Downhole fluid characterization based on changes in acoustic properties with pressure
Techniques for evaluating physical aspects of a formation fluid from within a wellbore include changing pressure on a sample of the formation fluid and transmitting at least one acoustic pulse through the fluid sample and analyzing acoustic information collected. Apparatus and methods for the evaluating involve using at least one acoustic transducer. Analyzing typically involves use of formulae that relate the fluid's equation of state and other properties to a change in the sound speed in the fluid as a function of pressure.
1. A tool for evaluating a fluid downhole, comprising:
a sample chamber adapted for having the fluid disposed therein, a pump for providing fluid communication with the fluid;
a transducer for providing acoustic communication with the fluid; and
a processor in data communication with the transducer and the pump;
wherein the processor is adapted to change the pressure of the fluid, excite the transducer and receive acoustic data and compute a rate of change of sound speed in the fluid.
2. The tool as in claim 1 , wherein the sample chamber comprises at least one of a sample tank and a sample line.
3. The tool as in claim 1 , further comprising electronics coupled to the transducer for generating the acoustic communication.
4. The tool as in claim 1 , further comprising electronics and a listening transducer coupled to the sample chamber for monitoring the acoustic communication.
5. The tool as in claim 1 , wherein a pressure in the sample chamber comprises pressure of between about 500 pounds per square inch (psi) and about 3,500 psi above a formation fluid pressure.
6. The tool as in claim 1 , wherein the sample comprises at least one of water, drilling fluid, mud, oil and formation fluids.
7. The tool as in claim 1 , wherein a source of pressure for changing the pressure comprises at least one of the pump and a remote supply.
8. The tool as in claim 1 , wherein the processor comprises a digital computer.
9. A method for evaluating a property of a fluid, comprising:
receiving the fluid into a sample chamber;
changing a pressure of the fluid sample;
transmitting at least one acoustic signal into the fluid sample; and
analyzing the at least one acoustic signal to evaluate the a rate of change of the sound speed in the fluid with pressure.
10. The method as in claim 9 , wherein the changing, transmitting and analyzing are performed for a plurality of pressure levels.
11. The method as in claim 9 , wherein the changing comprises at least one of substantially overpressurizing, overpressurizing, slightly overpressurizing, slightly underpressurizing, substantially underpressurizing, and completely relieving pressure.
12. The method as in claim 9 , wherein changing the pressure comprises pressurizing to at least one predetermined pressure.
13. The method as in claim 9 , wherein evaluating the properties of the fluid comprises solving the relationship:
P
=
P
0
+
A
(
Δ
ρ
ρ
)
s
+
B
2
!
(
Δ
ρ
ρ
)
s
2
where
P represents pressure applied to the fluid;
P 0 represents pressure in the sample chamber for an unpressurized state;
ρ represents a density of the fluid;
Δρ represents a change in the density of the fluid; and
A, B represent virial coefficients for the fluid.
14. The method as in claim 9 , wherein evaluating the properties of the fluid comprises solving the relationship:
B
A
=
(
2
ρ
c
(
ⅆ
c
/
ⅆ
P
)
)
;
where
dc/dP represents a derivative of a speed of sound c with respect to pressure, P applied to the fluid;
c represents the speed of sound in the fluid;
ρ represents the density of the fluid; and
A, B represent virial coefficients for the fluid.
15. The method as in claim 9 , wherein evaluating the properties of the fluid comprises solving the relationship:
Δ
A
c
=
-
[
M
c
2
(
B
/
A
+
1
)
]
ln
(
82.051
T
ρ
M
)
;
where
ΔA c represents a cohesive energy of the fluid;
M c represents an average molecular weight of the fluid;
T represents a temperature of the fluid;
ρ represents the density of the fluid; and
A, B represent virial coefficients for the fluid.
16. The method as in claim 9 , wherein evaluating the properties of the fluid comprises solving the relationship:
δ
=
[
(
ρ
c
2
(
B
/
A
+
1
)
)
ln
(
V
g
V
l
)
]
;
where
δ represents a solubility parameter for the fluid;
V g ,V l represent a molar volume of the fluid for a gaseous state and a liquid state, respectively;
c represents the speed of sound in the fluid;
ρ represents the density of the fluid; and
A, B represent virial coefficients for the fluid.
17. The method as in claim 9 , wherein evaluating the properties of the fluid comprises solving the relationship:
a
=
[
ρ
c
2
V
l
2
(
B
/
A
+
1
)
]
;
where
α represents a measure of attractive forces between molecules in the fluid;
V 1 represents a molar volume of the fluid for a liquid state;
c represents the speed of sound in the fluid;
ρ represents the density of the fluid; and
A, B represent virial coefficients for the fluid.
18. The method as in claim 9 , wherein evaluating the properties of the fluid comprises solving the relationship:
b
=
V
l
-
R
T
ρ
c
2
(
B
/
A
+
1
)
;
where
b bears a relation to the size of the molecules in the fluid;
R represents an ideal gas constant;
V l represents a molar volume of the fluid for a liquid state;
c represents the speed of sound in the fluid;
ρ represents the density of the fluid; and
A, B represent virial coefficients for the fluid.
19. The method as in claim 9 , wherein changing the pressure comprises maintaining the fluid in a liquid state.