In order to reach the goal of carbon neutrality set forth in the 2050 long-term Net Zero strategy, major Oil&Gas Operators face significant challenges, with operational excellence in asset management being a key pillar. Carbon Sequestration and Storage (CCS) operations are crucial to achieving carbon neutrality, with well barriers playing a fundamental role in guaranteeing long-term integrity when dealing with depleted reservoirs and legacy wells. This paper aims to quantify the long-term integrity of cements and tubulars when converting an oil/gas production well to CCS.
An innovative model is used to evaluate the long-term integrity of cements and tubulars through a semi-quantitative approach that defines a probability of barrier failure based on well-specific conditions. Cement degradation evaluation involves a qualitative and quantitative part, where the quality of cement operations is evaluated from CBL/VDL logs and an energetic approach, respectively. Tubulars degradation model is based on the evaluation of material corrosion rate, given by the contribution of sweet corrosion and sour corrosion mechanisms. The final values of failure probability of cements and tubulars barriers are used inside Eni proprietary e-Wise™ system in combination with other barriers for calculation of probability of well integrity loss through a fault tree analysis approach.
The semi-quantitative risk analysis approach is applied to a legacy well to assess the viability of conversion into CCS operations using the selected materials and barriers. Materials exposure to corrosion attacks is evaluated for fluid conditions, each with specific contents of H2S, CO2 partial pressures and water cut. The use of different Cement types with their physical properties and different tubing alloys is investigated to mitigate the exposure to corrosive environment. Tubing failure rates are compared with statistical values taken from international Oil&Gas database and the use of Corrosion Resistant Alloys (CRAs) allows a remarkable reduction of failure rate up to one order of magnitude, reducing the overall risk of well integrity loss for many of the well configurations studied.
The proposed model is able to assess the failure probability of tubular and cement barriers in a systematic and well-specific approach, leading to an improvement of the standard failure rates selection based on historical data. The approach allows the assessment of well-specific risk levels for new and converted wells for CCS operations, suggesting the adoption of additional barriers or best material selection.
