Ultrasonic logging methods and apparatus for measuring cement and casing properties using acoustic echoes
A method and apparatus for measuring parameters of a borehole using an acoustic logging system. By transmitting a high frequency acoustic signal essentially perpendicular to the major axis of a borehole casing using a transducer, the high frequency acoustic signal may produce one or more acoustic echoes. By receiving and analyzing a first acoustic echo from an inner wall of the borehole casing and at least one second acoustic echo from a back wall of the borehole casing, borehole parameters such as the impedance of the casing cement and the thickness of thicker borehole casings, up to and greater than 1 inch, may be determined.
1. A method using an acoustic logging system in a borehole having a borehole casing filled with fluid, the borehole casing being surrounded by casing grout, having an inner wall, having a back wall, and being excitable at a resonant frequency that induces reverberation, the borehole casing having a first impedance, the method comprising:
using a transducer of the acoustic logging system to transmit a signal in the direction of the borehole casing, the signal providing a frequency different from the resonant frequency of the borehole casing that induces reverberation, the signal producing acoustic echoes;
receiving a first of the acoustic echoes with the transducer from the inner wall of the borehole casing;
receiving a second of the acoustic echoes with the transducer from the back wall of the borehole casing, and
determining a naught impedance of the fluid in the borehole casing with the acoustic logging system;
determining, with at least one processor of the acoustic logging system, at least one borehole parameter based on at least one of the first and second acoustic echoes comprising determining a second impedance (Z 2 ) of the casing grout based at least in part on the naught impedance (Z 0 ) of the fluid, the first impedance (Z 1 ) of the borehole casing, and a ratio of amplitudes of the first and second acoustic echoes.
2. The method of claim 1 , wherein using the transducer to transmit the signal in the direction of the borehole casing comprises transmitting the signal to provide the frequency of at least 3 to 5 times the resonant frequency of the borehole casing.
3. The method of claim 1 , wherein using the transducer to transmit the signal in the direction of the borehole casing includes transmitting the signal perpendicular to the major axis of the borehole casing.
4. The method of claim 1 , wherein determining the second impedance of the casing grout is determined according to the equation:
Z
2
=
Z
1
*
(
K
-
EAR
K
+
EAR
)
where
:
K
=
(
4
*
Z
1
*
Z
0
Z
1
2
-
Z
0
2
)
EAR is an Echo Amplitude Ratio related to the ratio of the amplitudes measured by the acoustic logging system,
Z 1 is the first, acoustic impedance of the borehole casing which is a known constant,
Z 0 is the naught, internal fluid impedance of the fluid measured by a mud chamber in the acoustic logging system, and
Z 2 is the second impedance of the casing grout.
5. The method of claim 4 , wherein the Echo Amplitude Ratio comprises the ratio of the amplitude of the second acoustic echo to the amplitude of the first acoustic echo.
6. The method of claim 5 , wherein the Echo Amplitude Ratio is determined by adding the amplitudes of more than one of the second acoustic echoes from the back wall of the borehole casing and dividing the sum by the amplitude of the first acoustic echo.
7. The method of claim 1 , wherein determining the at least one borehole parameter further includes determining a thickness of the borehole casing.
8. The method of claim 7 , wherein determining the thickness of the borehole casing is determined according to the equation:
Thick
=
Δ
Techo
2
*
Caslow
where,
Thick is the thickness of the borehole casing,
ΔTecho is a time difference between receiving the first acoustic echo and receiving the second acoustic echo, and where
Caslow is an amount the borehole casing slows down a penetrating wave which is a known constant.
9. A downhole tool used in a borehole having a borehole casing filled with fluid, the borehole casing surrounded by casing grout, having an inner wall, having a back wall, and being excitable at a resonant frequency that induces reverberation, the borehole casing having a first impedance, the tool comprising:
an acoustic transducer configured to measure acoustic echoes in the borehole, the acoustic transducer configured to:
transmit a signal in the direction of the borehole casing, the signal providing a frequency different from the resonant frequency of the borehole casing that induces reverberation, the signal producing acoustic echoes;
receive a first of the acoustic echoes with the transducer from the inner wall of the borehole casing;
receive a second of the acoustic echoes with the transducer from the back wall of the borehole casing, and
a processor communicatively coupled to the acoustic transducer and having one or more instructions that when executed cause the processor to:
determine a naught impedance of the fluid in the borehole casing with the downhole tool; and
determine at least one borehole parameter based on at least one of the first and second acoustic echoes, comprising determining a second impedance (Z 2 ) of the casing grout based at least in part on the naught impedance (Z 0 ) of the fluid, the first impedance (Z 1 ) of the borehole casing, and a ratio of amplitudes of the first and second acoustic echoes.
