Technology Trends in Evaluating Cement Jobs Using Logging Tools
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Cement-job evaluation is a process that identifies whether the objectives of a cement-job operation have been achieved after cement placement. This paper reviews the technology trends in cement-job evaluation using logging tools and considers the main advantages and concerns associated with each technology. The technologies covered are acoustic tools, temperature logging, noise logging, resistivity logs, oxygen-activation logs, X-ray measurements, gamma/gamma density measurements, neutron/neutron logging, and fiber-optic measurements.
After a cementing operation has been performed and the cement has set, cement evaluation is conducted either through hydraulic testing or by using various well-logging methods. Verification of casing-cement quality by logging tools has been a contentious subject for the past few decades. On one hand, some experts claim that current technologies are not sufficient to show the sealing capacity and quality of casing cement. On the other hand, others claim that data processing and misinterpretation of the obtained data from cement-job evaluations are the crucial challenges, not the logging technologies. However, another critical element must be considered as a limiting factor—downhole condition. The downhole condition, which includes well parameters and well design, has a major influence on technology development and data interpretation. The well parameters include downhole temperature, downhole pressure, wellbore-fluid properties, casing size and thickness, cement thickness and type, fast formation, slow formation, and unconsolidated formation. Fig. 1 shows the most-likely well design and the worst-case scenarios that need to be considered when developing technology with an assumption that there is no formation washout.
Fig. 1—Typical well design and casing size and thickness to be considered during technology development. ID=inside diameter.
As shown in Fig. 1, cement sheaths may be relatively thin but they are very long, especially in the case of surface and intermediate casings. This means that extensive vertical defects along the borehole axis would be necessary to create a leakage pathway. Cement defects that can cause leakage include low top of cement (TOC), mud channels, chimneys, and microannuli.
Acoustic and ultrasonic logging tools are the standard tools used for indirect cement-quality measurement and for finding TOC. However, limitations exist concerning the measurement and interpretation accuracy that can be achieved using standard cement-bond logs. In addition, logs do not provide continuous, real-time, long-term monitoring of cement-barrier quality. Therefore, different technologies have been proposed that might have the potential to address the limitations of acoustic and ultrasonic logging tools.
The use of sonic logs for cement evaluation can be traced to the 1950s when, during formation evaluation by sonic logs, the occurrence of skipped cycles was noticed (transmitted energy is submitted to large attenuation) on 3-ft receivers when sonic logs were run in cased hole. Since then, this technique has been improved through the development of cement-bond-log and variable-density-log tools and ultrasonic pulse-echo and flexural-measurement techniques.
Logging-while-drilling sonic sensors, which are used for compressional- and shear-data acquisition, are used to find TOC through the casing or liners while running a drilling or cleanout assembly.
Ultrasonic pulse-echo techniques were introduced to the industry for cement evaluation in the early 1980s. These cement-mapping tools operate at much higher frequencies than acoustic tools. The principle of the ultrasonic technique is to cause a small area of the casing to resonate across its thickness. In pulse-echo techniques, a transducer, acting as both a transmitter and a receiver, sends out a short pulse of ultrasound and picks up the echo-containing resonance. The rate of decay of the resonance will be lower if there is fluid behind the casing, whereas cement will dampen the resonance faster.
A recent development targets the use of electromagnetic acoustic transducers (EMATs) for the generation of guided acoustic waves in the casing. For this technology, a Lorentz force is used to generate and measure acoustic waves directly. EMATs operate with a coil, a magnet, and a conductive casing and can function as both transmitter and receiver. EMATs generate two fundamental wave modes—shear horizontal and lamb flexural. In the shear-horizontal-wave mode, the particle motion is perpendicular to the direction of wave propagation; in the lamb-flexural mode, the particle motion is normal to the casing surface. The study of these wave modes is a direct measurement of the shear modulus of the solid material behind casing with a higher resolution compared with conventional acoustic techniques while eliminating sensitivity to the wellbore fluid or the need for physical contact of the transducers with the casing.
Temperature logs are used to detect temperature anomalies behind casing caused by cement hydration or leakage of fluids. Cement hydration occurs over a period of 6–12 hours after initial mixing of cement and is an exothermic chemical reaction that generates considerable heat. It is the temperature rise inside the well because of the heat conducted by the casing from the cement that is readily detected by temperature logs. Temperature logs recorded at a suitable point in time can be used to detect the TOC; however, complete verification of the seal quality of a primary-cementing operation is challenging. Detecting cement hydration must be performed before the heat created from hydration has dissipated. Therefore, this technique is not suitable for inspecting the casing cement later in the well life.
