Peter D. Carragher, Rose & Associates, LLP; Rosalie Constable, Constable Energy Consulting
Oil and Gas explorers continue to improve their skills at assessing the volumes and geological chance of success for prospects. However, exploration companies accept there will be failures and use the portfolio effect to ensure the value of the discovered volumes exceeds the program costs. Failures in exploration programs are generally an economic burden on the company.
We contend that the “portfolio effect,” accepting a certain number of failures, will not be acceptable to companies, regulators, or the broader societal interests in Carbon Capture, Utilization and Storage (CCUS). Assessing the risk in a CCUS project does require an assessment of geological success; although this is a necessary component, it is not sufficient for the complete assessment.
Therefore, we propose that a paradigm shift is required from the focus on geological and subsurface success to a broader assessment of failure throughout the life of a project. Also, operators must implement plans at all stages to monitor for and mitigate the possible failures.
CCUS projects can fail during any one of the Screening, Appraisal, Injection, or Closed-In stages. For example, in the Appraisal stage the seal and/or reservoir could be found to have poor properties. During the Injection stage, the injection rates of CO₂ could fall below the requirements, pressures could build, the model could fail to predict the plume direction, or seal failure to another stratigraphic level could occur. During the Closed-In stage, failures of seal, fault or well integrity could occur leading to a worse-case release back to the environment at either the seabed or atmosphere. There are known examples of each of these events.
A key difference with current subsurface risk assessment techniques is the requirement that the subsurface store will not fail for time scales varying from the near term to over 1,000 years. Although the IPCC proposed standard is that 99% of the injected mass will be retained for at least 1,000 years, we believe that operators, regulators, and society will require near term data that provides confidence in long term security of the project. This type of risk assessment is new to many in our industry. We’ll review some of the techniques developed to assess long term risk of disasters in the natural world.
A second key difference is that frequency of failures in, for example, wells and seals are likely to be very low, in the range of 1 percent per year and below. We contend that we cannot use expert judgement methods to differentiate subsurface risks between say 0.1% and 0.01% failure rates. At an annual failure rate of 0.1% per year in 1,000 years there is a 63% chance of one or more failures; at 0.01% per year there is a 10% chance of one or more failures.
We recommend using documented failure rate data from analogous wells and subsurface projects as a guide to the base rate probability of future failure in CCUS projects. We will show the effect of multiple failure modes on the time frames and stages of CCUS projects.