at prevailing ambient temperature and atmospheric pressure plus 0.5 bar, i.e. the vapour pressure is higher than 1.5 bar (abs) at ambient temperature. Example: nitrogen, carbon dioxide, typical low GOR non-sour oil flowline fluids, with H2S < 50 ppm.
• Category D: Flammable, unstable or toxic fluids which are gases or a mixture of gas and liquid with a TGLR>250 but H2S < 500 ppm in the gas phase at prevailing ambient temperature and atmospheric pressure plus 0.5 bar, i.e. the vapour pressure is higher than 1.5 bar (abs) at ambient temperature. Example: natural gas, liquid petroleum gas, ammonia, typical’ low risk sour facility’ or high GOR oil condensate and gas flowline fluids.
• Category E: Flammable, unstable or highly toxic fluids which are gases or a mixture of gas and liquid with TGLR > 250 and/or H2S = 500-1000 ppm in the gas phase (i.e. ‘high risk sour’ facility) at prevailing ambient temperature and atmospheric pressure plus 0.5 bar. i.e. the vapour pressure is higher than 1.5 bar (abs) at ambient temperature. Example: highly sour wellhead fluids.
• Category F: Flammable, unstable or highly toxic fluids which are gases or a mixture of gas and liquid with TGLR > 250 and/or H2S > 1000 ppm in the gas phase at prevailing ambient temperature and atmospheric pressure plus 0.5 bar, i.e. the vapour pressure is higher than 1.5 bar (abs) at ambient temperature. Example: acid gas, extremely sour wellhead fluids.

Pipelines carrying category A and B fluids are designed in accordance with ASME B31.4, while the pipelines carrying category C, D, E and F fluids are designed in accordance with ASME B31.8.

From this information you can see that "threshold" values of H2S are 500ppm and 1000ppm - as far as equipment isolation and piping design are concerned. But, if some other application is in question here (maybe you are evaluating the requirements for gas treating, I don't know), ANYTHING can be considered as sour or non-sour.

For the purpose of piping design and equipment/facility isolation philosophy, you can refer to the sources I cited above.
Regards,

[gvdlans]

Sour service environments are defined by NACE International specification MR0175 as "fluids containing water and hydrogen sulphide (H2S) that is at a total pressure of 0.4MPa (65psia) or greater, and if the partial pressure of hydrogen sulphide in the gas is greater than 0.0003MPa (0.05psia)".