Casing Stress Analysis Of Heavy Oil Thermal Production Well

Nowadays thin oil has become more and more scarce, and thus heavy oil recovery has been attached more importance . China has abundant heavy oil resources, but a mass of casing failures emerging in the heavy oil production have seriously restricted its expoitation.This thesis is to investigate the shaft stress distribution of thermal production wells, and to analyze the application effects of various cementing plan and the causes of casing failures. This is significant to prevent casing failures and design casings for thermal production wells.The well bore physical model was established on the basis of stimulating casing string load downhole. During the injection, the well bore was beared by ground stress, pressure of the steam and thermal stress due to the thermal expansion. In order to obtain the stress distribution of the innner well-bore, the load conditions of thermal prodution shafts are firstly calculated for solutions in the thesis, including using Orkiszewski method to discriminate steam flow patterns in the shaft, developing heat transfer model to calculate shaft temperature distribution, applying Huang Rongzun Model to calculate crustal stress. Based on the accurate calculation of loads, the current most commonly used prestress methods in thermal production well have been numerically simulated by finite element software ANSYS including conventional cementing, prestressed casing and thermal stress compensator method. The simulation results indicates that the conventionally designed casing will yield to different degrees below the packer in the conventional cementing; prestressed casing and thermal stress compensator method can keep the casing in a elastic state, but the safety margin is very small; the prestress method with casing protection measures deteriorates the state between cement sheath and crustal stress to different degrees. When investigating the measures to prevent casing failures, the author found that the thermal stress calculation in which only the effect of uniaxial thermal stress to casing is considered and the casing is under triaxial stress state is obviously not precise enough. In this thesis, a biaxial prestressed casing method considering bidirectional stress ellipse has been established, and the application results has been verified by numerical simulation. The simulation results show that the casing designed by biaxial prestressed method has a greater safety coefficient. Nevertheless, it still has the same weakness as the other prestressed casing method. The biaxial prestressed method also deteriorate the state both cement sheath and crustal stress, and the strength of cement should be inceased in order to guarantee the casing work normally.