| Since casing damage in Zhong No.18 well area which belongs to Fengcheng Oilfield launched in 2010 had occurred to 87 wells within 3?4 years. The major damage form is rupture failure, which affected the production and the late construction of oilfield seriously. Therefore, this paper aimed at geology, drilling and completion and production parameters conditions of Zhong No.18 well area to carry out the statistic analysis as well as the main factors of casing damage. On the basis of heat-solid coupling theory, this paper carried out the change of the temperature field and stress field in the perforated casing-cement ring-formation system in the process of steam injection, revealed the rupture failure mechanism of casing in low rounds steam injection, and analyzed the influence of perforation parameters, steel grade and steam injection parameters on the strength of casing, put forward the measures to prevent casing damage. The paper's main work is as follows:(1) Studied on the related information about casing damage of thermal recovery wells at home and abroad, collected 87 wells'casing damage information of Zhong No.18 well area in Fengcheng oilfield by analyzing the type of casing damage, casing damage location, casing specifications, steam injection time, steam injection parameters, the cementing quality and sanding situation, etc. Concluded the main factors that lead to casing damage are the high steam injection temperature and stress, the low steel grade and the severe sand production;(2) Combined with the main cause of casing failure under typical working condition of Zhong No.18 well area, established a heat-solid coupling finite element model of the perforated casing-cement ring-formation, and studied the temperature field, casing stress field and casing strength of the system. By comparing and analysising the change of stress distribution and strain values of the perforation and the casing on single round production conditions, conclusions are made that the stress near the perforation is greater than casing, because there is stress concentration phenomenon near the perforation. The plastic deformation inside the hole is more than 6%, and more than 500 MPa compressive stress change into 250 MPa tensile stress in the process of steam injection stage and soak stage. Cracking may occur inside the perforation in the low rounds steam injection, thus the mechanism of casing fracture failure in low rounds steam injection is founded;(3) Combined the thermal-solid coupling model with the typical product condition of Zhong No.18 well area, established perforating casing finite element model of thermal recovery well, analyzed the influence of perforating parameters, grade of steel casing, casing wall thickness and steam injection on the casing strength of thermal recovery wells, found out that the phase angle 120° do the minimal impact, which followed by 60°, because the hole density 16 hole/m and 20 hole/m do the minimal impact; Casing wall thickness is of little influence on the perforated casing of thermal recovery wells; The stress concentration phenomenon on perforation of P110 casing is superior to the performance on the TP90H and N80 casing, and P110 casing has the smallest plastic deformation in the steam injection stage and soak stage, but the stress change between adjacent hole of P110 casing should be focused on, for its possibility of being larger;(4) Put forward prevention measures on the basis of the casing damage characteristics of Zhong No.18 well area. Suggested adopting phase angle 120 °and hole density 20 holes/m, and improving casing steel level to 110 ksi to reduce casing low rounds cracking failure; improving the cementing quality, adopting advanced well completion technology of sand control to maintain the effective supporting surround the casing; using the prestressed well cementation and selecting thermal stress compensation tool to offset the thermal stress on casing. |
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