Abstract
The importance of well integrity barriers evaluation in Middle East has increased dramatically in the recent years. The focus has increased to verify zonal isolation down-hole under extreme conditions. Evaluation must address a broad spectrum of downhole conditions that include various casings thicknesses, different mud weights; various types and wide variety of cement systems. The shallow aquifers corrosion problem in Middle East demands placement of a competent light or ultralight cement sheath across these aquifers having lost circulation zones. This is prime objective in order to have first line of defense against long line ecltrochmemical corrosion problem seen in all historical fields. During cementing of such surface casings; it was observed fluid column in the hole dynamically dropping or very fresh mud close to surface contained trapped air. Conventional logging techniques were not providing evaluation because acquisition was not possible under such hostile environments.
In more than 80% of cases conventional cement bond log found to have sensitivity to micro-annulus; and required pressurizing the casing. Therefore it was increased need to evaluate cement sheath behind micro-annulus. Zonal verification was also much needed in case of low acoustic impedance cement systems having contamination with mud or tail mixing with neat. Such light or ultralight cement has essentially same response as free pipe has conventional logs; therefore best evaluation was required to avoid unnecessary costly squeeze jobs or side track decisions. Moreover; a major drawback prevailed in obtaining confident answers in high doglegs and horizontal well section due to tool centering issues with conventional rotating heads. In view of that industry recognizes the need of wireline evaluation that will not require any expert input to obtain onsite deliverables in a reasonable turn out time while the rig is waiting on for decision. As a consequence, there is an increasing need for barriers evaluation with a single wireline technology suitable and effective to log in all such terrains.
This paper discusses the application of a new tool in addressing the above mentioned challenges. The new tool incorporates the use of electromagnetic acoustic transducers (EMAT) in Integrity eXplorer (INTeX) tool to generate guided acoustic waves in the casing and to measure them as they propagate along the casing circumference. The INTeX tool consists of an arrangement of coils and magnets in close proximity to a conductive casing. The casing then becomes an integral 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/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 micro-annulus 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 field applications of INTeX tool.
