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

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