| dc.description.abstract | Drilling of wells for exploration and production of petroleum resources requires thorough analysis of well integrity. Well integrity can be defined as the application of technical, operational and organizational solutions to reduce the risk of uncontrolled release of formation fluids throughout the life cycle of a well. During drilling and intervention operations, the tubular is worn due to the mechanical interaction with the downhole equipment. This is known as tubular wear or casing wear. A fundamental part of an integrity analysis is to estimate casing wear and its effects on the strength of the tubulars. Modern operations demand highly accurate models for estimating casing wear and its effects. This research is designed to demonstrate how such solutions can help to ensure well integrity, reduce the risk of issues related to health, safety and environment, and enable cost-efficient measures. The thesis covers methods for improved wear estimation and models for estimating the effect of wear on the collapse strength of tubulars.
The first part of this thesis presents a methodology for estimating casing wear during drilling by utilizing real-time data and predictive modelling. A model for casing wear estimation is developed and tested on data from an operation on the Norwegian Continental Shelf. The study found the approach resulted in improvements to the wear estimate, with the utilization of real-time data resulting in a reduction of 12.9% when compared to logged wear. An analysis of the viability of applying the procedure in real-time during drilling found the computational efficiency to be sufficient for application, with the maximal runtime limited to approximately 10% needed in a worst-case scenario of the case study.
The second part of the thesis concerns the collapse strength of worn tubulars. A preliminary study of the collapse of unworn tubulars was performed. Here, the relevant models were evaluated using a statistical approach employing Monte Carlo analysis. Worn tubulars experience a reduction in collapse strength due to a reduction in wall thickness from wear. This reduction is usually in the form of a crescent-shaped wear groove. The finite element method (FEM) was applied to model the reduction in collapse strength. Numerous tubular collapse analyses were completed using an automated methodology for finite element analysis (FEA). The results were evaluated and used to generate generalized expressions for stress concentration factors (SCFs). Then the SCFs were used to derive expressions for the collapse strength of worn tubulars. Comparison to experimental results from 36 collapse tests shows an improvement to the state-of-the-art model where the new approach resulted in a mean of 1.003 and standard deviation of 0.077 compared to 1.039 and 0.079 for the state-of-the-art model, respectively. The new model is demonstrated to predict the stress response of worn tubulars accurately and precisely.
Lastly, a model for describing the wear characteristics tubulars made from corrosive resistant alloys has been developed based on original experimental data. This type of alloy has been found to experience excessive and unpredictable wear rates in field and laboratory conditions. An original experimental setup was used to measure the wear rate of 13Cr80 tubulars. Analysis of the resulting data demonstrated its high wear rate and distinctive wear characteristics. Furthermore, an adjustment to the state-of-the-art model for casing wear estimation has been proposed to account for the observed deviance in wear characteristics. | |
