深层地热井考虑水泥环损伤的套管预热应力设计

Research on Safety Evaluation Method of Casing in Geothermal Well

张智 冯潇霄

ZHANG Zhi FENG Xiaoxiao

油气藏地质及开发工程全国重点实验室·西南石油大学, 四川 成都 610500

National Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, China

深层地热资源相较于传统地热资源具有更大的开发潜力，但由于深层地热井井底温度更高，在开发开采中高温流体使套管受热膨胀，易引起套管屈服变形损坏。而目前对于深层地热井套管安全问题在理论研究和技术实践上均面临着诸多挑战。为此，根据厚壁圆筒理论和热弹性力学基本理论，利用对套管预先施加热应力降低套管的轴向热应力，考虑高温水泥石损伤后地层-水泥环-套管的接触问题，并以套管有效应力控制在相应温度下的最小屈服极限内为原则，根据套管柱强度设计安全系数，评价套管安全性能，建立深层地热井套管预热应力设计方法，缓解高温对套管热损伤的问题。结果表明，水泥浆的密度对损伤因子影响不大，水泥环的弹性模量对损伤因子起主导作用。在预热情况下，注水条件下(注水温度为65 ℃)在预热250 ℃以下能够满足设计条件，在地热生产条件下(地热温度为346 ℃)，需要超过200 ℃才能够满足安全设计要求。

Compared with traditional geothermal resources, deep geothermal resources have greater development potential. However, due to the higher bottomhole temperature of deep geothermal wells, the high temperature fluid in the development and exploitation causes the casing thermal expansion, which tends to cause casing yield deformation. At present, there are many challenges in theoretical research and technical practice for casing safety of deep geothermal wells. Therefore, according to the thick-walled cylinder theory and the basic theory of thermoelastic mechanics, the axial thermal stress of casing is reduced by applying thermal stress in advance, and the contact problem of formation-cement ring-casing after high temperature cement stone damage is considered. Based on the principle that the effective stress of casing is controlled within the minimum yield limit at the corresponding temperature, the safety factor of casing string is designed according to the strength of casing string, the safety performance of casing is evaluated, and the design method of casing preheating stress in deep geothermal wells is established to alleviate the problem of thermal damage of casing caused by high temperature. The analysis results show that the density of cement slurry has little effect on the damage factor, and the elastic modulus of cement ring plays a leading role in the damage factor. Under the water injection condition (water injection temperature at 65 ℃) preheat below 250 ℃ can meet the design conditions. Under the condition of geothermal production (geothermal temperature at 346 ℃), the safety design requirements is satisfied when preheat is above 200 ℃ to meet.

10.11885/j.issn.1674-5086.2021.09.29.02