高温高产气井自由套管热应力研究

Thermal stress model for the free-casing section to be used for high-temperature and high-yield gas wells

高温高产天然气开采所诱导的环空流体热膨胀及套管所受的热应力对井筒安全带来严峻挑战,套管爆裂及井口装置密封失效事件时有发生,因此,研究井口热应力及套管头升高十分必要。综合考虑井筒温度场、各层套管水泥返深、套管强度与力学稳定性等因素,建立了未封固井段自由套管轴向载荷及变形的计算方法,研究了高温高产气井热应力对套管及井口装置安全的影响。最后,以我国南海某气田为例,进行了井口热应力与套管头升高值的计算。结果表明,自由套管段的温度变化与各层套管水泥返深是影响井口热应力和套管头升高值的主要因素,井口热应力随表层套管和导管的水泥返深增加而增大,优化各层套管的水泥返深可以减小井口热应力;同时,随日产量增大,井筒温度升高,井口热应力和套管头升高值增加。

The paper is mainly concerned about the thermal stress caused by the temperature change in the case of the high-temperature and high-yield gas wells. As is known,thermal expansion of the annulus fluid and thermal stress of case pipes can likely impose serious challenges in wellbore safety due to the high stress caused by the high temperature and high yield they have to experience. Since it is often the case for the explosive burst and wellhead seal failure,it is of vital importance to study the wellhead thermal stress and the casing interface connection for the actual in-situ state. In hoping to reduce the impact of the thermal stress on the sealing of the case and the safety in production,the present paper intends to provide a calculation model of the thermal stress for the whole system of drilling intensity and the production steadiness. To meet the needs necessary for such a system production,we have managed to design a platform in structure and the pipeline to be used to ensure the production safety for the offshore oil and gas well system. In doing so,first of all,we have taken into careful account the free casing strength and the stability for drilling and well completion,by choosing the method for calculating the axial casing load and the likely casing head expansion and contraction of the casing surface,with the technology to be used in their production. And,secondly,we have proposed formulas for calculating the thermal stress and casing head expansion likely to be caused by the gas transferring process while considering the wellbore temperature and different casing string tops of the cement（ TOC） depths,etc. And,thirdly,in combination with the overall installation process of the casing head from drilling and operation of the gas transportation,the paper has made a detailed analysis of the effects of thermal stress on the production safety in the high-temperature and wells＇ high-yield. And,finally,a case study has been given to demonstrate the casing head expansion,in which we have made an exploration and analysis of all the necessary details involving the actual gas transportation in accordance with the thermal growth model. Thus,what we have done can be adopted to provide necessary references for the system＇s field application. And,in turn,the results of our research indicate that it can be used to optimize the TOC depths,the axial load reduction in the casing system to be caused by the high temperature elaboration.