四川盆地超深高含硫气田井下管柱完整性设计理念与方法

Downhole string integrity design for ultra-deep high-sulfur gas fields in the Sichuan Basin:Concept and methodology

四川盆地天然气“气大庆”建设过程中气井面临高温高压、H_(2)S/CO_(2)强腐蚀环境、管柱强度衰减等诸多技术挑战,管柱腐蚀穿孔、开裂等井下管柱完整性失效事件时有发生。为此,针对现有管柱腐蚀速率测试方法在井下精细流动模拟以及腐蚀环境设计上的不足,使用自研高温高压多相流动态循环腐蚀实验装置,综合考虑多种腐蚀介质下的酸性环境设计方法,还原了井下管柱的服役环境,对比了不同生产阶段腐蚀速率;并基于DCB实验,提出了高温高压强腐蚀环境下管柱断裂韧性的测试方法及管柱强度校核模型;最后,以四川盆地X井为例,得到了管材剩余服役年限与优化建议。研究结果表明:①对腐蚀环境与井下流动的精细化模拟,还原了不同服役阶段的腐蚀速率与特征,保证了腐蚀速率测定的准确性;②气井生产后期积水阶段管材平均腐蚀速率远大于生产初期携水阶段,且积水阶段点蚀现象极为严重,易造成管柱腐蚀穿孔甚至开裂;③C110管材的断裂韧性衰减严重,平均衰减比例达41.37%,必须针对应力腐蚀开裂现象开展管材DCB实验及强度校核;④在管柱服役寿命预测时,需综合考虑均匀腐蚀、点蚀及磨损对管柱强度的衰减作用,校核结果应满足安全服役的要求。结论认为,四川盆地超深高含硫气田是盆地天然气稳产、上产的主要对象,保障其安全高效开发对于四川盆地天然气“气大庆”建设具有重要现实意义,提出的井下管柱完整性设计理念及方法可为超深高含硫气田井下管柱完整性设计提供参考和借鉴。

In the construction process of"gas Daqing"in the Sichuan Basin,gas wells faces a series of technical challenges,such as high temperature and high pressure(HTHP),severe H_(2)S/CO_(2) corrosion environment,and string strength degradation,which lead to such accidents as string integrity failure(e.g.severe corrosion failure and cracking)from time to time.In view that the existing string corrosion rate measurement method is deficient in fine downhole flow simulation and corrosion environment design,this paper restores the service environment of downhole string and compares the corrosion rates in different production stages by using the independently developed cyclic corrosion experiment device of HTHP multi-phase flow state and comprehensively considering the design method of acidic environment in multiple corrosion media.Then,based on DCB experiments,a method for testing string fracture toughness in the environment with high temperature,high pressure and strong corrosion,and a string strength calibration model are proposed.Finally,the remaining pipe service life and the optimization suggestions are clarified with Well X in the Sichuan Basin as an example.And the following research results are obtained.First,the fine simulation of corrosion environment and downhole flow restores the corrosion rates and characteristics in different service stages and ensures the measurement accuracy of corrosion rate.Second,the average pipe corrosion rate in the water loading stage in the late period of gas well production is much higher than that in the water carrying stage in the initial period of production.In addition,the phenomenon of pitting corrosion is extremely serious in the water loading stage,which can easily cause string corrosion failure and even cracking.Third,the fracture toughness of C110 pipe degrades greatly with an average degradation proportion up to 41.37%,so it is necessary to carry out pipe DCB tests and strength calibration on stress corrosion and cracking.Fourth,when predicting the string service life,it is necessary to consider the degradation effect of uniform corrosion,pitting corrosion and abrasion on string strength comprehensively,and the calibration results shall meet the requirement of safe service.In conclusion,the high-sulfur gas fields in the Sichuan Basin are the main objects for stable natural gas production and natural gas production increase here,so their safe and efficient development is of important and practical significance to the construction of"Gas Daqing"in the Sichuan Basin.The proposed downhole string integrity design concept and method provide reference for the downhole string integrity design of high-sulfur gas wells.