Experimental Study on the Integrity of Cement Sheath Under Cyclic Loading
In the multistage fracturing process of shale gas wells,the information such as cement sheath tem-perature and strain used for evaluating the integrity of cement sheath is difficult to be directly obtained from the wellbore.Therefore,combined with the fiber Bragg grating sensing technology and full-scale physical simulation test device,tests were conducted to obtain the temperature and strain changes of cement sheath during the WOC(waiting on cement)and completion stages,and the influence of cyclic loads of pressure and temperature on the integrity of the cement sheath was analyzed.The study results show that during the WOC stage,the peak tempera-ture of the cement sheath is 1.9℃higher than the curing temperature,and the circumferential strain of the cement sheath is tensile strain,with a maximum value of 403.7×10-6 m/m;during the completion stage,under cyclic loading of 40 MPa and 50 MPa internal pressures,the cement sheath undergoes continuously accumulated plastic strain;when the accumulated plastic strain reaches the critical value of 87.1×10-6 m/m,the interfacial micro-an-nulus leads to annular gas leakage;under cyclic loading of temperature,the cement sheath undergoes circumferen-tial compressive strain,which increases the risk for occurrence of micro annulus.The study conclusions provide theoretical reference for field construction of multistage fracturing.
integrity of cement sheathcyclic loadfiber Bragg gratingmicro annuluscycle indexmul-tistage fracturing
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