Casing and tubing strings are generally considered to be exposed to static or quasistatic loads. Usually, this assumption is correct; however, in some cases (e.g., huff-and-puff heavy oil production, geothermal wells, deepwater high-pressure/high-temperature wells), changes in temperature or internal pressure can lead to variable loads and thus fatigue failure. This paper presents an integrated solution for fatigue life estimation and fatigue failure analysis during the tubular design of thermal wells.

The varying casing temperatures and temperature-dependent casing loads were obtained through numerical simulations of cyclic operations such as steam injection, soaking, and production. All these simulations were accomplished using commercial software, including a thermal flow simulator and a stress analyzer. The previously simulated casing loads were then used to calculate localized stress amplitude, strain amplitude, and mean stress. Finally the localized strain and stress values were used as input parameters of a strain-based fatigue model to estimate the lifetime (cycles) of the peak-load casing section. This fatigue model was implemented in a computer program and integrated with the aforementioned software.

A field case of a steam injection well was studied with the integrated thermal fatigue simulation. The casing fatigue life was predicted using four different fatigue life models. The results were compared with each other and with actual field data. The statistical fatigue life model exhibits best performance. The casing failure locations were analyzed and explained in terms of multistring stress analysis including casing-cement-soil interactions. It seems that the fatigue failure at the previous casing shoe or perforation is related to the axial load peaks at these places during the thermal cycles.

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