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BACKGROUND OF THE DISCLOSURE
It is often necessary to measure the rate of flow of fluid in a borehole.
The present apparatus is an ultrasonic Doppler measuring device providing
a fluid velocity measurement of fluid in a borehole. Consider a typical
example in which a cased well has been produced for some period of time.
Assume further that it may span two or three different horizons having
perforations and potentially producing fluids into the borehole. It is
necessary to measure the volume of the fluid flowing from each set of
perforations at the respective horizons. This can be typically compounded
even further when the formations produce different fluids. Assume that one
produces only petroleum products while another produces oil and water in a
known ratio. Alternately, assume that one of the formations produces
natural gas in a specified ratio. In such an example as this, the
commingled fluids will flow upwardly in the cased borehole. Routinely,
entrained bubbles are carried along with the fluid flow. The bubbles and
fluid flow at rates which may be the same or different depending on a
number of factors discussed below.
It is often desirable to measure the fluid flow velocity. One device used
for this is a type of flow meter which has a propeller affixed to some
kind of counting device. The velocity of the propeller driven by the fluid
flowing past the measuring device defines the flow rate by measuring the
velocity as the propeller is spun. This works in an acceptable range of
minimum and maximum flow velocities, but it does not work well at every
velocity. There are a number of factors which can obscure measurements and
create difficulties in making the measurements.
The present apparatus is a system that can be used to measure fluid
velocity in a variety of mixtures and circumstances. It is a system which
especially responds to scattered droplets of gas entrained in the form of
bubbles in the flowing fluid. Alternately, there are typically entrained
particles such as mill scale or other solid particles such as sand from
the formation. Whatever the source, there is a strong possibility that the
fluid will be something other than a pure fluid. Consider as one example a
well which produces natural gas commingled with other petroleum products.
The entrained gas bubbles are carried along with the produced fluid and
they may indeed even flow faster than the produced liquids. The bubbles
add another form of interface which scatters ultrasonic energy, a feature
exploited as described below.
The disclosure sets forth a pulsed ultrasonic Doppler system which takes
advantage of scattering from droplets and other particles in the flowing
fluid stream. It is a system ideally located at the lower end of a
centralized instrument body located in a cased well borehole, and wherein
the casing defines the fluid path for the mixture of fluids flowing up the
well. An instrument package of specified diameter is centralized by upper
and lower sets of centralizers so that the fluid produced by the
formation(s) flows up the casing and around the body which houses or holds
the instrument described below. In this circumstance, it is possible to
obtain the flow rate by directing an ultrasonic pulse from the measuring
instrument downwardly into the fluid flow directed at the volume of fluid
below the tool.
Pulses are formed at a selected repetition rate and have a short pulse
duration; they are transmitted downwardly into the flowing fluid and
impinge on reflective surfaces. Scattering occurs either by reflection or
refraction. This involves the interface between various matrials making up
the flow. For instance, droplets of oil and water will provide such an
interface. Gas bubbles in an otherwise liquid flow will also provide an
interface. Sand, mill scale and other particles of a solid nature also
provide such an interface. The scattering has the form of a reflected
signal after the transmitted pulse. It is therefore received at a time
interval thereafter, and encodes the movement of the scattering particles
in the fluid in the form of a Doppler shift. The Doppler shift can be
measured and calibrated to obtain fluid flow velocity.
Several velocities may be involved in the relative measurement. For
instance, the measuring tool can either be fixed or moving. The fluid can
either be stagnant or moving. One ordinary circumstance will find the
measuring tool moving downwardly while the fluid is flowing upwardly. The
rate of movement of the measuring tool is normally obtained at the surface
where it is measured as the logging cable for the measuring tool is
lowered into the cased well, and that measurement can be readily
subtracted from measurements of the fluid velocity relative to the tool.
Alternately, the velocity of the tool relative to the stationary
surrounding casing can be measured. In any event, such measurements can be
made.
BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the above recited features, advantages and
objects of the present invention are attained and can be understood in
detail, more particular description of the invention, briefly summarized
above, may be had by reference to the embodiments thereof which are
illustrated in the appended drawings.
It is to be noted, however, that the appended drawings illustrate only
typical embodiments of this invention and are therefore not to be
considered limiting of its scope, for the invention may admit to other
equally effective embodiments.
