GB2533378B

GB2533378B - Plug integrity evaluation method

Info

Publication number: GB2533378B
Application number: GB1422633.6A
Authority: GB
Inventors: Merciu Ioan-Alexandru
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2014-12-18
Filing date: 2014-12-18
Publication date: 2019-09-11
Anticipated expiration: 2034-12-18
Also published as: GB2533378A; WO2016099288A1

Description

Plug integrity evaluation method

Field of the invention

The invention relates to the field of wellbore evaluation methods and devices, and more specifically, but not limited to, plug integrity evaluation methods and devices.

Background

Hydrocarbon producing wellbores generally include a casing provided within the wellbore, wherein the casing is bonded to the wellbore by adding cement within the annulus formed between the outer diameter of the casing and the inner diameter of the formation. The material of the casing may be metal. A section of the wellbore may be closed by placing a cement plug across the inner diameter of the casing. The plug may also be made of other materials. The integrity of the cement plug needs to be evaluated after placing the plug, in particular whether the plug adheres solidly to the casing around the circumference of the casing. Established methods of testing the integrity of the plug include applying a large force or a mechanical impact onto the plug and estimating whether the plug can withstand the large force or impact. Problems with this method are that the test may be destructive when the impact causes fractures in the plug and that a platform may be required for supporting and controlling the testing tools.

Statement of invention

Aspects of the invention are set out in the claims. Preferred features are set out in the dependent claims.

Description of Figures

Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1a and 1b are illustrations of a wellbore with a plug;

Figure 2 is a flow diagram of a method;

Figure 3 is an illustration of a numerical model of a wellbore;

Figure 4 illustrates a recorded signal of an acoustic wave propagating through the model of Fig. 3;

Figure 5 illustrates a wave propagating through the casing; and

Figure 6 illustrates a set of transducers used for inducing a wave in the casing.

Specific description

Figure 1 a illustrates a wellbore with a plug in a first configuration of a casing and Figure 1b illustrates a wellbore with a plug in a second configuration of a casing. Figures 1a and 1b illustrate a casing 1 which has a cement layer 2 provided on the outside of the casing and further a cement plug 3 provided on the inside of the casing. The casing is a metal tubular. Figure 1b illustrates that there are also other sections of casing, such as casing 4, which has a cement layer provided both on the inside and on the outside and the evaluation methods described herein can be used for bother casing 1 and casing 4.

With reference to Figure 2, the inventors have appreciated that the integrity of a plug can be evaluated by inducing a first acoustic wave (S1) travelling through casing 1 in a first direction along the longitudinal axis of the casing and then detect a second acoustic wave (S2) travelling through the casing in a second direction along the longitudinal axis of the casing, wherein the second direction is substantially opposite to the first direction and wherein the second acoustic wave is an at least partial reflection of the first acoustic wave at said the location of the casing of plug 3. The inventors have realised that the bonding between the casing and the inner or outer cement layer causes a local variation in the impedance of the casing and that the reflected wave carries information about the bonding between the plug and the casing. Based on the detected wave, the integrity of the plug can be evaluated (S3).

Various devices may be used for inducing the acoustic wave in the casing. Examples are a mechanical hammer, a piezoelectric device, a flexural resonator, a wedge transducer or natural background noise. A particularly suitable example of a device for inducing the acoustic wave is an electromagnetic acoustic transducer (EMAT), which uses non-contact electromagnetic transduction to impart physical stress on the conductive metal casing. The EMAT can be used to induce a wave with a well-defined frequency. A plurality of EMATs may be used which are distributed around the axis of the casing, for example symmetrically around the axis in a transverse plane.

The acoustic wave induced in the casing may be one of the following types of waves: a compressional wave, a shear wave, a transversely polarized shear wave, a Lamb wave, a Rayleigh wave. These waves are elastic waves and a particular wave may be chosen to obtain particular information about the plug. In a more particular example, the EMATs will excite an extensional wave AO and flexural waves SO. The extensional waves (AO) will permit deeper depth of traveling when compared to other waves.

The frequency range of EMATs may be around a central frequency of 12 kHz, or between 10 and 15 kHz, but other types of transducers with other frequencies may be used. Other examples are transducers with a central frequency in the source signal at about 100, 200, 250, 265, 300 and 700 kHz. However, a limitation to the frequency is set by the length of time the transducers must listen for detecting the reflected signal, which may be between close to 0 to as long as 1 second, and the source signal preferably has a duration which is smaller than the length of time required for the reflection.

