SU1698864A1 - Acoustic well logging device using lamb waves - Google Patents

Abstract

The invention relates to geophysical studies of wells and can be used in logging through tubing to assess the permeability and other properties of formations exposed by drilling. The aim of the invention is to expand the field of application of the instrument by carrying out logging through the tubing. The purpose of the invention is achieved by making through holes in the case of the downhole tool at the installation site of a converter with a diameter of 0.7-0.8 of the case diameter. At the same time, the area of each piezoelectric element is not less than the cross section of the through hole, and the thickness of the plate placed in the center of the body perpendicular to the axis of the hole is not more than 0.3 of the thickness of the piezoelectric element. 1 hp f-ly, 3 ill. (L

Description

The invention relates to well logging and can be used in logging through tubing to evaluate permeability and other properties of formations exposed by drilling.

A downhole acoustic logging tool is known, which allows acoustic surveys of cased oil and gas wells with a diameter of 115 mm and more through tubing with an internal diameter of 50 mm. Borehole diameter

device 36 mm. A downhole tool consists of a housing in which radiating and receiving transducers are installed. The broadband radiating transducer operates in the 5-25 kHz frequency range, which allows studies to be performed on refracted (head) waves. The disadvantage of this device is the impossibility of conducting logging on low-frequency Lamb waves, for the occurrence of which it is necessary that the lengths of radiated waves D exceed the diameter of the well, ft / dc 1.

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For the considered device, the maximum wavelength is 60 mm, i.e. $ / dcЈ 0.5. The condition for the origin of the Lamb wave is not fulfilled, which reduces the information content of the acoustic studies, since the dynamic parameters of the Lamb waves characterize the permeability and crackability of the near-well zone.

A well-known downhole tool for acoustic logging on Lamb waves is known, consisting of housings containing a generator and an amplifier. The receiving and two radiating converters are located in the cases. The logging capability on Lamb waves is achieved due to the fact that in 1 spectrum of radiating j broadband transducers, frequencies of kHz are present. Elastic waves with low frequencies have wavelengths of 0.5-0.2 m, which, as a rule, are larger than the diameter of the wells under study. The diameter of the AKN-1 logging tool is 100 mm and is determined mainly by the overall dimensions of the radiating transducers. The disadvantage of this device is the impossibility of its use when conducting logging through the tubing, which also reduces the information content of acoustic well surveys.

The aim of the invention is to expand the area of change of the instrument by conducting logging through the tubing.

Figure 1 shows one of the diagrams of the downhole tool; figure 2 - cross section of the downhole tool in the installation location of the transducer; Fig. 3 is a view of wave patterns registered by the device in a well with a diameter of 200 mm.

The downhole device consists of a housing 1, a set of transducers 2, a generator 3 and an amplifier 4. At the installation site of the converter in the housing 1 of the device, a through hole 5, with a diameter dOTe c ° solid, is made. In the center of the body perpendicular to the axis of the hole there is a rigidly fixed elastic element 6 in the form of a plate, with thickness Lp and diameter

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lrno In order to increase the information content in terms of radiating transducers, acoustically the same is installed. , stiff plate 8, sealing hole.

The device works as follows

Using a switching device (not shown), part of the transducers 2 is connected to generator 3 and operates in the radiation mode.

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On both sides of the elastic element 6 are placed flat piezoelements 7, of thickness hn9 and diameter dnj. The piezoelectric elements are connected to the generator 3 in parallel and differently 55

The other part is connected to the amplifier k and operates in the mode of receiving elastic waves. Upon receipt of a rectangular pulse with a duration of на on the piezoelement 7, one of the piezoelectric elements is compressed, the other expands. There is a force directed perpendicular to the surface of the piezoelement and acting on the elastic element 6 and the volume of fluid in the hole, the parameters of which determine the minimum frequency of forced fluid oscillations in the well. The frequency of the generated elastic waves depends on the minimum frequency Ј, and by choosing Ј it is set such that the condition ft / dc 1 is satisfied, which allows acoustic logging on Lamb waves. An example of an oscillation wave packet of a 9 Lamb registered by one of the transducers is shown in FIG. 3. In order to increase the informativity of studies in the case of extended working layers, reduce the radiation frequency by increasing t. In certain types of cuts, it is necessary to know the shear wave attenuation in order to increase the efficiency of determining the permeability of the formations from the dynamic parameters of the Lamb waves. In this case, in order to increase the informativity of acoustic research in terms of radiating transducers, acoustically rigid plates are installed, hermetically closing the holes on one of the sides. In this case, a one-sided directed pressure on the borehole wall occurs, which creates the possibility of selective excitation and reception of shear waves 10 (Fig. 3).

