Cement Sheath Integrity: Simulation and Measures for Improvement at High Temperature Deep Well Conditions
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摘要: 研究水泥环最重要的一环就是要建立评价水泥环完整性的实验装置和方法。通过应力等效方式将实际井筒水泥环承受的工况载荷等效到模拟井筒水泥环上，以通气验窜方式检验水泥环失效情况，利用CT扫描技术探测水泥环内部结构，揭示其失效机理，建立一套水泥环完整性的实验装置及评价方法。对西部某油田深井高温高应力环境下水泥环受载进行模拟实验，当地层压力一定时，套管内压力从12 MPa升至57 MPa后再下降过程中，未见气泡；保持套管内压不变，降低地层压力，当压力降低到一定值时，检测有气泡冒出，观察发现水泥环上、下端面及侧表面未见明显裂纹，但与水泥环胶结紧密的套管能自由脱出，揭示了水泥环失效机理为水泥石韧性不足导致套管水泥环胶结面失效，产生了微环隙。根据CT扫描分析可得，水泥环试样界面孔隙体积减少67.97%，同时水泥环界面出现的环隙体积约为3 061 mm3，占界面总体积的9.51%，水泥环界面体积减少2.21%，证实出现了微环隙。从材料改性角度入手提高水泥石韧性，最终优选出加量为0.2%碳纤维水泥浆配方。该方法可为油田水泥环的评价及水泥环材料改性研究提供有力的支撑手段。Abstract: One of the most important purposes of cement sheath study is to establish experimental apparatus and procedure for evaluating the integrity of cement sheath. The working load of cement sheath in a wellbore can be simulated on an artificial cement sheath using equivalent stress method. With the help of core holder, downhole temperature and stress environment can be simulated. By pressurizing the artificial sheath with gas to see if there is gas channeling, the cement sheath integrity can be verified. CT scanning can be used to probe the internal structure of cement sheath, hence to bring to light the failure mechanism of cement sheath. Based on these ideas, a whole set of equipment was developed and a procedure established to evaluate the integrity of cement sheath. Simulation experiments have been carried out using the equipment and the procedure on cement sheaths found in high temperature high stress deep wells in an oilfield in west China. The results of the experiments showed that, at a certain formation pressure in high stress environment, when casing pressure was first increased to a certain level and then decreased, the cement sheath was damaged and even became failed. Once the channels for gas channeling were formed, no matter how low the pressure was inside the casing, gas bubbles can be detected. It was observed that no visible fractures were found on both ends and the side surface of the cement sheath, but the casing string that was tightly bonded with the cement sheath can now fall out freely, indicating that it was the lack of toughness of set cement that resulted in the failure of the bonding between the casing and the cement sheath, producing a micro annular space for the casing to fall. Data acquired with CT software showed that the interface pore volume of the cement sheath sample was reduced by 67.97%, the volume of the micro annular space was 3061 mm3, 9.51% of the total sheath volume, and the total volume of cement sheath was reduced by 2.21%, proving the existence of micro annular space. By modifying the material added to cement slurry, the toughness of the set cement can be improved, and a cement slurry with 0.2% carbon fiber was formulated. The method described in this paper can be used to evaluate the quality of cement sheath, and as a helpful supporting means in modifying cement slurry additives.
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Key words:
- Well cementing /
- Cement sheath /
- Integrity /
- Evaluation apparatus /
- Material modification
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