The reflected signal is detected using acoustic detectors. The EMAT may also be used for detecting the reflected signal, thereby being arranged to both induce and detect acoustic signals. After the first acoustic wave is induced in the casing, the acoustic wave will travel through the casing away from the acoustic source, both upwards towards the surface and downwards towards the plug. Any impedance changes in the casing, due to cement bonding or due to a variation in material surrounding the casing, will cause a reflection of the acoustic wave. The time delay between the first wave and the detected reflected wave is an indication of the distance between the acoustic source and the point of reflection, with a larger time delay corresponding to a larger distance from the source. The velocity of the guided wave inside the casing is known and the distance between the source and detection location can be calculated as the travel time divided by half and multiplied by the velocity. An algorithm may use an amplitude threshold in the time domain or an envelope (using the Hilbert transform) may be displayed in order to select the reflected signal from the recorded signal because the signal reflected at the plug will have a larger amplitude when compared to the signal reflected at an area without a plug.

Once the reflected signal has been recorded and sections of the signal have been ‘mapped’ to sections of the casing, the bond between the plug and the casing can be evaluated. A high attenuation of the acoustic signal generally corresponds to a strong bonding, while a low attenuation generally corresponds to weak bonding. A transition between a section of the casing with weak bonding to cement or no bonding and a section with a strong bonding will cause a variation in the refractive index of the casing material and thereby cause a reflection. The recorded waveform may be evaluated in the frequency domain by applying a fast Fourier transform.

In some abandoned wells, the casing is cut at a portion of the plug such that the formation is exposed and the plug can be formed against the formation to avoid any leak paths along the casing. In that configuration, the cut surface of the casing will cause a strong reflection of the acoustic wave.

Advantages of this configuration of inducers and detectors are that the devices are all located above the plug, thereby avoiding the need to have any devices below the plug, and that the measurements are non-destructive. Moreover, no expensive platform would be required in an off-shore application because a boat or other vessel with suffice for supporting and controlling the devices.

Figures 3 to 5 illustrate models and acoustic waves from a section of the wellbore without a plug across the inner diameter. The claimed invention defines a method and system for inspecting a section of a well which includes a plug, and Figures 3 to 5 and the accompanying description therefore fall outside the scope of the claims but are provided for a better understanding of the invention. Figure 3 illustrates a simplified model of a casing 31 which ends in a casing head 32, and which is surrounded by sediment 33 and has a surrounding cement layer 34 provided further downhole than the location where the sediment starts. In this model, there is no plug and the acoustic generation and detection, called transducer, take place at a single location 35. The model which is used for calculating the propagation of the wave through the casing is 3D Axisymmetric Cylindrical Elastic Finite Difference Simulation. In the model a ring of ultrasound transducers is used at location 35. An excitation signal with a frequency bandwidth from 7 to 20 kHz is used.

Figure 4a is a graph the acoustic wave which is recorded at location 35 in Figure 3, with time delay on the horizontal axis and amplitude of the detected signal on the vertical axis. Figure 4b is a more detailed graph of a section of Figure 4b in which the reflection from the cement layer 34 is detected. After the initial pulse (41) which generates the first acoustic wave, the part of the wave which is travelling upwards and towards the casing head 32 is partially reflected. As the casing head 32 is close to the transducer when compared to the sediment and cement layer, the partial reflection of the casing head (42) is detected after a short time delay. The partial reflection is relatively strong compared to later reflections because the end of the casing provides a prominent change in impedance. After a further time delay, the reflection of the sediment is detected (43) and, finally, the reflection of the top of the cement layer is detected (44). The attenuation of the recorded reflection from the top of the cement layer with respect to the induction signal is -25.7dB.

Figure 5 is a diagram showing the propagation of a sound wave though sediment 33. The sound wave partially escapes from the casing while travelling through the casing. The sediment is material resting at the top of cement. The sediment behaves in a similar way as a fluid for leaky P modes escaping from the casing. The sediment contains loose matter with small particles from mud or other particles and there is no cementation between the particles. Figure 5a illustrates a first acoustic wave travelling down a casing, Figure 5b illustrates a partial reflection of the first acoustic wave at the top of a cement layer and Figure 5c illustrates the reflected wave travelling upwards through the casing.