10. The downhole tool of claim 9 , wherein the acoustic transducer is configured to transmit the signal in the direction of the borehole casing to provide the frequency of at least 3 to 5 times the resonant frequency of the borehole casing.
11. The downhole tool claim 9 , wherein the acoustic transducer is configured to transmit the signal perpendicular to the major axis of the borehole casing.
12. The downhole tool of claim 9 , wherein the second impedance of the casing grout is determined according to the equation:
Z
2
=
Z
1
*
(
K
-
EAR
K
+
EAR
)
where
:
K
=
(
4
*
Z
1
*
Z
0
Z
1
2
-
Z
0
2
)
EAR is an Echo Amplitude Ratio related to the ratio of the amplitudes measured by the downhole tool,
Z 1 is the first, acoustic impedance of the borehole casing which is a known constant,
Z 0 is the naught, internal fluid impedance measured by a mud chamber in the downhole tool, and
Z 2 is the second impedance of the casing grout.
13. The downhole tool of claim 12 , wherein the Echo Amplitude Ratio comprises the ratio of the amplitude of the second acoustic echo to the amplitude of the first acoustic echo.
14. The downhole tool of claim 12 , wherein the Echo Amplitude Ratio is determined by adding the amplitudes of more than one of the second acoustic echoes from the back wall of the borehole casing and dividing the sum by the amplitude of the first acoustic echo.
15. The downhole tool of claim 9 , wherein to determine the at least one borehole parameter, the one or more instructions further cause the processor to determine a thickness of the borehole casing.
16. The downhole tool of claim 15 , wherein the thickness of the borehole casing is determined according to the equation:
Thick
=
Δ
Techo
2
*
Caslow
where,
Thick is the thickness of the borehole casing,
ΔTecho is a time difference between receiving the first acoustic echo and receiving the second acoustic echo, and where
Caslow is an amount the borehole casing slows down a penetrating wave which is a known constant.
17. A downhole tool used in a borehole having a borehole casing filled with fluid, the borehole casing surrounded by casing grout, having an inner wall, having a back wall, and being excitable at a resonant frequency that induces reverberation, the borehole casing having a first impedance, the tool comprising:
a means for transmitting a signal in the direction of the borehole casing to provide a frequency different from the resonant frequency of the borehole casing that induces reverberation, the signal producing acoustic echoes;
a means for receiving a first of the acoustic echoes from the inner wall of the borehole casing;
a means for receiving a second of the acoustic echoes from the back wall of the borehole casing;
a means for determining a naught impedance of the fluid in the borehole casing with the tool; and
a means for determining at least one borehole parameter based on at least one of the first and second acoustic echoes comprising means for determining a second impedance (Z 2 ) of the casing grout based at least in part on the naught impedance (Z 0 ) of the fluid, the first impedance (Z 1 ) of the borehole casing, and a ratio of amplitudes of the first and second acoustic echoes.
18. The downhole tool of claim 17 , wherein the means for transmitting the signal comprises means for transmitting the signal in the direction of the borehole casing to provide the frequency of at least 3 to 5 times the resonant frequency of the borehole casing.
19. The downhole tool of claim 17 , wherein the second impedance of the casing grout is determined according to the equation:
Z
2
=
Z
1
*
(
K
-
EAR
K
+
EAR
)
where
:
K
=
(
4
*
Z
1
*
Z
0
Z
1
2
-
Z
0
2
)
EAR is an Echo Amplitude Ratio related to the ratio of the amplitudes measured by the downhole tool,
Z 1 is the first, acoustic impedance of the borehole casing which is a known constant,
Z 0 is the naught, internal fluid impedance measured by a mud chamber in the downhole tool, and
Z 2 is the second impedance of the casing grout.
20. The downhole tool of claim 19 , wherein the Echo Amplitude Ratio comprises the ratio of the amplitude of the second acoustic echo to the amplitude of the first acoustic echo.
21. The downhole tool of claim 19 , wherein the Echo Amplitude Ratio is determined by adding the amplitudes of more than one of the second acoustic echoes from the back wall of the borehole casing and dividing the sum by the amplitude of the first acoustic echo.
22. The downhole tool of claim 17 , wherein the means for determining the at least one borehole parameter further comprises means for determining a thickness of the borehole casing.
23. The downhole tool of claim 22 , wherein the thickness of the borehole casing is determined according to the equation:
Thick
=
Δ
Techo
2
*
Caslow
where,
Thick is the thickness of the borehole casing,
ΔTecho is a time difference between receiving the first acoustic echo and receiving the second acoustic echo, and
Caslow is an amount the borehole casing slows down a penetrating wave which is a known constant.