Investigation of electromagnetic method for identifying casing damage in thermo-mechanical coupling field

Oil casings in deep wells are subjected to high-temperature fluids and complex geological stresses over extended periods, leading to potential damage. This study investigates the feasibility of employing an electromagnetic method to detect early-stage stress concentrations of oil casings under high-temperature conditions. Finite element simulations using a three-point bending method are conducted to analyse the distribution and concentration of stress under varying temperatures and loads. An electromagnetic apparatus featuring a four-legged probe is developed for experimental validation under high-temperature conditions. The effects of temperature, stress, and excitation frequency on the magnetic flux response of the oil casing are examined, revealing a correlation between magnetic flux signals and stress states. Experimental results demonstrate that frequency division detection effectively identifies stress concentration areas. The magnetic flux exhibits a three-stage nonlinear evolution characterised by rapid-response (dφ/dT < −5 Wb/℃), steady-transition (−5 Wb/℃ < dφ/dT < −0.5 Wb/℃), and horizontal-saturation (dφ/dT > −0.5 Wb/℃). This study confirms that the electromagnetic detection in the thermal coupling field can accurately reflect the stress characteristics of casings in high-temperature wells, providing a theoretical basis for the early non-destructive detection of deep casings.