Logging tool

1. A logging tool for determining properties of a fluid surrounding the tool arranged downhole in a casing comprising a wall and having a longitudinal extension, the logging tool having a substantially longitudinal cylindrical shape with a longitudinal axis and, when seen in cross-section, a periphery, wherein the logging tool comprises:

a plurality of electrodes arranged spaced around the longitudinal axis in the periphery of the tool so that the fluid flows between the electrodes and the casing wall, wherein the tool is free of grounded electrical means or electrical means having a fixed potential arranged as guards between the electrodes; and

the logging tool configured to measure the capacitance of the fluid surrounding the outside of the tool between two electrodes in all combinations giving n*(n−1)/2 capacitance measurements for n electrodes.

2. The logging tool according to claim 1 , wherein the logging tool has a space between every two electrodes, which space is substantially filled up with a non-conductive means, the non-conductive means is made of a substantially non-conductive material and/or a non-conductive gas.

3. The logging tool according to claim 1 , further comprising a positioning device for determining a position of the logging tool along the longitudinal extension of the casing.

4. The logging tool according to claim 3 , wherein the positioning device is a driving unit.

5. The logging tool according to any claim 1 , further comprising a centralisation device for centralising the logging tool in the casing.

6. The logging tool according to claim 5 , wherein the centralisation device is a driving unit.

7. The logging tool according to claim 1 , wherein the electrodes are arranged in a front end of the tool at a distance from a tip of the tool of less than 25% of a total length of the tool.

8. The logging tool according to claim 1 , further comprising an orientation device for determining an orientation of the tool in the casing.

9. The logging tool according to claim 8 , wherein the orientation device is an accelerometer.

10. The logging tool according to claim 1 , wherein the tool comprises at least eight electrodes.

11. The logging tool according to claim 1 , wherein the means for measuring provides a continuous measurement of the capacitance between the electrodes.

12. The logging tool according to claim 1 , wherein the means for measuring measures a capacitance between two electrodes at a rate of at least 1 measurement of the capacitance per second.

13. The logging tool according to claim 1 , wherein the measurement of the capacitance between two electrodes is performed at a potential (V) over two electrodes and with a frequency of at least 1 MHz.

14. The logging tool according to claim 1 , further comprising a printing circuit directly connected with the electrodes without the use of cords or cables.

15. The logging tool according to claim 3 , wherein the electrodes are positioned between the tip of the tool and the positioning device.

16. A method for using a logging tool for determining properties of a fluid surrounding the tool arranged downhole in a casing comprising a wall and having a longitudinal extension, the logging tool having a substantially longitudinal cylindrical shape with a longitudinal axis and, when seen in cross-section, a periphery, wherein the logging tool comprises a plurality of electrodes arranged spaced around the longitudinal axis in the periphery of the tool so that the fluid flows between the electrodes and the casing wall, wherein the tool is free of grounded electrical means or electrical means having a fixed potential arranged as guards between the electrodes, and the logging tool configured to measure the capacitance between two electrodes in all combinations giving n*(n−1)/2 capacitance measurements for n electrodes, the method comprising the steps of:

measuring the capacitance of the fluid surrounding the outside of the tool between two electrodes,

calculating the permittivity distribution, and

creating an image of the fluid flowing around the tool as a cross-sectional view transverse to the longitudinal extension of the tool.

17. A method for using a logging tool for determining properties of a fluid surrounding the tool arranged downhole in a casing comprising a wall and having a longitudinal extension, the logging tool having a substantially longitudinal cylindrical shape with a longitudinal axis and, when seen in cross-section, a periphery, wherein the logging tool comprises a plurality of electrodes arranged spaced around the longitudinal axis in the periphery of the tool so that the fluid flows between the electrodes and the casing wall, wherein the tool is free of grounded electrical means or electrical means having a fixed potential arranged as guards between the electrodes, and the logging tool configured to measure the capacitance between two electrodes in all combinations giving n*(n−1)/2 capacitance measurements for n electrodes, the method comprising the steps of:

measuring the capacitance of the fluid surrounding the outside of the tool between two electrodes,

calculating the permittivity distribution from the following equations and Linear Back Projection:

S

~

ijk

=

S

ijk

∑

k

⁢

S

ijk

C

~

ij

=

C

ij

-

C

min

C

max

-

C

min

ε

~

LBP

=

S

~

T

⁢

C

~

wherein S ijk is a sensitivity at a point k in a set of points,

S

~

ijk

=

S

ijk

∑

k

⁢

S

ijk

is a normalized sensitivity for each point k in the set of points, C ij is the capacitance between the two electrodes, C min is the capacitance when only gas is present between the two electrodes and the wall, C max is the capacitance when only water is present between the two electrodes and the wall,

C

~

ij

=

C

ij

-

C

min

C

max

-

C

min

is a normalized capacitance between the two electrodes, {tilde over (∈)} LBP is a normalized permittivity distribution, {tilde over (∈)} LBP ={tilde over (S)} T {tilde over (C)}is a sensitivity matrix, and {tilde over (∈)} LBP ={tilde over (S)} T {tilde over (C)}is a normalized capacitance, and

creating an image of the fluid flowing around the tool as a cross-sectional view transverse to the longitudinal extension of the tool.

18. A method for determining a permittivity profile of a cross-sectional view of a fluid in an annulus using an electrode arrangement in the form of a set of electrodes arranged along a periphery of a cylindrical logging tool, comprising the steps of:

making a set of capacitance measurements constituted by one capacitance measurement for each combination of two electrodes from the set of electrodes,

determining the permittivity profile by:

providing a first calibration set (∈ min , C min ) constituted by a set of capacitance measurements for each combination of two electrodes from the set of electrodes when the annulus is filled with a first known fluid,

providing a second calibration set (∈ max , C max ) constituted by a set of capacitance measurements for each combination of two electrodes from the set of electrodes when the annulus is filled with a second known fluid different from the first known fluid,

providing a sensitivity matrix associated with the electrode arrangement, and

calculating the permittivity from the following equations:

S

~

ijk

=

S

ijk

∑

k

⁢

S

ijk

C

~

ij

=

C

ij

-

C

min

C

max

-

C

min

ε

~

LBP

=

S

~

T

⁢

C

~

wherein S ijk is a sensitivity at a point k in a set of points,

S

~

ijk

=

S

ijk

∑

k

⁢

S

ijk

is a normalized sensitivity for each point k in the set of points, C ij is the capacitance between the two electrodes, C min is the capacitance when only gas is present between the two electrodes and the wall, C max is the capacitance when only water is present between the two electrodes and the wall,

C

~

ij

=

C

ij

-

C

min

C

max

-

C

min

is a normalized capacitance between the two electrodes, {tilde over (∈)} LBP is a normalized permittivity distribution, {tilde over (∈)} LBP −{tilde over (S)} T {tilde over (C)}is a sensitivity matrix, and {tilde over (∈)} LBP −{tilde over (S)} T {tilde over (C)}is a normalized capacitance.

19. The method according to claim 18 , further comprising the step of:

creating an image based on the calculations.

20. The method according to claim 19 , further comprising the step of:

storing the set of capacitance measurements constituted by one capacitance measurement for each combination of two electrodes from the set of electrodes on a data storage media.

21. The method according to claim 18 , further comprising the step of:

storing {tilde over (∈)} LBP a data storage media.

22. The method according to claim 18 , further comprising the step of:

storing a representation of {tilde over (∈)} LBP on a data storage media.

23. A detection system comprising the logging tool according to claim 1 and a calculation unit for processing capacitance measurements measured by the electrodes.

24. A downhole system comprising the logging tool according to claim 1 and a driving tool.