The principle of the downhole tool design is that the Lamb wave shaping is not affected by the directional characteristic of the radiating element, and it is important that

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The other part is connected to the amplifier k and operates in the mode of receiving elastic waves. Upon receipt of a rectangular pulse with a duration of на on the piezoelement 7, one of the piezoelectric elements is compressed, the other expands. There is a force directed perpendicular to the surface of the piezoelement and acting on the elastic element 6 and the volume of fluid in the hole, the parameters of which determine the minimum frequency of forced fluid oscillations in the well. The frequency of the generated elastic waves depends on the minimum frequency Ј, and by choosing Ј it is set such that the condition ft / dc 1 is satisfied, which allows acoustic logging on Lamb waves. An example of an oscillation wave packet of a 9 Lamb registered by one of the transducers is shown in FIG. 3. In order to increase the informativity of studies in the case of extended working layers, reduce the radiation frequency by increasing t. In certain types of cuts, it is necessary to know the shear wave attenuation in order to increase the efficiency of determining the permeability of the formations from the dynamic parameters of the Lamb waves. In this case, in order to increase the informativity of acoustic research in terms of radiating transducers, acoustically rigid plates are installed, hermetically closing the holes on one of the sides. In this case, a one-sided directed pressure on the borehole wall occurs, which creates the possibility of selective excitation and reception of shear waves 10 (Fig. 3).

The principle of the downhole tool design is that the formation of Lamb waves is not affected by the directional characteristic of the radiating element, and the possibility of creating low-frequency controlled oscillations of the fluid in the well, which by virtue of inertia and viscosity of the latter excitations acquire a flat front and turn into a pressure wave. Such oscillations are created by an elastic element in the form of a plate with flat piezoelements exciting it, operating in the mode of displacements and radiating directly into the well. The frequency of radiation is largely dependent on the size of the added mass of the medium surrounding the elastic element. At dOTe / dnp 0.7-0.8, the value of the added mass is equal to the mass of the liquid, the particle displacement velocity in which is equal to the displacement velocity on the front wall of the piezoelectric element.

Thus, if the diameter of the device is defined by two standards: 3 and 42 mm, then the diameters of the holes are chosen, respectively, in the intervals: 25.2-28.8 mm and 29, -33.6 mm.

When an elastic element is made of a material whose acoustic and density characteristics are close to those of the piez elements, the structure of the elastic element and piezo elements is acoustically homogeneous. As the material of the elastic element using copper or its alloys. Piezoelectric ceramics of the PZT-23 type, etc., are used as piezoelements. With a thickness of an elastic element of not more than 0.3 kPe, the highest radiation amplitudes are provided, while maintaining the integrity of the piezoelectric elements and the design of the mobile system as a whole. The diameter of the piezoelectric elements must be close to or equal to the diameter of the hole, in addition to lowering the frequency of the moving system, to provide a flat front of the radiated wave and to weaken the excitation of the head waves due to the expense

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waves from the edges of the piezoelectric elements.

Claims (2)

The device provides acoustic logging at Lamb waves through tubing, suppressing head waves propagating in 9 boreholes using the traditional method of exciting acoustic waves used in standard acoustic logging devices, as a result, the identification of the Lamb wave and tracking of its lines is simplified. 5 base correlations in various lithotypes of rocks. The device also makes it possible to additionally excite shear waves in the rocks studied. Invention Formula 1, A downhole acoustic logging tool for Lamb waves, comprising a housing with low-frequency radiating and receiving transducers, an excitation unit and an amplifier, characterized in that, in order to expand the field of application of the instrument by conducting logging through tubing (NDT) ), at the place of installation — each converter case has a radial through-hole with a diameter of 0.7–0.8 of the case diameter and is equipped with an elastic element in the form of a plate mounted in the center of the case perpendicular to the axis of In this case, piezoelements are placed on both sides of the plate, the surface area of each of which is equal to at least the cross section of the through hole, the plate thickness is less than 0.3 the thickness of the piezoelectric element, and the piezoelectric elements are connected in opposite directions. 2. The device according to claim 1, of tl and h. Is y and so that, in order to adjust the low-frequency component of the acoustic impulse, the excitation unit is designed as a generator of rectangular impulses. five 0 0 five Wj № $ 01 O l Ndi U 2 gpf  7 q v gpf