Blowout Probability Estimation for CCS New and Converted Wells and Risk Assessment on Well Barriers Integrity
Authors: Zausa Fabrizio , Rossi Nicola , Morcella Tomas H. 
Published: June 2024

2050 long-term Net Zero strategy imposes great challenges for major Oil&Gas Operators for which operational excellence in asset management is a key pillar. Among the most important contributions, Carbon Sequestration and Storage (CCS) operations are crucial to reach the goal of carbon neutrality. Well barriers play a fundamental role in guaranteeing a long-term integrity when dealing with depleted reservoirs and legacy wells. Within this context, the objective of the paper is to quantify the long-term integrity well barriers under different time and environmental conditions when converting an oil/gas production well to CCS.

The long-term integrity is evaluated employing a comprehensive risk assessment approach, incorporating a semi-quantitative model for cement and tubular materials along with flow paths study under different conditions to conduct a sensitivity analysis comparing the reliability and integrity of new wells versus converted wells in Carbon Capture and Storage (CCS) projects. The model considers the key components of well barriers and operational status to evaluate their performance under varying conditions.

A risk analysis approach is applied to a set of legacy wells to assess the viability of conversion into CCS operations using the selected materials and barriers. Each case is selected with the aim of covering different well barriers settings, wellhead configuration (surface or subsea), completion type (single, dual completion) and Xmas Tree type (vertical, horizontal) and environmental conditions (specific contents of H2S, CO2 partial pressures and water cut).

To estimate the final blowout probabilities Fault Tree Analysis is utilized to systematically identify and assess potential failure modes and their associated probabilities. The final values of failure probability of the different well barriers (DHSV, production packer, xmas-tree valves, etc.) are used inside Eni proprietary e-wise™ system in combination with the material conditions for calculation of probability of well integrity loss through a fault tree analysis approach.

A sensibility analysis is done to quantify the impact of design choices, material selection, and operational variables on the overall risk profile of new wells and converted wells in CCS applications. Factors such as wellbore conditions, cement properties, and tubing specifications are considered in assessing the vulnerability of well barriers to failure.

The proposed model assesses, with a systematic and well-specific approach, the blowout probability of a wide variety of well conditions in case of long-term applications. Results from the sensitivity analysis aim to provide insights into the comparative risk levels associated with different well types, aiding decision-makers in optimizing well design and conversion strategies for CCS projects. This information is essential for stakeholders in the Oil & Gas industry, enabling them to make informed decisions regarding the selection of well types, design specifications, and operational practices that minimize the risk of barrier failure, ultimately enhancing the safety and success of CCS initiatives.