FIELD OF THE DISCLOSURE

This disclosure generally relates to borehole tools, and in particular to methods and apparatuses for conducting ultrasonic downhole measurements.

BACKGROUND OF THE DISCLOSURE

Drilling wells for various purposes is well-known. Such wells may be drilled for geothermal purposes, to produce hydrocarbons (e.g., oil and gas), to produce water, and so on. Well depth may range from a few thousand feet to 25,000 feet or more. In hydrocarbon wells, downhole tools often incorporate various sensors, instruments and control devices in order to carry out any number of downhole operations.

Typical acoustic logging tools may include, by way of example, a televiewer which comprises a rotating ultrasonic acoustic transducer that operates in a frequency range on the order of 100 kHz or more. Higher acoustic frequencies are preferred in order to achieve better resolution in the confined space of a borehole. In operation, the televiewer may rotate at a desired rate, such as 5 to 16 rotations per second, for example, to continuously scan the borehole sidewall as the televiewer is drawn up the borehole at a rate which may be around 3/16 inches to ⅜ inches per scan. A beam of acoustic pulses is launched along the normal to the borehole sidewall as the transducer scans the interior surface of the borehole. The pulse rate depends upon the desired spatial resolution such as 1500 pulses per second or 128 to 256 pulses per scan. The insonified borehole sidewall returns pulses reflected therefrom, back to the transducer on a time-multiplexed basis. The reflected acoustic signals are detected, amplified and displayed to provide a continuous picture of the texture and structure of the borehole sidewall. Other application include determination of the goodness of a cement bond to a steel casing as well as monitoring the integrity of the casing itself.

The diameter of a borehole logger may be on the order of 2⅞% in (7.3 cm), so that it can be run into relatively small boreholes. However, many borehole diameters are on the order of 10-14″ (25.4-35.6 cm) or more, so that the length of the acoustic-pulse trajectory from the transducer, through the borehole fluid to the borehole sidewall, may be up to 10″ (25.4 cm). In the normal course of events, the borehole fluid may be contaminated by drill cuttings, air bubbles and foreign matter which severely attenuate the acoustic energy by scattering because the physical dimensions of the contaminants are comparable to the wavelength of the wavefields emitted by the transducer.

SUMMARY OF THE DISCLOSURE

In aspects, the present disclosure is related to methods and apparatus for estimating downhole parameters relating to an earth formation intersected by a borehole, such as, for example, borehole geometry, bed boundary distance and orientation, fracture distance, orientation, and extent, dip, and so on.

Method aspects may include ultrasonic borehole logging using an ultrasonic borehole imaging tool in a borehole intersecting the earth formation. Methods may include adjusting a focus for an ultrasonic beam generated from a single-transducer ultrasonic assembly of the ultrasonic imaging tool; using a receiver to generate measurement information responsive to an ultrasonic signal caused by the ultrasonic beam; and estimating a parameter of interest from the measurement information. Methods may include adjusting the focus in dependence upon environmental conditions, the environmental conditions comprising at least one of: i) standoff between the ultrasonic imaging tool and a wall of the borehole; and ii) borehole annulus conditions. Methods may include adjusting the focus in substantially real-time. The ultrasonic beam may be focused with a focal zone at the borehole wall configured to produce a beam spot size of a selected diameter.

Adjusting the focus may include selecting a lens having at least one focal parameter corresponding to the environmental conditions from a set of interchangeable ultrasonic lenses, wherein each lens of the set has focal parameters unique within the set corresponding to particular environmental conditions; coupling the lens with a transducer of the ultrasonic imaging tool; and generating the ultrasonic beam using the lens. Methods may include, upon a change in environmental conditions: removing the lens; selecting a second lens from the set; coupling the second lens with the transducer; and generating the measurement information with the second lens. Methods may include, upon a change in a state of operation of the lens: removing the lens; selecting a replacement lens; coupling the replacement lens with the transducer; and generating the measurement information with the replacement lens.

Adjusting the focus may include identifying at least one focal parameter corresponding to the environmental conditions; adjusting a variable focus lens assembly comprising a lens to modify a focal parameter of a lens responsive to the identified at least one focal parameter; and generating the ultrasonic beam using the lens after the adjusting. Modifying the at least one focal parameter may include altering the curvature of the lens.

The lens assembly may include the lens and a transducer assembly. Modifying the at least one focal parameter may include changing an amount of at least one fluid between the lens and the transducer assembly to alter the curvature of the lens. Changing the amount of the at least one fluid may result in a lower pressure between the lens and the transducer assembly than an exterior of the lens assembly.

Embodiments include apparatus for ultrasonic borehole logging in a borehole intersecting the earth formation. Apparatus may include an ultrasonic borehole imaging tool comprising: a single-transducer ultrasonic assembly comprising an adjustable focus lens assembly; a receiver configured to generate measurement information responsive to an ultrasonic signal caused by the ultrasonic beam; and at least one processor configured to estimate a parameter of interest from the measurement information. The adjustable focus lens assembly may include a variable focus lens assembly comprising a lens to modify a focal parameter of the ultrasonic beam. The at least one processor may be configured to: identify at least one focal parameter corresponding to environmental conditions; and adjust the variable focus lens assembly to modify the focal parameter responsive to the identified at least one focal parameter.

Examples of some features of the disclosure may be summarized rather broadly herein in order that the detailed description thereof that follows may be better understood and in order that the contributions they represent to the art may be appreciated.

