Downhole acoustic transducer, downhole probe and tool comprising such a transducer

A downhole ultrasonic probe comprising a single downhole ultrasonic transducer used to transmit and/or receive ultrasonic waves in a hydrocarbon well where a fluid is present, the downhole ultrasonic transducer comprising:

• a metal housing defining an internal cavity isolated from the fluid of the hydrocarbon well by a membrane wall made of metal or metal alloy;
• a piezoelectric element mounted inside the internal cavity, the piezoelectric element having a front side mechanically coupled on the membrane wall;
• wherein:
• the internal cavity is at a pressure unrelated to a downhole hydrocarbon well pressure;
• a back side of the piezoelectric element is arranged to be free to oscillate in the internal cavity so as to generate a high acoustic impedance mismatch between the piezoelectric element and the internal cavity at the back side and to maximize acoustic transmission at the front side;
• a thickness of the membrane wall is such that there is a common resonance between the membrane wall and the piezoelectric element thereby achieving high acoustic transmission through the membrane wall, and such that the membrane wall is suitable to resist to the downhole hydrocarbon well pressure;
• and wherein said downhole ultrasonic transducer is positioned at a tip of an elongated and flexible protective tube.

A downhole ultrasonic probe (50) comprising multiple downhole ultrasonic transducers, each downhole ultrasonic transducer being used to transmit and/or receive ultrasonic waves in a hydrocarbon well where a fluid is present, each downhole ultrasonic transducer comprising:

• a metal housing defining an internal cavity isolated from the fluid of the hydrocarbon well by a membrane wall made of metal or metal alloy;
• a piezoelectric element mounted inside the internal cavity, the piezoelectric element having a front side mechanically coupled on the membrane wall;
• wherein:
• the internal cavity is at a pressure unrelated to a downhole hydrocarbon well pressure;
• a back side of the piezoelectric element is arranged to be free to oscillate in the internal cavity so as to generate a high acoustic impedance mismatch between the piezoelectric element and the internal cavity at the back side and to maximize acoustic transmission at the front side;
• a thickness of the membrane wall is such that there is a common resonance between the membrane wall and the piezoelectric element thereby achieving high acoustic transmission through the membrane wall, and such that the membrane wall is suitable to resist to the downhole hydrocarbon well pressure
• and wherein said downhole ultrasonic transducers are angularly distributed as an array around a cylindrical housing, each downhole ultrasonic transducer defining a measuring angular sector around the probe, each downhole ultrasonic transducer being connected to a tool control circuit, the tool housing constituting a ground for the tool control circuit.