FIG. 1 shows the measuring instrument of the present disclosure suspended
in a flowing well for making fluid flow velocity measurements;
FIG. 2 is an enlarged detail view of the lower end of the tool shown in
FIG. 1 illustrating details of construction of the ultrasonic pulse
transmitter and receiver mechanism;
FIG. 2 is an enlarged view of the transducer showing signal propagation and
return; and
FIG. 4 is a transmitter and receiver timing chart.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Attention is directed to FIG. 1 of the drawings where the numeral 10
identifies the apparatus to the present disclosure. The sonde 10 is
supported on a logging cable 12 extending into a cased well 14, the sonde
being located in the center of the casing by lower and upper centralizers
16 and 18. The centralizers have the form of bow springs which are located
around the periphery to enable centering of the sonde to thereby direct
fluid flow pass the sonde in a surrounding annular flow space. The logging
cable 12 extends to the surface where it passes over a sheave 20 and is
directed to a drum 22 where it is spooled for storage. The cable can be
several thousand feet in length. The are one or more conductors in the
cable which connect with a surface located electronic system including a
power supply and the various and controls for use of the sonde. In
addition, the equipment at the surface connects to a recorder 26 which is
provided with a depth measurement from a mechanical or electronic depth
measuring apparatus 28 operative from the sheave. This enables the data to
be recorded as a function of depth in the borehole.
The present apparatus is lowered to various depths in the well so that a
formation 30 which produces fluid can introduce fluid flow below the sonde
10 where the fluid flows through a set of perforations at 32 into the
well. The fluid flows upwardly and is produced from the wellhead.
Typically, several perforations enable production of the fluid from the
formation 30. Typically, the well will pass through one or more such
producing formations. Indeed, it is possible to have fluid production from
two or three different horizons along the depth of the well. In the latter
instance, it is possible that the formations produce different volumes and
perhaps different mixtures of fluid. As an example, assume that the
formation 30 produces only oil. Another formation may produce a mixture of
oil and water. Another formation may produce oil mixed with natural gas
which is entrained as bubbles in the fluid production. It is not unusual
for the formation to produce at least a measure of sand, and indeed, large
volumes of sand may also be produced along with the production of the
fluids.
The latter is mentioned as one example among many of the materials which
flow with the formation fluids directed upwardly in the well and which
flow around the sonde toward the surface. In the typical instance,
production of mixed oil and water produces oil droplets entrained in the
water as a result of the differences in surface tension between the two
fluids. Alternately, natural gas bubbles may be produced and they are
carried along with the fluid flow. It is useful to the present apparatus
to note this mixture and to especially recognize that the interface
between two of the materials serves as a reflective surface. Expanding on
this thought, the interface between oil and water defines a reflective
surface. In like fashion, the interface between the produced natural gas
and the surrounding liquid that carries it is also such a reflective
interface. Another example includes solid particles such as mill scale
from the surface of the drill pipe making up the cased well. Other
examples include the produced sand.
The foregoing variety of exemplary flowing materials points out something
of the variety of flowing fluids which are measured by the present
apparatus. The present apparatus is responsive to the reflective
interfaces defined in the flowing materials. The reflective interface thus
can have a large variety and can be, for example, the interfaces mentioned
above and any other interface tending to reflect or refract ultrasonic
signals transmitted through the flowing fluid. The flowing fluid is not
simply a monolithic singular liquid (e.g., a specified weight of crude oil
with various lighter and heavier constituents) but is in fact a mixture
and especially a mixture having reflecting interfaces. So to speak, the
reflective interfaces at or on grains of sand, gas bubbles, oil droplets
entrained in water, etc. serve as a scattering reflective surface for
purposes to be described.
Attention is now directed to FIG. 2 in conjunction will FIG. 1. The lower
portion of the sonde is constructed with a surrounding fluid tight housing
36. The housing 36 encloses the electronic package (not shown) which is
provided to operate the transmitter described hereinbelow. Also, it
encloses the receiver and the circuit for extracting the Doppler shift
information as will be detailed. FIG. 2 further shows the housing 36 to
have the form of an elongate tubular member which has an internal cavity
for receiving a motor 38. The motor 38 is connected to suitable support
mounting or bracket which holds the motor in position. The motor has a
shaft which extends through a transverse bulk head to provide pressure
tight isolation. Furthermore, the motor 38 is connected to a rotating hub
40 which is rotated at a controlled velocity by the motor. For instance,
the rotational rate can be one turn per minute. This can be changed, that
is speeded up or slowed down, by the incorporation of a gear box connected
with the motor housing. In any event, the hub 40 serves as a mounting. It
is shaped in the form of a tubular plug at the end of the cylindrical
housing. An internal seal member is provided so that fluid from the
exterior does not leak to the interior. Further the hub 40 supports a
piezoelectric transmitting element 42. It is mounted in the body of the
plug and has an exposed external face which is directed downwardly in the
well to provide a transmitted ultrasonic pulse. The plug is rotated so
that the ultrasonic beam described below sweeps across the cross sectional
area of the pipe which makes up the well.
