PSI - Issue 22

Yihua Dou et al. / Procedia Structural Integrity 22 (2019) 33–42 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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the risks mentioned above, the maximum allowable pumping pressure and the maximum production pressure difference are analyzed according to the remaining strength of perforated casing. 5.1. Determine the maximum allowable pumping pressure with the remaining strength of perforated casing  102 guns, 16 hole/m, 90 ° phase angle perforation is adopted in this well. According to the casing diameter, wall thickness, steel grade, perforating parameters, the coefficient of remaining strength after perforating is 0.793 for 51/2" × 9.17mm P110 casing considering the weakening of the perforation, perforated hole edge stress concentration and fracture cracking tendency. That means after perforating, casing burst strength decreased from 87.1 MPa to 69 MPa (0.793 × 87.1 MPa).Considering the fact of perforated hole edge crack (with the additional stress coefficient of 1.3) and the viscous friction resistance of fracturing fluid flow, combined with table 6, the maximum allowable pumping pressure determined by the remaining strength of perforated casing is shown in Fig.7. The maximum pumping pressure should be in the green area in Fig. 7.

Fig. 7 the maximum allowable pumping pressure determined by remaining strength of perforated casing

5.2. Analysis of the maximum allowable production pressure difference

As mentioned above, due to the weakening effect of perforated holes, the casing strength decreases after perforating. In the later stage of drainage, if the pressure difference is too large to exceed the remaining collapse strength, the perforated casing will be collapsed. Therefore, the maximum allowable production pressure difference is determined by the collapse strength of the perforated casing. Due to the remaining strength coefficient of 0.793 for the perforated casing of this well, the collapse strength of casing decreased from 76.4 MPa to 60 MPa (0.793× 76.4 MPa) after perforating. Considering the impact of perforating cracking, then cement ring outside casing and the safety factor of collapse strength as 1.5, the maximum production pressure difference is recommended to be 40 MPa. 6. Conclusion (1) Perforating casing samples were obtained by using common perforating gun and projectile combination in the simulated well according to the actual perforating plan. The stress concentration coefficient around the actual perforated hole was measured through physical experiment and verified by finite element method, with the error less than 8.7%. The stress concentration factor around the actual perforated hole is about 1.30. (2) The formula for calculating the stress intensity factor and its revised factor of the casing hole is obtained by regularizing the perforated hole into a semi-elliptical crack on the surface. Then the safety of the casing in the perforating section is judged by the double criterion of stress intensity and stress intensity factor. For the thin-wall casing, the formula of remaining collapse strength of perforated casing is obtained by applying the theory of rod pipe similarity and considering the stress concentration. (3) The research in this paper could help to guide the perforation design, subsequent well testing and stimulating, control the parameters during well services, and ensure the safety and wellbore integrity of the perforating casing. Acknowledgements This research was funded by the Chinese National Natural Science Foundation (grant number 51374171).