The importance of cement evaluation has steadily increased as the oil and gas industry strives to ensure and verify zonal isolation. Evaluation must address a broad spectrum of downhole conditions that include various casing thicknesses, mud weights, and cement types. The most-challenging condition occurs with lightweight (low-density) cements, where the standard cement-bond systems have virtually the same response in free and cemented pipe. In these conditions, found in many new well completions, cement evaluation is extremely problematic. To address this industry challenge, a new sensor and system has been developed for use downhole.
The new tool incorporates the use of electromagnetic acoustic transducers (EMATs) to generate guided acoustic waves in the casing and to measure them as they propagate along the casing circumference. EMATs consist of an arrangement of coils and magnets in close proximity to a conductive casing. The casing then becomes a final part of the transducer system. The acoustic excitation is achieved by driving currents through the coils, which creates eddy currents in the casing. These eddy currents, in the presence of a constant magnetic field, create the Lorenz forces that generate the acoustic waves. The EMATs are then used to measure the induced waves. This system generates and measures the signals directly in the casing, eliminating any need for fluid coupling or physical contact and enabling operation in all fluid and gas environments. By varying the magnetic field and coil structure, different acoustic modes may be created and measured.
The most valuable of the guided modes are the horizontal shear or SH waves, which cannot be generated by conventional compressional transducer systems. These waves propagate along the casing, with their particle displacement perpendicular to the wave propagation and parallel to the casing surface. SH waves respond directly to the shear modulus of the material that is directly coupled on the backside of the casing, enabling direct detection of a solid adhered to the casing. The Lamb or flexural modes are other guided waves that can be generated by the EMATs. These modes can be incorporated with the SH modes to detect a microannulus condition without the need for multiple passes and pressure applied to the casing. EMAT sensors are incorporated into a pad system in a coplanar configuration, enabling azimuthally sectored compensated attenuation measurements for the various wave types.
In this paper, we look at the theoretical background, physics, and modeling of these new EMATs as well as the various guided-wave modes they can generate. We then review additional laboratory measurements and test-well data that demonstrate the capability of this new approach for cement evaluation. These results clearly validate the new system's ability to address cement evaluation across the broad spectrum of downhole environments.