Summary Mechanical properties such as Young's modulus, Poisson's ratio, and strength are important parameters for evaluating the cement sheath integrity. However, the current triaxial testing methods for the mechanical properties of intact-cylinder cement specimens ignore the “ring” shape of the wellbore cement sheath and its nonuniform 3D stress distribution, which may lead to deviations from the actual mechanical properties of the wellbore cement sheath that can affect the theoretical analysis results. In this study, we designed a hollow-cylinder cement specimen to address the aforementioned conditions and conducted uni, tri, and multiaxial compression tests to analyze the differences in the deformation, damage, and mechanical failure of the hollow- and intact-cylinder cement specimens. It was found that under the same confining pressure, the Young’s modulus of the hollow-cylinder cement specimen with hole pressure was approximately 1.2 times that of the intact-cylinder cement specimen; however, the difference in Poisson’s ratio was not significant. The uniformity of the radial and circumferential stress distributions in the hollow-cylinder cement specimens was the main factor affecting the volumetric strain curves and the damage threshold. Under tri and multiaxial compression conditions, the deviatoric stress of the hollow-cylinder cement specimens was higher than that of the intact-cylinder cement specimens. Compared with the Mohr-Coulomb criterion, the twin-shear unified strength theory can more accurately reflect the relationships between the radial, circumferential, and axial stresses and the strength of the hollow-cylinder cement specimens. Through a failure test of the cement sheath based on a self-developed wellbore simulation device, it was verified that the mechanical properties of the hollow-cylinder cement specimens can better reflect the failure law of the wellbore cement sheath compared with intact-cylinder cement specimens. The findings of this study can contribute to the understanding of the mechanical behavior of wellbore cement sheaths.
The failure of wellbore sealing will cause leakage of greenhouse gases, such as carbon dioxide and methane, which will harm oil and gas recovery and environmental safety. Cementing is an important part of wellbore sealing. Only good cementing can keep the wellbore seal for a long time and improve the well life. In this study, we considered the construction of a horizontal shale oil well in eastern China as the background and analysed the rheological properties of the annulus fluid. We developed a displacement motion model and a calculation model for the annulus dynamic equivalent circulation density, and numerical simulations were used to study the impact of the dosage and injection sequence of the pad fluid on the displacement efficiency and annulus dynamic equivalent circulation density. The results show that when the pad fluid is composed completely of flushing fluid, the displacement performance is better than that of the spacer. By increasing the dosage of the flushing fluid from 0.3 times the annular volume to 1.0 times, the displacement efficiency can be increased by 3.3%, and the retention of the drilling fluid is also reduced by 3.6%. However, it can lead to a significant reduction in the annulus dynamic equivalent circulation density and increase in the risk of leakage. After adding the spacer, the structure of the flushing fluid–spacer provides the optimal injection sequence. Considering the application status in the field example well, it was shown that it can not only ensure the safety of cementing operations, but also improve the displacement efficiency. The results of this study have important theoretical significance and application value and can provide guidance for the optimisation design of the engineering scheme.
Summary Shale gas development usually uses large displacement horizontal well and staged fracturing technology to increase operation production. The complex environmental and construction conditions often lead to wellbore sealing integrity problems in the shale gas production process. This study shows a new method for evaluating the sealing integrity of shale gas cement sheath interfaces, which aims to understand the failure mechanism during shale gas production and to propose countermeasures that can effectively improve the sealing integrity of cement sheath interfaces in shale gas cementing. The study results showed that the oil contamination of cement sheath interface will greatly weaken its sealing performance. After repeated cyclic loading, the sealing performance of the conventional and expanded cement sheath assemblies is damaged, and a gas channel is formed, which is caused by the combination of microcracks and microgaps. Furthermore, oil contamination of the cement sheath interface will accelerate its sealing failure. The addition of an expansion agent is helpful to solve the problem of microgap destruction, and the fibers or whiskers can alleviate the problem of tensile cracking. The field application in the three wells proved that the toughened expanded cement slurry significantly improved the sealing integrity of the cement sheath interface in shale gas wells. The research results can evaluate and predict the sealing performance of the cement sheath interface in shale gas wells under the conditions of staged fracturing and have some directional significance for the cement slurry system optimization in the field.
Abstract After cement slurry is injected into the a well, weight loss will occur before the cement sheath is formed, so that the pressure of the cement slurry column will gradually decrease, which will increase the risk of micro annulars at the casing-cement sheath-formation interface. In this study, one cement sheath sealing integrity test platform integrated with curing and testing was established to simulate the hydration weight loss process in the cement sheath setting. The sealing integrity of the cement sheath under stage pressurization and rapid depressurization was tested. Using the theory of thick-walled cylinder of an elastoplastic mechanics, the development of the elastoplastic interface of the cement sheath under the change of casing internal pressure was calculated, and the calculation method of equivalent crack width were established. The research results show that, the greater the setting pressure loaded during the curing process on the cement sheath, the higher the stress that the cement sheath can withstand during the later operation. After the cement sheath body is destroyed, as the internal pressure of the casing decreases, the width of the micro cracks increases. The research results of this paper can provide an important reference for designing mechanical properties of cement sheaths and improving the sealing ability of cement sheaths.
Carbon capture and storage (CCS) technology has become one of the most cost‐effective and promising ways to achieve global climate change mitigation goals. The CO 2 storage capacity in depleted oil and gas reservoirs is currently the safest and most economical option. The long‐term sealing of the cement–formation interface in CO 2 storage wells (CSW) is essential to avoid CO 2 leakage. The filter cake easily affects the interfacial sealing ability, which is a thin impermeable layer formed by drilling fluid residue or solid particles remaining on the borehole wall due to permeable formation. Filter cake removal is essential for favorable cementation and production efficiency. This study highlights the current developments and the main challenges for oil‐ and water‐based mud filter cake removal during the cementing and completion process. Moreover, research ideas and recommendations for various types of filter cake removal are proposed to provide a reference for future work. This information helps deepen understanding with respect to the different filter cake removal options available in the CCS industry and provides a knowledge base that can facilitate the improvement of the cementing quality and production efficiency of existing systems to combat battle CSW safety and production efficiency problems.
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