The use of acoustic (e.g., ultrasonic) measurement systems in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications is well known. In known systems an acoustic sensor, typically with a substantially homogenous piezo-ceramic structure on board, operates in a pulse-echo mode in which it is utilized to both send and receive a pressure pulse in the drilling fluid (also referred to herein as drilling mud). In use, an electrical drive voltage (e.g., a square wave pulse) is applied to the transducer, which vibrates the surface thereof and launches a pressure pulse into the drilling fluid. A portion of the ultrasonic energy is typically reflected at the drilling fluid/borehole wall interface back to the transducer, which induces an electrical response therein. Various characteristics of the borehole, such as borehole diameter and measurement eccentricity and drilling fluid properties, may be inferred utilizing such ultrasonic measurements. 

 

While prior art acoustic sensors have been used in various downhole applications (as described in the previously cited U.S. Patents), their use, particularly in logging while drilling (LWD) and measurement while drilling (MWD) applications, tends to be limited by various factors. As used in the art, there is not always a clear distinction between the terms LWD and MWD, however, MWD typically refers to measurements taken for the purpose of drilling the well (e.g., navigation) whereas LWD typically refers to measurement taken for the purpose of analysis of the formation and surrounding borehole conditions. Nevertheless, these terms are hereafter used synonymously and interchangeably.

 

 

The present invention addresses one or more of the above-described drawbacks of prior art acoustic sensors used in downhole applications. Referring briefly to the accompanying figures, aspects of this invention include a downhole tool including at least one acoustic sensor having a piezo-composite transducer. The piezo-composite transducer may be configured, for example, to withstand demanding downhole environmental conditions. Various exemplary embodiments of the acoustic sensor further include a matching layer assembly for substantially matching the acoustic impedance of the piezo-composite transducer with that of the drilling fluid and for providing mechanical protection for the transducer and/or a backing layer for substantially attenuating ultrasonic energy reflected back into the acoustic sensor. Exemplary embodiments of the downhole tool of this invention include three acoustic sensors disposed substantially equidistantly around the periphery of the tool.

 

Exemplary embodiments of the present invention advantageously provide several technical advantages. Various embodiments of the acoustic sensor of this invention may withstand the extreme temperatures, pressures, and mechanical shocks frequent in downhole environments. Tools embodying this invention may thus display improved reliability as a result of the improved robustness to the downhole environment. Exemplary embodiments of this invention may further advantageously improve the signal to noise ratio of downhole acoustic measurements and thereby improve the sensitivity and utility of such measurements.

 

In one aspect the present invention includes a downhole measurement tool. The downhole measurement tool includes a substantially cylindrical tool body having a cylindrical axis. The tool further includes at least one acoustic sensor deployed on the tool body, the acoustic sensor including a piezo-composite transducer element with anterior and posterior faces. The piezo-composite transducer is in electrical communication with an electronic control module via conductive electrodes disposed on each of the faces. The piezo-composite transducer element includes regions of piezoelectric material deployed in a matrix of a substantially non piezoelectric material, the regions extending through a thickness of the transducer element in at least one dimension. In exemplary variations of this aspect, the acoustic sensor includes a laminate having a composite backing layer, at least one matching layer, and a barrier layer deployed at an outermost surface of the acoustic sensor.

 

In another aspect, this invention includes an acoustic sensor having a piezo-composite transducer element. Further aspects of this invention include a method for fabricating a downhole measurement tool and a method for fabricating an acoustic sensor.

 

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should be also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

 

Exemplary backing layers may also include substantially any suitable powder material, such as tungsten powers, tantalum powders, and/or various ceramic powders. In one useful embodiment, tungsten powders having a bimodal particle size distribution may be utilized. For example, one exemplary backing layer includes a mixture of C-8 and C-60 tungsten powders. The particle size of C8 is in the range from about 2 to about 4 microns while the particle size of C60 is in the range from about 10 to about 18 microns.