A Novel Tool for Cement Evaluation
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The annular casing cement is an important part of the well barrier throughout the life cycle of a well. With the increasing number of plug and abandonment (P&A) operations, increased attention is now given to annular cement evaluation and the ability to prove adequate zonal isolation. If an existing annular barrier can be verified, heavy and time-consuming operations to restore the annular seal can be avoided. One of the concerns when it comes to cement integrity is the frequently occurring microannulus at the casing-cement interface. How such a microannulus forms and how it affects the integrity of the well, has been presented in this thesis. The acoustic logging tools used for cement evaluation today have also been presented. Their individual strengths and deficiencies have been discussed, and their response to a microannulus has been highlighted. A novel logging tool for cement evaluation the Annulus Verification Tool (AVT) has been developed at the Department of Geoscience and Petroleum at the Norwegian University of Science and Technology (NTNU). The tool is meant to complement the acoustic tools used today, to improve evaluation of the cement sheath's sealing capability, especially in cases where a microannulus is detected or suspected. The AVT applies a radial force on the inner casing wall while recording the displacement of the casing. This gives a measurement of the stiffness of the casing and any surrounding material, explaining how the tool can detect the presence of an annular cement. By measuring the casing displacement with high resolution, a microannulus can be both detected and its size can be quantified. This enables an evaluation of the microannulus's effect on the integrity of the annular cement. A prototype of the AVT has been constructed and an experimental set-up has been designed to allow for initial testing of the tool. This includes construction of full-scale diameter samples representing a typical production casing cement job, with the possibility to generate a uniform microannulus of a known size at the casing-cement interface. The experimental testing performed, has shown that the AVT is able to differentiate a casing supported by an annular cement sheath from a free pipe, due to the stiffness contrast. This makes the tool able to detect the presence of an annular cement even in cases where a microannulus exists. Results also showed that a microannulus gives a characteristic logging response, and that the microannulus size can be quantified with good accuracy. Inferring the quality of the cement sheath itself proved to be challenging, as the logging response seemed to depend on other factors than solely the mechanical properties of the casing, cement, and formation. Experimental tests performed with tool eccentricity and tilting has shown that the AVT is prone to improper centralization within the casing. Numerical simulations (finite element analysis) have been performed to further increase the understanding of the AVT's logging response in specific cases. This was done using the geometry of the experimental set-up, to be able to directly compare with the experimental results and aid for an increased understanding of these. The simulated free pipe stiffness was found to match decently with the experimental results, while the simulated well-cemented stiffness was found to be significantly higher than for the experimental results. Further, the simulation results showed that a casing without annular cement support is deformed almost exclusively by ovalization, rather than compression of the steel. This confirmed the underlying principles behind determining the microannulus size from the AVT logging response. Experimental testing of the AVT should be continued, to further evaluate the uncertainty related to the microannulus size calculations. Testing on more complex casing-cement-formation samples should also be performed. The work on the numerical simulations should be continued, to resolve the discrepancy in the well-cemented stiffness. Simulations should also be used further to evaluate potential damage to the casing and cement, imposed by logging. In a longer perspective, the AVT should be made ready for downhole application. Suggested modifications (compared to the prototype) have been presented, as well as the AVT's possible role in future cement evaluation.