Summary In permanently abandoned wells, cement plugs serve as a well-barrier element that is essential for providing long-term zonal isolation. Poorly plugged wells might provide leakage pathways that pose risks to the surrounding environment. It is well-known that the final quality of set cement is influenced by the cement placement, but the influence of downhole-pipe roughness to the hydraulic sealing of a cement plug has not been specifically investigated. This paper presents an experimental study assessing the cement-plug sealing of neat- and silica-cement systems placed in pipes with three different levels of surface roughness. The experimental test uses a small-scale laboratory setup, consisting of a test cell filled with a cement plug, which can simulate realistic wellbore curing and operating conditions. By subjecting the cement plug to gas differential pressure, the setup detects any gas leak through the cement plug and measures the leak rate. As a complement to the experimental study, a computational-fluid-dynamics (CFD) simulation is conducted to study fluid flow around the cement plug. Experimental tests detect an instant gas leak in all samples at low differential pressure (<0.01 bar) through leak sources at the cement/steel interface, presumed to be microannuli. Furthermore, experimental results discover that placing cement plugs in rough-surface pipes could reduce the gas-leak rate, and the most significant reduction was found from samples cured in the roughest pipe. The CFD simulation results, along with pressure-buildup-curve observation, show that the leak path of samples placed in rough-surface pipes tends to be tortuous and less connected, thereby adding resistance for gas to flow.

PrefaceThis thesis is a one of my fruitful result of a keen work for the past one semester. This thesis is also part of the requirement for a master degree in Petroleum Engineering, Department of Petroleum Technology and Applied Geophysics, Norwegian University of Science and Technology (NTNU). The study described herein began in spring 2016 to the extent of 30 educational points. Apart from the efforts of myself, the success of this study depends on the guideline of many others. I take this opportunity to thank these people.First, I express my deep gratitude to my supervisor, Sigve Hovda, for the time and constructive response to the work. His enthusiasm working in this project has preserved the optimistic attitude and made this project possible. I would also like to thank Erik Skogen for the supportive discussion of well logging which I found very valuable for this work. I would also like to thank AGR company for providing access to iQx software, including the access to the data. And, I would also like to thank MATLAB © 2015The MathWorks, Inc.,Natick, Massachusetts, United States, for supporting the computation within the study. Among all of the benefits provided by Institute Petroleum of Technology (IPT) NTNU, my curiosity of interdisciplinary working on this project together with these people have benefited myself more than most. Despite the academic support, I would thank my all of my friends from Petroleum Engineering/Petroleum Geosciences, especially cohort 2014, who became my loyal partners during the long journey of 2 years of the master degree. I would also thank my friends from Indonesia who bring the joy during the rough hour working on my thesis.Most importantly, none of this could have happened without my family. To my mom, the person that I adore the most, I sent my deepest love and thank for being there all the times for me. I would also to thank my dad, sister, and brother who never stop giving me comfort even though we are miles away and I am forever grateful.Trondheim, 2016-06-09 Anisa Noor Corina i AbstractThis thesis presents an automatic real-time analysis of lithology interpretation through a method of statistical analysis: kernel probability density method. The goal of this thesis is to develop a method for interpreting and predicting lithology from the borehole geophysical data in real time. Prior to the development, the data is explored to check the data quality and the requirement of data correction. In addition, from exploratory data analysis, the data characteristics can be observed thus the best-fit classification method can be selected. The study focuses on the univariate analysis of gamma-ray data in classifying shale and non-shale lithology. In addition to univariate analysis, a preliminary study of bivariate analysis is also provided in this thesis. The bivariate analysis combines the gamma-ray and the neutron data.Within the study, the models of probability density are constructed by using kernel estimator. The data source for the models are extracted from 3 wells in the North ...

A cement plug is widely applied for permanent abandonment phase to provide long-term zonal isolation against fluid flow. Maintaining cement plug integrity is a challenging task, and loss in cement sealing poses risks to the surrounding environment and surface safety. It is well-known that the cement performance is affected by cement material and downhole conditions. Nevertheless, investigations linking these influencing factors with the sealing of cement plugs are still limited, especially with the lack of proper equipment in the past. In the present work, a small-scale laboratory setup has been constructed to test the sealing ability of a cement plug. It has unique features that can simulate plugging operations at the downhole conditions and preserve the cement curing condition. By testing using this setup, it is possible to measure the minimum differential pressure required for gas to flow across the cement plug and the gas leak rate. The silica cement mixture was selected as the plug material, prepared using silica flour. Investigation of silica cement under the influence of expanding agent additive and various curing temperature was carried out. It was found that adding an expanding agent improved the sealing of cement plugs. Moreover, samples cured at a high temperature were less resistant to gas flow with the leak path observed at the cement/steel interface, indicating debonding.

Hydrated bentonite is considered an alternative subsurface sealing/plugging material for deep geo-energy wells. However, the knowledge related to this application and the corresponding properties of bentonite is still lacking. This includes the mechanical properties at the interface of bentonite plugs with the adjacent materials (surrounding rock or casing steel) and the mechanical stability of plugs under downhole in-situ conditions. In this work, we performed experiments investigating the interface shear properties and shear strength of a bentonite plug under various settings for deep geo-energy applications, such as hydrocarbon and geothermal wells. The interface’s shear properties against various adjacent materials and fluid conditions were characterized. The influence of chemical exposure, the salinity of the curing fluid, fluid pH, pressure, and temperature on bentonite’s mechanical stability was evaluated in a small- and large-scale setting. The latter was performed using realistic casing sizes and placement methods, relevant for the field application. The experimental results show that the averaged shear strength of the bentonite plug interface is 13.3 kPa and 9.1 kPa when cured in freshwater and seawater, respectively. The increase in strength with increasing curing pressure, temperature, and fluid pH was characterized for the first time. The interfacial properties of cohesion and friction angle vary with different surrounding materials. They are also influenced by the saturating condition and salinity of the saturating fluid. Based on the experimental results, a bentonite plug with a minimum length of 15–43 m placed in casings of 7–5/8″ to 13–3/8″ would be sufficient to meet the necessary criteria of the Dutch regulators as an isolating material for well abandonment.

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