Abstract
During the thermal recovery of heavy oil when using cyclic steam injection technology, a microannulus tends to form at the cementing interface subjected to high temperature and pressure during steam injection, and large temperature and pressure differences after injection can lead to wellbore integrity failure. In this study, a thermomechanical coupled finite element casing‐cement‐formation model of a thermal recovery wellbore is established. The deformation of the wellbore during both the steam injection stage and the steam shutdown stage is analyzed. The microannulus formation mechanism at the cementing interface of the wellbore is studied. During steam injection, under the large thermomechanical coupling load, the wellbore generates a high stress that leads to elastic‐plastic deformation. In the steam shutdown stage, with the load on the wellbore decreasing, elastic deformation recovers mostly, while plastic deformation continues. If the plastic deformation is large enough, a microannulus will form at the cementing interface. Increasing the elastic moduli of the casing, cement, and the formation can enlarge their plastic deformation during steam injection. The increase of plastic deformation of the cement or formation can enlarge the microannulus of the casing‐cement interface or the cement‐formation interface correspondingly in the steam shutdown stage.
Highlights
Heavy oil geological reserves far exceed other conventional crude oil reserves
During the steam injection stage, with the wellbore temperature rising, thermal expansion deformation of the casing, the cement, and the formation occurs, and the formation exerts a certain constraint on the expansion deformation; thermal stress in the casing-cement-formation system and elastic-plastic deformation occur correspondingly. erefore, the temperature field of the wellbore is the basis of thermal stress and deformation analysis
With the increase of the distance from the center of the borehole, the temperature decreases from 250°C to 40°C of the inner borehole to 25°C of the far formation. e average temperature of the near-borehole area during the steam injection stage is much higher than that during the steam shutdown stage. erefore, this large temperature difference can lead to a large thermal stress in the near-borehole area during the steam injection stage
Summary
Heavy oil geological reserves far exceed other conventional crude oil reserves. Heavy oil has great potential for exploitation. Eir results show that the change of temperature and pressure in the wellbore will cause breakage of cement and produce a microannulus at the interface between the cement and the casing or between the cement and the formation, resulting in failure of wellbore integrity [11, 12]. Fan et al constructed a numerical model of a shale gas wellbore assemblage by using a step finite element method and analyzed the influence of in situ stress, formation elastic modulus, and cement properties on the microannulus of the cementing interface during the fracturing process [21]. An elastic-plastic model of the casing-cementformation system of thermal recovery well wellbores is established by using the finite element method. The cementing interface is calculated, and the influence of the elastic moduli of the casing, cement, and the formation on the microannulus size is analyzed
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