中深层地热井开采对套管与水泥环性能影响的研究

Research on the influence of middle-deep geothermal well exploitation on the performance of casing and cement sheath

摘要:

地热井开采过程中井筒热流体造成的瞬时高温条件下，对套管的力学性能有较大的影响。本文基于套管-水泥环-地层岩石稳态热传导数学模型计算井筒温压场，建立中深层地热开采放热与套管相互作用力学评价的数学模型，并对地热开采放热对套损的影响进行有限元计算和试验模拟，得到以下结论：在地热开采井筒放热作用下，使套管产生了较大的椭圆度，削弱了套管的抗挤强度。当内压达到90 MPa、套管内温度为30 ℃时，在地层径向压力不等的情况下，内壁面应力分布极不均匀，应力最大差值为560 MPa，应力沿周向分层分布，受到地层径向压力大的一侧出现大范围的应力集中区域。随着温度的升高，套管的位移云图变化不大，最大位移值略有增加。当温度达到120 ℃后，套管处于较为恶劣的热力环境，热应力的变大导致套管塑性变形严重，同时内压增大到120 MPa时，套管内壁整体呈高应力状态，更易导致壁面出现塑性破坏。研究成果对研究地热开采放热对套损的影响规律，对评估套管的受力环境、水泥环强度、水泥环微间隙宽度等参数提供指导意义。

关键词:地热开采;套管;水泥环;塑性破坏;中深层地热井

Abstract:

In the process of geothermal well exploitation， the transient high temperature caused by wellbore thermal fluid has a great impact on the mechanical properties of casing. This paper calculates the wellbore temperature and pressure field based on the steady-state heat conduction mathematical model of casing cement sheath formation rock， establishes the mathematical model for the mechanical evaluation of the interaction between middle-deep layer geothermal exploitation heat release and casing， and carries out the finite element calculation and experimental simulation of the influence of geothermal exploitation heat release on casing damage. The following conclusions are obtained： under the action of geothermal exploitation wellbore heat release， the casing has a large ovality， which weakens the collapse strength of the casing. When the internal pressure reaches 90MPa and the temperature in the casing is 30℃， the stress distribution on the inner wall is extremely uneven under the condition of different formation radial pressures， with the maximum difference of 560MPa. The stress is distributed in layers along the circumference， and a large range of stress concentration areas appear on the side subject to the formation radial pressure. With the increase of temperature， the displacement contour of casing changes little， and the maximum displacement increases slightly. When the temperature reaches 120℃， the casing is in a relatively harsh thermal environment. The increase of thermal stress leads to serious plastic deformation of the casing. At the same time， when the internal pressure increases to 120MPa， the inner wall of the casing is in a high stress state as a whole， which is more likely to lead to plastic failure of the wall. The research results can provide guidance for the influence of heat release from geothermal exploitation on casing damage， and for the evaluation of casing stress environment， cement sheath strength， cement sheath micro gap width and other parameters.

Key words:geothermal exploitation;casing;cement sheath;plastic failure;middle-deep layers geothermal well