The effect of stress on MFL signals has been investigated for pipeline applications, specifically for cases of uniaxial tension and compression, biaxial loading, and dents. The MFL response to stress is often complex and a function of residual and applied stresses, magnetizing field strength and direction, defect geometry and material magnetic properties.
One of the challenges with MFL logging is associated with achieving accurate measurements of the extent and penetration of defects, even without high casing stress. Methods to evaluate casing integrity and maximum allowable operating pressure rely on accurate descriptions of defect geometry. The presence of high casing stress adds additional uncertainty to the MFL interpretation.
The objective of this study is to better understand the effects of stress on MFL measurements in casing, as well as to subsequently characterize of corrosion defects based on those measurements. Large-scale testing will be carried out in the lab to investigate the effects of stress on MFL tool response under conditions representative of downhole storage applications, specifically with casing metallurgy, post-yield loads and casing logging tools. The test plan includes axial tensile loading of a casing joint test specimen to examine the magnitude of the stress effects on the MFL response and the effects on the characterization of real and manufactured corrosion defects. Depending on the results, there may be an opportunity to begin to develop correction factors for stress effects.
The research began in October, 2014, and it is expected to conclude by the 3rd quarter of 2015. PRCI is collaborating with MFL tool vendors to ensure full utilization of the test results.