Logging while drilling (LWD) is a rapidly developing in-situ measurement technique due to its advantages of saving time and cost, less mud invasion during measurements, and the application in deviated or horizontal wells. Unlike the measurements in a vertical well, the LWD tools are inevitably off-centered in deviated or horizontal boreholes due to gravity. In this study, we investigate the wave propagation of unipole acoustic LWD with eccentric tools. The unipole transducer in a fluid-filled borehole can be characterized as a single point source deviated from the symmetric axis. The signals of unipole transmitters with arbitrary azimuths can approximately be equivalent to a linear combination of monopole, dipole, and quadrupole sources. According to analyses of the wave separation, it is concluded that the dipole components are dominant in the full wave arrays of unipole tools. The formulation of responses at the opposite azimuth with respect to a unipole transmitter reveals that the unipole data by this gathering mode would have less sensitivity to the tool positions. Numerical simulations by 3D finite difference further confirm this. For conventional multipole LWD including dipole and quadrupole logging, it is quite hard to evaluate the true shear velocity of the formation once the tool is highly off-centered in the borehole. For the unipole data gathered at the opposite azimuth to the source, however, the signal properties are very stable, with little dependence on either the tool eccentricity or the source orientation in horizontal wells. Both the fast and the slow shear velocities of an anisotropic formation can therefore be measured with good accuracy according to inversion tests in this study. Numerical examples further show it is feasible to evaluate the formation shear anisotropy by the unipole data during the tool rotating.


Unipole Sonic Logging While Drilling With Eccentric Tools and Anisotropy Evaluations

In-depth understandings of corrosion mechanisms are crucial to the safe and cost-effective operations of structures and equipment. Since the most straightforward and endangering consequence of corrosion is material loss, corrosion rate has been one of the most widely regarded parameters for evaluating corrosion phenomena. There exist several standard laboratory techniques for calculating corrosion rates, such as coupon weight loss measurement, linear polarization resistance measurement and electrochemical impedance spectroscopy. However, the conversions of the intermediate variables that are measured by these techniques (e.g. mass changes, corrosion currents, etc.) into corrosion rates necessitate a number of assumptions which would render the results prone to uncertainty.

In recent years, ultrasonic testing with permanently installed transducers has emerged as a promising approach for monitoring corrosion in the field. It is much more convenient to implement than the other techniques and is able to measure wall-thickness losses (WTLs) of components in a more determinate manner. In this work, the author has optimized UT to become a novel research tool for measuring corrosion rates in the lab. On the non-working surface of each corrosion specimen that is under investigation, a piezoelectric transducer is permanently bonded and continuously excites ultrasonic waves towards the working surface of the specimen and captures the reflections. The time-of-arrival’s of the reflections can then be converted into the WTL of the specimen via a simple mathematical relationship.

At first, a stability test was performed and an unprecedented measurement precision of 40 nm was demonstrated. Galvanostatic and open-circuit corrosion experiments then followed and it was shown that ultrasonic measurements of the WTLs of the specimens were in outstanding agreements with optical measurements and analytical predictions. While corrosion rates could be calculated from WTLs by linear regression, the presence of noise would manifest uncertainty in the rates that are calculated as such. Therefore, a statistical approach was introduced to quantify the confidence levels of linearly fitted corrosion rates. Through statistical analysis, it was found that corrosion rates of 0.1 – 0.2 mm/year could be detected within 1 – 2 hours. Such a response time renders the technique introduced a powerful research tool for carrying out corrosion related investigations.


A Novel Ultrasonic Technique for in situ Quantitative Evaluation of Corrosion Rates

No proceedings papers in 2020. Video only.

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