Fluid flow through a leakage pathway generates noise with two measurable parameters—intensity and frequency. Noise intensity, also known as acoustic intensity, is defined as the energy carried by the sound wave per unit area. In the context of leakage, noise intensity depends on fluid-flow rate and the differential pressure driving it, while noise frequency depends on the geometry of the leakage pathway. As a rule of thumb, when fluid flows with ease through a large area, a low-frequency noise is generated, whereas fluid flowing with difficulty through a narrow space generates a high-frequency noise.
The idea behind evaluating formation resistivity through casing is to conduct a low-frequency alternating current from one electrode to another positioned farther along the casing. Under this configuration, a very small portion of the current leaks from the casing to the adjacent rock, and the casing current can be studied. The difference in the vertical current at Electrode 1 and at Electrode 2 is the leakage current that left to the formation. From the leakage current and the potential of the casing, the apparent resistivity of the formation can be calculated.
In this technique, oxygen is irradiated with high-energy neutrons from a neutron generator and, consequently, the oxygen forms an unstable isotope of nitrogen with a 7.13-second half-life. When the nitrogen decays back to oxygen, a gamma ray is generated. These generated gamma rays are detected, recorded, and analyzed. The near and far detectors record the generated gamma radiation from the decay of the activated nitrogen in water. The requirement for a calibration measurement in a zero-flow zone of the well is one of the challenges associated with this technique, as is the steady-state nature of the measurement.
In order to evaluate casing cement, the X-rays need to penetrate the casing. Therefore, X-ray or high-energy X(γ)-ray generation is required for the ray to penetrate the casing, investigate the cement, and be backscattered back through the casing to a detector. Detection of backscattered photons depends on the source energy and flux, the distance to the voids or defects, downhole temperature, and the density of intervening material. X-ray interaction with the intervening material could be in the form of coherent scattering, incoherent scattering, fluorescence, or the Auger effect.
Gamma/Gamma Density Measurements
Gamma/gamma density measurement is based on absorption of gamma rays as a function of density. This technique is often referred to as the gamma ray density. In this technique, gamma rays are emitted by a source and the backscattered gamma rays are sensed by a detector. The intensity of the backscattered gamma rays is related to the density of the intervening material.
In this technique, neutrons are generated by use of a particle accelerator. A high-voltage current passes through a material called the “filament,” which is a source of deuterium. As the current passes through it, the filament will heat up and, consequently, deuterium atoms are released. The released deuterium atoms are bombarded by electrons generated by a high-voltage current passing through a cathode. The current heats up the cathode, and electrons are then released. By creating an electric field, the released electrons are directed toward the deuterium atoms. Collisions cause the deuterium atoms to lose electrons, resulting in positive-charge atoms. At this time, a high-voltage current passes through a material that is called a “target,” which is a source of tritium. When current passes through it, the target will heat up and release tritium atoms. By creating a strong electric field, collisions between tritium atoms and the positive-charge deuterium atoms result, producing helium atoms and neutrons.
Neutrons are able to travel hundreds or even thousands of meters through air. They can easily penetrate iron and steel; however, they are stopped by hydrogen-rich materials such as water. When an individual neutron collides with the nuclei of atoms, it loses energy. When the neutron reaches an intermediate energy level, it is called an epithermal neutron. As epithermal neutrons collide with more nuclei, they lose additional energy and come to a general equilibrium with the surrounding nuclei. At this level, they are called thermal neutrons. Thermal neutrons are easily captured by nearby nuclei, causing gamma rays to be released. The number of neutrons captured and gamma rays released is proportional to the amount of hydrogen in the medium.
Pulses of light generated by a laser are sent through an optical fiber and are reflected repeatedly from the fiber walls. The fiber and its coating form a wave guide with total internal reflection such that light is not lost through the fiber walls. A sensor or combination of sensors can be placed along the fiber and record measurements of pressure, temperature, seismic, mechanical stresses, chemicals, flow, and other properties.
Technology Trends in Evaluating Cement Jobs Using Logging Tools
01 May 2018
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