Influence of Casing Inner Pressure
Fault slipping could happen during or after fracturing, as the casing’s inner pressure is different in different stages. Figure 15 and Figure 16 show the variation of diameter along the casing under different inner pressures. It can be seen that with the increase of inner pressure, the reduction of the casing’s inner diameter decreases, which indicates that higher inner pressure is beneficial to maintain the casing’s integrity.
Figure 15. Variation of diameter under different casing inner pressures.
Figure 16. Magnified area.
It should be noted that the casing’s inner pressure has little impact on the first concave area in the curves, while it has a relatively significant impact on the second concave area. When the casing’s inner pressure is zero, the reduction of diameter is the greatest (4.90 mm), and compared with the reduction when the casing’s inner pressure is 115 MPa (the reduction is 4.06 mm), the increase of the diameter reduction is 20.7%. The reason for this is mainly because the casing’s inner pressure has a dramatic impact on the equivalent stiffness of the casing string. The higher the casing’s inner pressure, the higher the equivalent stiffness.
Influence of Casing Thickness
With the increase of casing thickness, the shear resistance strength increases. Because the research object contains two casings, including intermediate and production casing, the thickness of both types of casings are changed, so as to evaluate the effectiveness of increasing the casing thickness.
Figure 17 shows the variation of the casing’s inner diameter under different production casing thicknesses. It can be seen that with the increase of the casing thickness, the reduction of casing’s inner diameter is almost unchanged in the first concave area, but decreases clearly in the second concave area (Figure 18).
Figure 17. Variation of diameter under different production casing thickness.
Figure 18. Magnified area.
Figure 19 shows the reduction of the casing’s inner diameter with the variation of the intermediate casing thickness, and it can be seen that with the increase of the casing thickness, the reductions of the casing’s inner diameter decreases nearly linear in both concave areas. Comparing the two methods, increasing the thickness of the intermediate casing above the casing shoe is the more effective.
Figure 19. Variation of diameter under different intermediate casing thickness.
Influence of Cement Sheath Mechanical Parameters
The mechanical parameters of the cement sheath can be adjusted by using different cement slurry formulas, and have an impact on the reduction of the casing’s inner diameter. Figure 20 shows the variation of diameter along the casing under different elasticity moduli of the cement sheath. It can be seen that the influence on the casing’s inner diameter in the two concave areas is different.
Figure 20. Variation of diameter under different elasticity moduli.
In the first concave area, with the increase of the elasticity modulus of the cement sheath, the reduction of the casing’s inner diameter decreases. But in the second concave area, the variation of the diameter has a reverse rule (Figure 21). Figure 22 and Figure 23 show the influence of the Poisson ratio on the casing’s inner diameter. It can be seen that with the increase of the Poisson ratio, the reduction of diameter increases, especially in the second concave area, which indicates that the lower Poisson ratio is beneficial for decreasing the reduction of the casing’s inner diameter.
Figure 21. Magnified area.
Results Comparison and Mitigation Method
The method in this study presented a way to evaluate the reduction of casing’s inner diameter. Using to the microseismic measurements, microseismic moment magnitude can be obtained. And based on the study of Chen et al. , slip distance can be calculated. Then, by using the model proposed in this study, the diameter of a casing’s inner wall after fault slipping can be computed.
Figure 22. Variation of diameter under different Poisson ratios.
The computed results and the measurement results by using MFC tools were compared, as shown in Figure 24. It can be seen that the numerical method in this study has an accuracy of up to 90.17%, and it can be used to select the soluble bridge plug after casing shear deformation, achieving the purpose of overcoming the problem that MFC measurement is costly, as mentioned above.
Figure 23. Magnified area.
According to the above analysis, it could be seen that decreasing the slip distance was the best way to protect the casing. In order to mitigate or eliminate fault slipping, the interface between the Nisku and Ireton formations should avoid being opened during the operation of well cementation. As a consequence, the well structure was optimized. The depth of the intermediate casing shoe was reduced to approximately 3600 m, and the depth was more than 100 m above the interface between the Nisku and Ireton formations. The method was applied to nine wells, eight wells did not occur casing shear deformation, which was proved to be the most efficient and economical way. And the engineering practice supported the correctness of the analysis in this study.
Figure 24. Comparison of measurement and computed results.
Casing shear deformation occurring during multistage fracturing was monitored by using MFC tools, and the cause of the shear deformed points located at the interface between the Nisku and Ireton formations was analyzed. A new investigation based on the MFC measurement results was carried out, and the impact of influential factors on the reduction of a casing’s inner diameter was studied. The following conclusions were drawn:
(1) MFC surveys were carried out to monitor the casing deformation occurring during multistage fracturing in Simonette, Canada. Statistical data showed that shear deformation was the main type of casing deformation, and the shear deformed points can be classified as two types according to the positions of occurrence: shear deformed points located at the position of the interface between the Nisku and Ireton formations (75%), and in the horizontal segment (25%).
(2) The cause of casing shear deformation occurring at the interface between the Nisku and Ireton formations was analyzed. When the interface between the different layers was opened during the operation of well cementation, the friction coefficient between the layers decreases dramatically. During multistage fracturing, the fault was activated and slipped along the interface, which was verified by the microseismic data.
(3) A numerical model has been developed to analyze the reduction of the casing’s inner diameter. The simulation results showed that: (a) fault slipping caused the reduction of casing inner diameter, and the maximum change appeared at the position of the interface of the two formations; (b) the cross-section of the casing calculated by the numerical model was similar to the shape reflected by MFC data in that particular position.
(4) A sensitivity analysis was carried out and the influence of slip distance, the casing’s inner pressure, the mechanical parameters of the cement sheath, and the intermediate and production casing thickness on the reduction of the casing’s inner diameter were analyzed. According to the numerical analysis results, decreasing the slip distance, maintaining high pressure, decreasing the Poisson ratio of the cement sheath, and increasing the casing thickness were beneficial for protecting the integrity of casing. Furthermore, the effectiveness of increasing the intermediate casing thickness is greater than increasing that of the production casing.
(5) Measurement results were compared with computed results to verify the method proposed in this study. The numerical method in this study has an accuracy of up to 90.17%, which can be used as basis for choosing soluble bridge plugs. In addition, the well structure was optimized, and the depth of the intermediate casing shoe was reduced to approximately 3600 m, which was more than 100 m above the interface between the Nisku and Ireton formations. The effectiveness of this method was verified by engineering in practice, as eight of nine wells did not incur casing deformation after implementation of the method.
Y.X. contributed to developing the mathematical model, performed the data analysis, and wrote the manuscript; J.L. (Jun Li) and G.L. supervised the research and edited the manuscript. J.L. (Jianping~Li) and J.J. analyzed the engineering data.
“Study on failure mechanisms and control methods of wellbore integrity of shale gas horizontal wells” (U1762211);“Optimum research of non-uniform cluster perforation along the long horizontal section in heterogeneous shale reservoirs” (51674272).
This research was financially supported by the Key Program of National Natural Science Foundation of China “Study on failure mechanisms and control methods of wellbore integrity of shale gas horizontal wells” (U1762211), the National Natural Science Funds “Optimum research of non-uniform cluster perforation along the long horizontal section in heterogeneous shale reservoirs” (51674272).
Conflicts of Interest
The authors declare no conflict of interest.
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