Getting Reliable On-Site H2S and CO2 Concentrations for Anti-Corrosion Measures in Gas Wells

摘要

This paper demonstrates the importance of valid on-site measurements of hydrogen sulphide (H2S) and carbon dioxide (CO2) in gas wells in relation to corrosion management, notably for the proper selection of tubular goods, where material choice can result in cost changes of millions of dollars. The problems of loss of H2S during sample storage are well known, providing a strong justification for on-site measurements. This work has looked at the ways to measure and validate both H2S and CO2 on-site. Conflicting values recorded on-site confirm the difficulty of getting reliable H2S and CO2 measurements. H2S measurements at three gas-condensate wells were made on bottom hole samples involving a special non-reactive wireline sampler service. The intention was to obtain more accurate reservoir concentrations by avoiding anticipated losses through reaction or absorption on the production tubing. However, comparative surface measurements on the separator gas stream gave higher concentrations in nearly all cases, and a thorough investigation of the data confirmed that the separator gas measurements, made using simple equipment, were significantly more reliable. In addition to the measurements themselves, it is demonstrated that well history, well conditioning and producing interval must be taken into account when interpreting measurements. For example, one test measured H2S production only after an acid fracturing treatment, implying that the original formation fluid contained extremely low or possibly no H2S. The paper makes recommendations for on-site measurements and their validation including the use of two independent methods for H2S determination.