Measuring and testing – Volume or rate of flow – Using differential pressure
Reexamination Certificate
2002-02-04
2004-08-17
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Using differential pressure
Reexamination Certificate
active
06776054
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of flow meters for multiphase mixtures. In particular, the invention relates to flow meters for oil and water mixtures in hydrocarbon boreholes.
BACKGROUND OF THE INVENTION
The measurement of oil and water flow rate in each producing zone of an oil well is important to the monitoring and control of fluid movement in the well and reservoir. In addition to a flow meter, each zone may have a valve to control the fluid inlet from that zone. By monitoring flow rates of oil and water from each zone and reducing flow from those zones producing the highest water cut (i.e., ratio of water flow rate to total flow rate), the water production of the entire well can be controlled. This, in addition, allows the reservoir oil to be swept more completely during the life of the well.
Ideally, a flow meter in such an installation should satisfy several criteria: 1) it should be extremely reliable and operate for years at downhole temperature and pressure; 2) it should operate in both stratified (near-horizontal) and dispersed flow regimes over a wide range of total flow rate and cut; 3) it should not require that the completion be oriented azimuthally in any particular way during installation; 4) it should not require licensing of radioactive sources and, finally; 5) the flow meter should allow small changes in water cut and flow rate to be detected.
Typically, downhole flow meters determine the holdup (volume fraction of oil or water) and the velocity of the oil phase, the water phase, or both. The flow rate of water is then determined from the product of water holdup &agr;
w
, the pipe area A, and the velocity of water U
w
. An analogous relation holds for oil flow rate. In general, the velocities of water and oil are different. The slip velocity (difference in oil and water velocities) depends on many parameters, such as the inclination angle of the flow pipe (i.e. deviation), roughness of the pipe wall, and flow rates of the two phases. In general, one must measure the holdup and velocities of both oil and water to determine oil and water flow rate uniquely. In practice, sometimes one measures the velocity of only one phase and uses a theoretical or empirically determined slip law to obtain the other. This has a number drawbacks including inaccuracies due to differences conditions used as inputs to the model and the actual conditions downhole.
A common method to determine the velocity of a fluid is to measure the rotation rate of a turbine blade in the flow stream. In single phase flow, the rotational velocity of the turbine is simply related to the velocity of the flow. However, in mixed oil and water flow the response of the turbine can be so complicated as to be uninterpretable.
Another method of velocity measurement uses tracers. A tracer is injected into the phase of choice (oil or water) and, at a known distance downstream, a sensor detects the time of passage of the tracer. The velocity is computed from the known distance and time of travel. One disadvantage of the tracer method for permanent downhole use is the need for a reservoir of tracer material and a mechanical tracer injector. The reservoir limits the number of measurements and the injector, being a mechanical device, is prone to sticking and failure.
Another method of velocity measurement uses local capacitance or resistance sensors. This method is appropriate for flow regimes in which one phase is dispersed as droplets in another continuous phase. As a droplet passes one of the sensors, a signal is produced for a time duration related to the speed of the droplet. Given knowledge of the droplet size by other means, the velocity of the droplet can be deduced. One disadvantage of this method is that it does not work at all in a stratified flow regime, since it relies on the existence of bubbles.
There are other methods of flow measurement that can be used, which are not described herein, but are familiar to those skilled in the art.
Another method of velocity measurement uses a Venturi. In single phase flow, a Venturi generally obeys the Bernoulli equation which relates volumetric flow rate Q to fluid density &rgr; and pressure drop from the inlet to the throat of the Venturi:
Q
=
C
2
Δ
p
/
ρ
(
1
A
throat
2
-
1
A
inlet
2
)
Equation
1
where C is the discharge coefficient which is approximately unity but depends on the geometry of the Venturi, &Dgr;p is the pressure drop from Venturi inlet to throat, and A
throat
at and A
inlet
are the throat and inlet cross sectional areas, respectively. The same equation can be used to determine the combined oil and water flow rate where the density in this case is the average mixture density in the throat of the Venturi. In practice, the square root in the equation makes it relatively insensitive to errors in both the density and pressure determinations.
A common method to determine the holdup in a flow of oil and water is to measure the average density of the fluid. Since oil at downhole pressure and temperature typically has a density which is smaller than that of water (around 0.7 g/cm
3
compared to 1.0 g/cm
3
), the oil and water holdups &agr;
o
and &agr;
w
can be determined proportionately from the mixture density by the relations
α
o
=
ρ
w
-
ρ
mix
ρ
w
-
ρ
o
Equation
2
α
w
=
ρ
mix
-
ρ
o
ρ
w
-
ρ
o
Equation
3
A common method to determine the mixture density is to measure the hydrostatic pressure of a column of fluid with a gradiomanometer. This device relies on having a component of the gravitational force vector along the axis of the flow pipe. This type of device, however, fails when the flow pipe is horizontal because the gravitational force vector is perpendicular to the pipe axis.
Another method to determine holdup uses capacitor plates to measure the bulk dielectric constant of the fluid. This method is used for flow regimes in which the water is dispersed in bubbles within an oil-continuous medium. It fails in stratified flow or in flow regimes in which the oil is dispersed in bubbles within a water-continuous medium.
Another method to determine holdup uses electrodes or an inductive coupling to measure the bulk resistance of the fluid. This method is used for flow regimes in which the oil is dispersed in bubbles within a water-continuous medium. It fails to work properly in stratified flow or in flow regimes in which the water is dispersed in bubbles within an oil-continuous medium.
Another method to determine holdup uses arrays of capacitor plates or resistance electrodes to measure dielectric constant or resistance in the fluid immediately surrounding the sensor. The accuracy of this method depends on the number of sensors in the array. The disadvantages with this method are that small probes are prone to damage and fouling and the probes are invasive to the pipe, preventing other tools or devices from passing by them freely.
Mixers of various types have been used to mix the oil and water, so as to effectively reduce the slip and allow for more accurate determination of the flow rates. Some mixers are simply small orifices in plates of suitable material. Others comprise more elaborate fins having certain twists or curled shapes. There are a number of disadvantages, however, in using conventional mixers when trying to measure the flow rates of oil and water downhole. For example, the mixer often obstructs the borehole, such that it may be difficult to pass certain equipment such as production logging tools, etc. Mixers also can produce unacceptable amounts of pressure loss. Additionally, mixers are prone to excessive wear with age.
It is possible to measure the pressure differential upstream and downstream of a conventional mixer in an attempt to determine the total flow rate of oil and water. This technique, however, has a number of drawbacks. For example, the accuracy of the flow rate determined by this method is likely to be much lower than using a Venturi, and, i
Ferguson John William James
Fitzgerald John Barry
Goodwin Anthony Robert Holmes
Meeten Gerald Henry
Pelham Sarah Elizabeth
Batzer William B.
Patel Harshad
Ryberg John J.
Schlumberger Technology Corporation
Wang William L.
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