Towards Robust Multi-Casing Evaluation Using Pulsed Eddy Current Sensors

Electromagnetic Pulsed Eddy Current (PEC) sensing, for multi-casing corrosion evaluation in Oil and Gas industry, can be categorized as either collocated or non-collocated methods. Our recent 3D finite-element modeling- and advanced processing-based studies [7-8] showed that although collocated sensors, having localized sensitivities, give an intuitive time-to-depth (qualitative) corrosion display, they lack the sensitivity to fully resolve casings behind the second pipe in addition to suffering badly from eccentering and changing pipe properties. Inversion of multiple spacings and multi-frequency non-collocated measurements in centralized settings [2] gives good results for up to four centered pipes but the quality of results is severely compromised by casing and tool eccentering (limiting maximum utilizable frequency for interpretation), and by ghosting effect (double indication of same heterogeneity – limiting reliability near casing collars). In this work, the impact of approaching anomalies and eccentering, as typically encountered while logging a multi-casing oil and gas well, will be analyzed in detail through 3D modeling studies and advanced processing schemes introduced to address these issues.