Criteria and risk of integrity loss for wells with sustained casing pressure.
Theodore Yao, Andrew Wojtanowicz
Vol. 34, no. 2 (2017), s. 639-653, [1]
Sustained casing pressure (SCP) represents a major issue because of its large scale occurrence and risks to health, safety, and the environment. Present regulatory assessment of sustained casing pressure is mostly qualitative with implicit risk formulation. It currently holds that wells with casing head pressure that can be bled-down to zero and is followed by slow 24-hour pressure buildup are below acceptable level of risk. This study introduces new quantitative metrics of well integrity loss risk – the instant cement sheath leak rate of 15 scf/min (barrier integrity), and the total annual environmental gas discharge of 6 tons of volatile organic compounds (VOC) per year. Field data from 19 wells reportedly affected by sustained casing pressure (SCP) are examined with a SCP testing software to assess whether or not wells with pressure that is able to bleed to zero would meet the proposed criteria. Using modeling and software tools developed by Xu and Wojtanowicz (2001) and Kinik and Wojtanowicz (2011), it is determined that three of the 19 wells (15.8%) examined would fail the instant leak rate criterion. On the total discharge criterion, assuming the wells' annuli above the cement top filled out with sea-water, it was also found, again, that 15.8 percent of the wells would fail the total environmental discharge criterion. Moreover, for the worst-case scenario of absolute open gas flow (empty annulus above the cement top), five of 19 wells (26.3%) would fail the criterion. It is shown that – statistically, the bleed-down of casing pressure to zero gives a 90-percent confidence of the well passing the proposed criteria. Furthermore, no clear correlation was observed between pressure build-up and the barrier integrity or the environmental gas discharge criteria, thus questioning the 24-hour pressure buildup relevance as a risk indicator.