Well integrity evaluation using acoustic and electromagnetic measurements

Abstract

The evaluation of downhole well integrity is an important business for the oil and gas industry. It is driven primarily by the need to optimize production while maintaining a safe environment despite the inexorable corrosion of casing strings and other harsh downhole conditions. Furthermore, with aging infrastructure, evaluating well integrity to plan plug and abandonment or slot recovery projects is critically important. Many technologies have been developed to address the challenges of evaluating casing and cement integrity under varied downhole conditions. These technologies are necessarily growing increasingly more sophisticated to meet the requirements of evaluating multiple cemented casing strings. In particular, wireline cased-hole logging techniques based on electromagnetic (EM) and acoustic techniques have been applied to casing corrosion and cement integrity evaluation. The acoustic measurement techniques deployed for well integrity evaluation typically include the sonic frequency range of tens of kilohertz up to the ultrasonic frequency range of megahertz. The frequency ranges are appropriate for characteristic resolutions of decimeters down to millimeters for sonic and ultrasonic measurements, respectively. These techniques are sensitive to the elastic properties of materials in the well completion and crucial for assessing zonal isolation. In the domain of evaluating corrosion in one or multiple casing strings, the traditional measurement approach uses EM techniques, which are sensitive to the metal thickness, the electrical conductivity (the inverse of electrical resistivity), and the magnetic permeability of ferromagnetic pipes. The spectrum of common EM casing inspection techniques encompasses a frequency range between quasi-static (direct current) up to about 100 kHz. More recently, transient EM or pulsed-field eddy current tools have been introduced. Specific physics and challenges associated with well integrity measurements must be understood and considered in existing and emerging technology solutions.

Uniform Circular-Array-Based Borehole Pulsed Eddy-Current System for Asymmetry Defect Inspection in Downhole Casings

“…Notably, the size of the borehole PEC system is restricted by the physical limitations of the downhole conditions and will have influence on the inspection performance. Given a ramp signal with a transmitting current of 0.5 A and a turn-off time of 30 µs, the secondary magnetic field can be obtained after turning off the excitation, with which the nondestructive testing of downhole casings can be realized [1]. Taking MFDs on the cylindrical surface with r = 15 mm as examples, the depth-azimuth images of the normalized MFD with azimuth from −180 • to 180 • at sampling times of 10 ms (early time) and 20 ms (late time) are shown in Figures 2a and 2b, respectively, where the horizontal axis denotes the circumferential azimuths, and the vertical axis represents the borehole (longitudinal) depth of the PEC model.…”

“…Recently, the inspection of wellbore casings has been extensively investigated with an increasing concern for safety in oil and gas production [1][2][3][4]. There are several techniques for detecting and quantifying various defects in wellbore casings.…”