Figure 6 illustrates a tool which can be used for the method described herein. The tool is a wireline logging device and consists of a set of at least two rings of 8 transducers in the plane perpendicular to the longitudinal axis of the casing. The transducers are decentralized with respect to the longitudinal axis of the casing in order to cover a maximum circumferential area of the casing. For example, a final tool can consist of 4 rings and 32 transducers in total. The transducers can be EMATs. In Figure 6 is illustrated a tool body 61 with spring loaded arms 62 which urge transducers 70 towards the wall of the casing 64. The transducers do not need to be in contact with the wall of the casing to induce an acoustic signal in the casing. The tool will be able to induce a signal even if there is debris 65 deposited on the wall of the casing between the casing and the transducers. The casing is fixed to the geological formation 66 by way of cement 67 provided within the annulus between the casing and the formation. A cement plug 68 is provided within the casing below the tool. Well fluid 69 may be present on top of the plug. The tool includes EMATs 70 attached to spring arms 71. There may be fluid 72 in the annulus between the casing and the formation. Figure 6a illustrates a top view of the tool.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which fall within the scope of the appended claims.

Claims

CLAIMS:

1. A method of inspecting a section of a well, wherein the well comprises a metal tubular and a plug across the inner diameter of the tubular in said section of the well, the method comprising: inducing a first acoustic wave travelling through the tubular in a first direction along the longitudinal axis of the tubular; detecting a second acoustic wave travelling through the tubular in a second direction along the longitudinal axis of the tubular, wherein the second direction is substantially opposite to the first direction, and wherein the second acoustic wave is an at least partial reflection of the first acoustic wave at said section of the well; and evaluating the integrity of the plug based on the detected wave.

2. The method according to claim 1, further comprising using at least one acoustic transmitter placed within the tubular to excite the first acoustic wave.

3. The method according to claim 1 or 2, further comprising using at least one receiver placed within the tubular to detect the second acoustic wave.

4. The method according to claim 1, further comprising using a plurality of transmitters provided within the tubular for inducing the first acoustic wave and wherein the plurality of transmitters are distributed symmetrically around the longitudinal axis of the tubular in a plane perpendicular to the longitudinal axis of the tubular.

5. The method according to claim 1 or 4, further comprising using a plurality of receivers provided within the tubular for detecting the second acoustic wave and wherein the plurality of receivers are distributed symmetrically around the longitudinal axis of the tubular in a plane perpendicular to the longitudinal axis of the tubular.

6. The method according to any preceding claim, wherein the first and second acoustic waves are ultrasonic waves.

7. The method according to claim 6, wherein the acoustic waves are substantially within a frequency range from 10 to 15 kHz.

8. The method according to claim 3 when dependent on claim 2, wherein the at least one transmitter and the at least one receiver are located within the tubular closer to the surface when compared to the plug.

9. The method according to claim 5 when dependent on claim 4, wherein the plurality of transmitters and the plurality of receivers are located within the tubular closer to the surface when compared to the plug.

10. The method according to any one of the preceding claims, wherein the well further comprises a casing provided around the tubular and wherein the method further comprises using the detected acoustic wave to determine the bonding between the casing and the tubular.

11. The method according to any one of claims 8 or 9, wherein the step of evaluating the integrity of the plug comprises determining the bonding between the plug and the tubular.

12. The method according to any one of the preceding claims, further comprising estimating the impedance variation within the tubular in said section based on said detecting of the second acoustic wave.

13. The method according to any one of claims 8 or 9, wherein the step of detecting comprises detecting said second acoustic wave at a plurality of time delays from the first acoustic wave for evaluating the integrity of the plug at a plurality of sections of the plug.

14. The method according to claim 5, wherein the step of evaluating the integrity of the plug comprises evaluating the integrity of the plug at a plurality of locations in a plane perpendicular to the longitudinal axis of the tubular using measurements of the plurality of receivers.

15. A system for inspecting a section of a well, the system comprising: a transmitter arranged to induce a first acoustic wave travelling through a metal tubular within the well in a first direction along the longitudinal axis of the tubular; a receiver arranged to detect a second acoustic wave travelling through the tubular in a second direction along the longitudinal axis of the tubular, wherein the second direction is substantially opposite to the first direction, wherein the second acoustic wave is an at least partial reflection of the first acoustic wave at said section of the well; and a processor arranged to calculate properties of a plug provided across the inner diameter of the tubular at said section of the well based on an output of the receiver.

16. The system according to claim 15, wherein the transmitter and receiver are both provided by a transducer.

17. The system according to claim 15 or 16, comprising a plurality of transmitters and receivers.

18. The system according to any one of claims 15 to 17, comprising a plurality of transducers arranged symmetrically with respect to the longitudinal axis of the tubular in a plane perpendicular to the longitudinal axis of the tubular.