Introducing a New Logging–While–Drilling Ultrasonic Borehole Imaging Technology Using Oil–Based Mud in Mature Field, Offshore Abu Dhabi
Authors: Goraya Yassar , Alfelasi Ali Saee , Khemissa Hocine , Al Dhafari Bader Mohamed , Ashraf Muhammad , Khaled Islam , Al-Mutwali Omar , Okuzawa Takeru , Aki Ahmet , Montasser Ahmed , Fares Wael , ElGammal Ahmed 
Published: November 2020

Acquisition of high-resolution images for reservoir characterization from logging-while-drilling (LWD) technologies has historically been limited to water-based mud (WBM) applications. The introduction of LWD ultrasonic technologies means high-resolution images and the associated analysis are now available in both WBM and oil-based mud (OBM) applications.

This paper details the first deployments of a 4¾-in. LWD ultrasonic imaging service in a mature field, offshore Abu Dhabi, and the assessment of images in both WBM and OBM wells. The 4¾-in. ultrasonic tool combines both borehole size and shape measurements with high-resolution radius and reflection amplitude images. The ultrasonic sensor uses four transducers that operate in a pulse-echo mode. By firing simultaneously, the transducers provide a total of 2,000 travel time and reflection amplitude measurements each second, enabling the creation of high-resolution images, even at high-logging speeds. The methodology described was used to evaluate the suitability of the LWD ultrasonic measurements to enhance reservoir understanding, along with LWD azimuthal formation density and azimuthal high-resolution resistivity image measurements in WBM applications.

The 4¾-in. ultrasonic borehole imaging technology was deployed while drilling with OBM/WBM to acquire ultrasonic images to capture the subtle geological features that often control the reservoir properties, but whose characterization was challenging previously due to technology limitations. The long lateral was logged with ultra-sonic imaging while drilling through Cretaceous carbonates, traversing through different layers going up and down in stratigraphy; showcasing subtle variations with complementing images that helped understand the vug distribution, bioturbation, faults, and dissolution seams, in addition to the bedding boundaries.

High-resolution borehole shape analysis was performed to understand the impact of stresses on the well trajectory, made possible with the high-definition, multisector images. The resolution of the reflection amplitude images, in particular, enables identification of drilling-induced features on the surface of the borehole, highlighting the measurement's value in understanding the impact of the bottom hole assembly (BHA) on the quality of the wellbore. The travel time measurements provide detailed evaluation of the borehole size and shape, with the three-dimensional (3D) visualization of the wellbore illustrating the ability of the service to identify borehole enlargement and breakout. These findings demonstrate the suitability of the service to address wellbore stability issues in real time.

This paper details the first use of the ultrasonic service in Abu Dhabi. Comparison of images from the new ultrasonic imaging service with established LWD technologies highlights the suitability of the radius and reflection amplitude images to provide enhanced formation evaluation analysis in both WBM and OBM applications. Log examples show the high-resolution images are able to identify bedding features and fractures and provide assessment of borehole size and shape for wellbore stability evaluation.