Assessing Flow Potential Risk in Shallow Zones

Abstract Maintaining integrity is a major concern throughout the lifecycle of a well, especially during the decommissioning stage. Regulatory requirements mandate that barriers be established to prevent the flow of fluids to the surface, and that these barriers reconstitute the cap rock above intervals deemed to have flow potential. This can lead to extremely complex projects in scenarios where shallow zones distant from the reservoir are classified as having flow potential. Accurate characterization of shallow zones can have an outsize impact on the costs and operational challenges of the abandonment. Additionally, because decommissioning constraints must already be considered from as early as the drilling stage, these characterizations may influence decisions ranging from well design and completion to the field development project. Characterizing the flow potential of shallow zones is inherently challenging, due to the scarcity and uncertainty of the data from these intervals. An exceedingly conservative approach can lead to abandonments that are not only costly, but that might also carry unnecessary operational hazards. On the other hand, not being sufficiently conservative increases the risk of an eventual leakage to the environment. Hence, the development and application of a consistent risk-analysis methodology is essential for operators. The methodology presented is based on an ALARP approach and aims to quantify the likelihood and severity of an eventual outflow of fluid from the well after its abandonment, thus determining the level of risk associated with each interval and allowing for appropriate actions to be taken. The severity classification of a leakage event is based on the characteristics of the fluid present in the interval and on a stochastic estimate of the eventual volume that would flow to surface, supported by an analytical model that couples reservoir and wellbore flow behavior. Meanwhile, the likelihood is defined by the probability that such a leak would occur, based on a set of direct and indirect evidence available, and on the level of confidence associated with the data. The application of this methodology has proven to be quite robust. It has streamlined the evaluation of individual intervals across various scenarios by providing a systematic approach that different teams can easily and consistently follow and has increased confidence in the decision-making process. Consequently, several recent wells were able to be decommissioned through simpler, safer configurations, and the drilling of several prospective wells was made viable due to less exigent isolation requirements. Given the uncertainty related to the flow potential of shallow intervals and the consequences of underestimating or overestimating the risks associated with their characterization, it is vital that practical experiences and novel modeling strategies be shared and discussed by the community, to enhance the safety of well operations.