A new LWD ultrasonic imager for use in both water- and oil-based muds uses acoustic impedance contrast and ultrasonic amplitude measurements to obtain high-resolution structural, stratigraphic and borehole geometry information. Following extensive testing in the Middle East and the US, this paper presents results from the first European deployment of the new 4.75-in. high-resolution ultrasonic imaging tool.
An ultrasonic transducer, which operates at high frequency, scans the borehole at a high sampling rate to provide detailed measurements of amplitude and traveltime. A borehole caliper measurement is made, based on the time of arrival of the first reflection from the borehole wall. A second measurement detects formation features and tectonic stress indicators from the change in signal amplitude. The amplitude of the reflected wave is a function of the acoustic impedance of the medium. Resulting impedance maps have sufficient resolution to detect sinusoidal, non-sinusoidal and discontinuous features on the borehole wall.
Breakouts, drilling-induced fractures, and tensile zones were used for stress direction determination. Breakout identification was obtained both from amplitude images and oriented potato plot cross sections derived from traveltime measurements.
The orientation of natural fractures is parallel at the maximum stress direction, indicated by drilling-induced fractures and tensile zones. The World Stress Map confirms the maximum stress direction determination.
It was also possible to detect certain key-seat zones and investigate borehole conditions to prevent issues during the subsequent casing job.
The new LWD ultrasonic imaging technique represents an important alternative to density and water-based mud resistivity imaging, which has several limitations. Unlike the resistive imaging LWD tool that is very sensitive to standoff, the higher tolerance of the ultrasonic imaging tool enables the amplitude and traveltime ultrasonic images to contain fewer unwanted artifacts.
Li, Jing (PathFinder---Schlumberger) | Boonen, Paul Mathieu (Boonen-Petro, LLC) | Dawber, Mike (PathFinde Energy Services) | Lee, Rick (Pathfinder - A Schlumberger Company) | Kennedy, Dave (Northeastern University) | Hollmann, Joseph
With the rapid growth of horizontal drilling, azimuthal LWD bulk-density images (and also photo-electric effect images) have proven to be indispensable tools for the identification of bed boundaries, estimating bed dips, and determining the steering direction of the drilling assembly. The inherent low resolution of the density measurement has, however, typically limited the interpretation of these images to the analysis of structural scale features. The ability to image finer scale detail is governed to a large extent by sampling density.
Sampling density depends upon the sampling frequency of the instrument, the rate of penetration of the drilling assembly and the rotary speed of the drillstring. Conventional downhole image processing schemes average raw measurements either in time or depth before sectoring them into relatively large circumferential sectors. This averaging process conserves tool memory and smooths noisy data but inevitably results in the loss of high spatial frequency instrument responses associated with fine scale geological features. Instead of compressing data, we present a method that preserves all the raw measurements stored in the tool memory and maps them to a grid around and along the borehole. Statistical noise is mitigated and a specially designed interpolation scheme is used to fill empty grid locations before analyzing the data and smoothing the results. This methodology allows the creation of high resolution images with up to 256 circumferential sectors and a depth increment as small as 0.6 inches.
The technique is equally applicable to borehole image data acquired by any type of logging tool providing that the raw measurements are frequently sampled and stored in the tool memory. For example, high resolution borehole
caliper images have also been created from ultrasonic transducer measurements of the same tool used to acquire the data presented in this paper. Azimuthal bulk-density data acquired in this manner allows for the opportunity to produce optimized images in oil-based mud and overcomes the limitations of LWD micro-resistivity imaging tools in non-conductive borehole fluids. This technique has been successfully applied to wells drilled in both the UK North Sea and US land. Field examples presented in this paper illustrate the benefits of the application in imaging thin beds, sedimentary bedding structures, coal seam bedding internal structure, faults and fractures.