Integrated Gas Source Analysis for Wells with Sustained Casing Pressure: A Case Study from Sour Gas Fields in Sichuan Basin, China

Sustained casing pressures are observed in many gas-field wells in Sichuan Basin. The presence of corrosive sour gas i.e. H2S and/or CO2 in the annulus, coupling with the none corrosion-resistant alloy (CRA) casing materials, may damage the casing and lead to loss of containment, which could be a disaster to the asset, local communities, and the environment. It is thus of significance to identify the gas source for the wells with sustainable casing pressure. Several wells in the study area were identified as risky wells with an initial risk screening exercise. The further determination of gas source, either sweet gas in the shallow formation or sour gas in the deep formation, would have a big impact on the risk levels and subsequent mitigation plans. An integrated forensic study was conducted to investigate the gas source, including annulus gas composition analysis, isotope fingerprinting, casing pressure profile construction and analysis, casing pressure bleed-off and build-up operations, cement quality evaluation, temperature and noise logging analysis, etc. In this paper, the integration of various tools and data enabled a quality diagnosis of the gas source for two risky wells. It is believed the most likely gas source for the first well is from the shallow sweet gas formation. The risk was then revised down to Low-Medium, and the well was temporarily plugged and abandoned (P&A) for future use. For the second well, the gas source is confirmed from the deep sour gas formation, and thus the risk is relatively high. A permanent P&A program was performed, and both the sour and sweet gas layers were successfully isolated per the standard barrier requirement to protect community and environment. In conclusion, most of the subsurface measurements have limitations and thus result in the imperfect data. The key to solving the subsurface puzzle is to consolidate all the perceptions together after reviewing the problem from different angles. The integration of multiple types of data in this study, including geological and engineering data, static and dynamic measurements, etc., allows a mostly likely interpretation of gas source to support the key operations decision. The results from integrated data analysis have helped improve the risk assessment, guide the development of risk mitigation plans and operations procedures to eliminate and/or reduce the risk of these wells and secure the subsurface integrity.