International Journal of Earth Sciences and Engineering

Xianyong Zhang ¹, Aiguo Yao ¹

Affiliations
1 China University of Geosciences (Wuhan), Wuhan, Hubei, CN

Abstract

In high temperature geothermal well of drilling, when hot dry rock (HDR) meets the lower temperature drilling fluid, larger thermal stresses will be produced due to a large temperature gradient in borehole surrounding rock. Thermal stresses combined with non-uniform in-situ stress and wellbore pressure, which will endanger the stability of surrounding rock. A stress analytical model of borehole surrounding rock in HDR section was established, according to thermal and elastoplastic theory and finite element analysis of temperature distribution of surrounding rock. The stability factor is established to judge the stability of borehole surrounding rock, according to Mohr-Coulomb failure criterion. The stability of borehole surrounding rock in high temperature geothermal well of Yangbajing area in China is analyzed by solving the analytical model. When borehole with little internal pressure, the position of highest stability factor is the direction of the azimuth angle of 0° and 180°, and the position of lowest stability factor is the direction of the azimuth angle of 90° and 270°, while positions of highest stability factor and lowest stability factor are opposite when borehole with a very high internal pressure. The stability factor of the borehole increases first and then decreases with the increase of borehole internal pressure, and turning points in different direction are difference. With the increase of the distance from the axis of borehole, the stability factor of borehole surrounding rock is difference in different azimuth angle and different borehole inner pressure.

Keywords

High Temperature Geothermal Well, Borehole Rock, Thermal Stress, Analytical Model, Stability Factor.

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Xianyong Zhang ¹, Aiguo Yao ¹

Affiliations
1 China University of Geosciences (Wuhan), Wuhan, Hubei, CN

Abstract

In thermal recovery of heavy oil wells, stresses of cement sheath in the oil reservoir section are greater than that of conventional wells, which are due to the action of high temperature and pressure during steam injection. An analytical mechanical model is established, considering pressure and temperature loads coupling effects. The axial stress increase is more significant with the increase of temperature, compared with the radial stress and tangential stress increases, and the effect of temperature increase on the maximum shear stress of cement sheath becomes significant when temperature rise higher than 160°C. When temperature rise above 120°C, effects of temperature increase on maximum shear stress of cement sheath with different elastic modulus are differences, and reducing the elastic modulus of cement sheath is helpful to reduce the maximum shear stress in higher temperature rise. At the same time, when the temperature rise relatively small (below 160-180°C), the effect of steam injection pressure on maximum shear stress is obvious. The axial stress and the maximum shear stress of the cement sheath are increased after casing pre-tension stressed cementing, because the spring back effect of casing will lead to the production of additional axial stress of cement sheath.

Keywords

Cement Sheath, Temperature Changes, Analytical Mechanical Model, Stresses, Elastic Modulus.

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