Mechanical characteristics of well cement under cyclic loading and its influence on the integrity of shale gas wellbores

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This paper focuses on the effect of repeated loading and unloading conditions on cement sheaths during staged fracturing. Triaxial compression and cyclic loading/unloading tests were designed for cement at room temperature and elevated temperatures of 95 °C and 130 °C. Based on the mechanical characteristics of cyclic loading and unloading, in the staged fracturing simulation process, numerical simulation of the cement sheath in both shallow and deep formations was performed. The following observations were made. (1) The elastic modulus of the cement was high at room temperature but lower under 95 °C and 130 °C. However, the deformation performance was greatly improved at higher temperatures, showing significant mechanical properties of an elastoplastic material. (2) In the cyclic loading test, the residual strain of the cement increased as the number of cycles at room temperature increased, and the unloading secant modulus was larger than that of the initial loading segment. The cement cumulative residual strain was larger under a higher temperature, and there was a significant “rebound hysteresis” in the unloading curve; the unloading secant modulus value also increased. The numerical simulation of staged fracturing produced the following conclusions. (1) The cement sheath in the shallow formation undergoing fracturing was at risk of circumferential tensile failure, and the severity of tensile failure increased with an increasing number of fracturing stages. (2) The cement sheath in the deep formation was at risk of plastic failure under compression, and the cement sheath gradually entered the plastic yield state as the number of fracturing stages increased.