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We have developed a new 3D acoustic logging tool (3DAC). To examine the azimuthal resolution of 3DAC, we have evaluated a 3D finite-difference time-domain model to simulate a case in which the borehole penetrated a rock formation boundary when the tool worked at the azimuthal-transmitting-azimuthal-receiving mode. The results indicated that there were two types of P-waves with different slowness in waveforms: the P-wave of the harder rock (P1) and the P-wave of the softer rock (P2). The P1-wave can be observed in each azimuthal receiver, but the P2-wave appears only in the azimuthal receivers toward the softer rock. When these two types of rock are both fast formations, two types of S-waves also exist, and they have better azimuthal sensitivity compared with P-waves. The S-wave of the harder rock (S1) appears only in receivers toward the harder rock, and the S-wave of the softer rock (S2) appears only in receivers toward the softer rock. A model was simulated in which the boundary between shale and sand penetrated the borehole but not the borehole axis. The P-wave of shale and the S-wave of sand are azimuthally sensitive to the azimuth angle variation of two formations. In addition, waveforms obtained from 3DAC working at the monopole-transmitting-azimuthal-receiving mode indicate that the corresponding P-waves and S-waves are azimuthally sensitive, too. Finally, we have developed a field example of 3DAC to support our simulation results: The azimuthal variation of the P-wave slowness was observed and can thus be used to reflect the azimuthal heterogeneity of formations.
We have developed a new 3D acoustic logging tool (3DAC). To examine the azimuthal resolution of 3DAC, we have evaluated a 3D finite-difference time-domain model to simulate a case in which the borehole penetrated a rock formation boundary when the tool worked at the azimuthal-transmitting-azimuthal-receiving mode. The results indicated that there were two types of P-waves with different slowness in waveforms: the P-wave of the harder rock (P1) and the P-wave of the softer rock (P2). The P1-wave can be observed in each azimuthal receiver, but the P2-wave appears only in the azimuthal receivers toward the softer rock. When these two types of rock are both fast formations, two types of S-waves also exist, and they have better azimuthal sensitivity compared with P-waves. The S-wave of the harder rock (S1) appears only in receivers toward the harder rock, and the S-wave of the softer rock (S2) appears only in receivers toward the softer rock. A model was simulated in which the boundary between shale and sand penetrated the borehole but not the borehole axis. The P-wave of shale and the S-wave of sand are azimuthally sensitive to the azimuth angle variation of two formations. In addition, waveforms obtained from 3DAC working at the monopole-transmitting-azimuthal-receiving mode indicate that the corresponding P-waves and S-waves are azimuthally sensitive, too. Finally, we have developed a field example of 3DAC to support our simulation results: The azimuthal variation of the P-wave slowness was observed and can thus be used to reflect the azimuthal heterogeneity of formations.
Cement evaluation is commonly thought of as running a cement bond log (CBL) and attempting to interpret the results to determine if there is isolation in the wellbore. Oftentimes that interpretation is made in isolation with little or no information on what occurred during the drilling and cementing of the well, or the cement systems used. Evaluating cement in older wells where the drilling report states, "ran casing, cemented same" can be particularly challenging. Cement evaluation is much more than a CBL. Understanding the objectives of the cement job, the design limitations imposed by those objectives and the resulting slurry and job designs are all integral parts of cement evaluation. Oftentimes the selection of a specialty cement system to meet specific well requirements can dictate how the cement can be evaluated. To properly evaluate a cement sheath, knowledge of the cement job, slurry designs and the limitations of the evaluation technique must be understood. To attempt to perform a cement evaluation based solely on the log output from a CBL, or any log, invites considerable error and bias into the resulting interpretation. This paper reviews various methods of cement evaluation, from job data, casing and formation pressure testing through sonic and ultrasonic logging. The assumptions associated with each technique are outlined and the discussion includes the limitations of the various techniques along with cautions on how misinterpretation of the results can lead to assumptions of cement integrity that may not be appropriate. The impact of new boutique cement designs, which incorporate high concentrations of inert materials to give the set cement unique properties, is discussed. The ability of specific logging techniques to evaluate the presence of these slurries is presented. Data on selected boutique cement systems where conventional UCA strength data is not representative of the crush strength of the cement due to the incorporation of specialty materials is included. An overview of cement evaluation, and a risk based discussion of what technique may be most appropriate based on the cementing objectives is presented. Methods of reducing risk uncertainty in cement evaluation are discussed along with the "validity" of the various data sets available to the engineer to perform a proper cement evaluation on the well. Understanding the objectives of the cement job sets the boundary conditions for the designs, and from those designs the ability to evaluate the resulting cement placement and well isolation can be determined. Setting the evaluation methodology and understanding the type of information required to apply that methodology can improve the quality of the evaluation.
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.
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