Acoustic measurements above a plate carrying Lamb waves

📝 Abstract

This article presents a set of acoustic measurements conducted on the Statoil funded Behind Casing Logging Set-Up, designed by SINTEF Petroleum Research to resemble an oil well casing. A set of simple simulations using COMSOL Multiphysics were also conducted and the results compared with the measurements. The experiments consists of measuring the pressure wave radiated of a set of Lamb waves propagating in a 3 mm thick steel plate, using the so called pitch-catch method. The Lamb waves were excited by a broadband piezoelectric immersion transducer with center frequency of 1 MHz. Through measurements and analysis the group velocity of the fastest mode in the plate was found to be 3138.5 m/s. Measuring the wave radiated into the water in a grid consisting of 8x33 measuring points, the spreading of the plate wave normal to the direction of propagation was investigated. Comparing the point where the amplitude had decreased 50 % relative to the amplitude measured at the axis pointing straight forward from the transducer shows that the wave spread out 3.2 mm after propagating 140 mm in the plate.

💡 Analysis

This article presents a set of acoustic measurements conducted on the Statoil funded Behind Casing Logging Set-Up, designed by SINTEF Petroleum Research to resemble an oil well casing. A set of simple simulations using COMSOL Multiphysics were also conducted and the results compared with the measurements. The experiments consists of measuring the pressure wave radiated of a set of Lamb waves propagating in a 3 mm thick steel plate, using the so called pitch-catch method. The Lamb waves were excited by a broadband piezoelectric immersion transducer with center frequency of 1 MHz. Through measurements and analysis the group velocity of the fastest mode in the plate was found to be 3138.5 m/s. Measuring the wave radiated into the water in a grid consisting of 8x33 measuring points, the spreading of the plate wave normal to the direction of propagation was investigated. Comparing the point where the amplitude had decreased 50 % relative to the amplitude measured at the axis pointing straight forward from the transducer shows that the wave spread out 3.2 mm after propagating 140 mm in the plate.

📄 Content

Proceedings of the 39th Scandinavian Symposium on Physical Acoustics, Geilo, Norway, Jan. 31 – Feb. 3, 2016 Acoustic measurements above a plate carrying Lamb waves Andreas Sørbrøden Talberg 1, Tonni Franke Johansen 1,2 1 Norwegian University of Science and Technology, NTNU 2 SINTEF ICT Contact email: andreasstalberg@gmail.com Abstract This article presents a set of acoustic measurements conducted on the Statoil funded Behind Casing Logging Set-Up, designed by SINTEF Petroleum Research to resem- ble an oil well casing. A set of simple simulations using COMSOL Multiphysics were also conducted and the results compared with the measurements. The experiments consists of measuring the pressure wave radiated of a set of Lamb waves propagat- ing in a 3 mm thick steel plate, using the so called pitch-catch method. The Lamb waves were excited by a broadband piezoelectric immersion transducer with center frequency of 1 MHz. Through measurements and analysis the group velocity of the fastest mode in the plate was found to be 3138.5 m/s. Measuring the wave radiated into the water in a grid consisting of 8x33 measuring points, the spreading of the plate wave normal to the direction of propagation was investigated. Comparing the point where the amplitude had decreased 50 % relative to the amplitude measured at the axis pointing straight forward from the transducer shows that the wave spread out 3.2 mm after propagating 140 mm in the plate. 1 Introduction According to the Norwegian Petroleum Directorate there have been drilled more than 1500 exploration wells and more than 4200 development wells on the Norwegian Continental Shelf since the discovery of oil in 1966 [1]. Due to the severity of uncontrolled hydrocar- bon migration from oil or gas bearing reservoirs to the surroundings, both during produc- tion as well as when the well has been plugged for abandonment, it is crucial to cement the well properly. After a cement job has been completed, a proper inspection has to be conducted to evaluate the cement sheath. The operation of placing a permanent plug in a well that has reached the end of its lifetime on offshore exploration wells in Norway, can contribute with 25% of the total drilling costs [2]. Due to the extra cost for the responsible company and the environmental consequences of an improperly executed cement job, the field of cement job evaluation is of great interest. Well Cementing When placing a cement sheath, different criteria have to be fulfilled. The main objective of primary cementing, the stage where cement is placed in the annulus between the casing and the borehole wall, is to create zonal isolation. This is achieved by creating a hydraulic seal between both the cement and borehole wall, as well as between the casing and the ISBN 978-82-8123-016-3 1 Proceedings of the 39th Scandinavian Symposium on Physical Acoustics, Geilo, Norway, Jan. 31 – Feb. 3, 2016 Figure 1: Examples of cement slurry dis- placement problems in the annulus. Figure 2: Illustration of the particle dis- placement in Lamb wave modes. The ar- rows indicate the direction of the wave propagation. cement. There should also be no fluid channels inside the cement [3]. Figure 1 shows dif- ferent problems that may occur when a cement sheath is placed improperly in a wellbore. Causes of a poorly conducted cement job can be divided into two categories: flow prob- lems of mechanical origin like poor centralization and washouts, and degradation of the cement during the curing stage where the cement may be polluted by fluid or gas [4]. The production capacity of a well may never reach its full potential without complete isolation in the wellbore. To control if the cement sheath has been placed properly, a cement job evaluation has to be conducted. Cement Job Evaluation: The Pitch-Catch Technique Since the 1960s when Zemanek and Caldwell first applied the ultrasonic pulse-echo tech- nique to well logging with their tool named the Borehole Televiewer (1969), the ultrasonic tools have been an important tool in the field of cement job evaluation. The ultrasonic tools usually operate in the frequency range of 200 to 700 kHz, much higher than the acous- tic tools. The frequencies are chosen to coincide with the ressonance frequency of the casing [5]. The so called pitch-catch (P-C) method used e.g. by Schlumberger in their Isolation Scanner (2005) [6, 7] is the technique used in this paper. The concept of the pitch-catch method involves the Lamb wave and the conditions for the following derivations are that the plate, in which the Lamb waves are excited, is an infinitely unbounded plate lying in the xy-plane. For sufficiently high frequencies, 80 kHz or more, the pulse from the ultrasonic transducer interacts with a small area of the casing, making the approximation of treating the part of the casing as a part of an infinitely unbounded plate appropriate [8]. A plate supports two infinite sets of Lamb wave modes whose velocities in the plate depend on

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