The ultrasonic transmitter 42 is an element which is installed having an
exposed face directed somewhat downwardly. As shown in the drawings, it is
directed downwardly at an angle of approximately 45.degree. from the
vertical. When rotated, it will sweep out a conic area which encompasses
the entire cross sectional area of the pipe. It is possible to direct the
beam downwardly at a different angle by positioning the transmitting
antenna element at perhaps 15.degree. to 30.degree. from the vertical
angle. It is important to direct the beam somewhat downwardly in the well.
The interplay of this beam direction in conjunction with the flow path
should be understood. Specifically, the sonde 10 is something of a plug or
restriction placed in the well. It causes the flow to redirect into the
annular space within the well and on the exterior of the housing 36. This
speeds up the flow rate so that the flowing fluids pass around the sonde
and thereby enhances the apparent relative velocity. It provides more of a
contrast. It tends to cause a focused collection of bubbles and the like
in the annular flow space so that there are more reflective particles,
bubbles, etc. moving through the beam transmitted in the flowing well
fluids. The precise position of the beam can be adjusted over a range so
that the upward flow is correctly measured by reflecting from a sufficient
number of bubbles that a meaningful response is obtained. Should there be
absolute stillness, i.e. no flow, and should the fluid filling the well be
without entrained particles droplets, etc., the ultrasonic beam will not
have any scattering surface which reflects it. Fortunately, in the
ordinary circumstances of use, the present apparatus practically always is
able to obtain a responsive signal because, in practically every
application, the fluid flowing past the measuring device will expose the
flow and the entrained bubbles, reflected droplets, sand particles, etc.
will be sufficient to create a back scatter and received ultrasonic
signal.
In FIG. 2 and 3 of the drawings, the numerous bubbles are indicated by the
numeral 48. The beam is shown at 50. The beam intercepts the bubbles that
flow through the beam and provides back scattering which returns a
reflected scattered beam back to the transmitting element 42. It is
preferably switched on and then off so that the pulse transmitted
therefrom has a finite duration. It is used as a receiver also. After
transmission, it is used in a receiver mode so that the receive signal can
be recognized. If desired, separate antennas can be provided for
transmitting and receiving, but convenience is usually served by combining
the two elements as one. This enables a relatively small structure to be
used to measure the flow velocity of the entrained particles. Usually, the
entrained particles are carried at the same velocity as the medium in
which they are formed. That is, the entrained particles are a good
indicator of fluid flow velocity.
The beam as shown in FIG. 2 is therefore directed at an angle with respect
to the axis of the cased well, and this angular position of the beam
assures that it will intercept an adequate number of bubbles and provide
the appropriate backscatter. After the transmitted pulse is formed and the
transmitter is switched off, the equipment is switched to a receive mode
so that the received signal can be observed at transducer 42 and the
received signal is then processed by amplification and subsequent
recording.
The transmitted pulse and the received pulse back scattered by the
droplets, bubbles, particles, etc. define a difference in frequency which
is related to the Doppler shift. The Doppler shift derives from the
relative change of position during the transmitted pulse, and it is
therefore proportional to the flow velocity. Should the velocity be zero,
and should the particles be merely suspended without moving in a zero
velocity fluid medium, the equipment will provide an indication that there
is no velocity for the particles. On the other hand, it will also provide
an indication which is proportionate to the velocity of the produced
fluids flowing through the well.
It may be necessary to obtain calibration data. For instance, the tool 10
may be used in a stationary mode, or it can be lowered or raised as
required. When lowered or raised during measurements in such
circumstances, the measured velocity will have an error equal to the
velocity of the sonde 10 during measurements. If the velocity of the sonde
10 is known, that factor can be removed from the data by simple arithmetic
to delete that velocity component. In use of the present system, the
velocity of the flowing fluid and the droplets are typically the same.
This is especially so where the fluid production continues for an interval
and the entrained bubbles, droplets, etc. are carried there along and are
produced at a fairly stabilized rate which is approximately proportional
to the rate at which well fluids are recovered. While it may be important
to separate oil from water and sand at the surface, the mixture which is
observed in the well from production is valuable in that the mixture
provides an indication of velocity as a result of the entrained particles
and droplets. Because of the differences at the interface between the
materials, backscatter occurs and the transducer 42 is thus used as a
receiver element.
The present invention supports the instrument circuitry within the sonde 10
as shown in FIG. 1. That is connected so that both transmitter and
receiver are enabled to operate through the transducer 42. If desired,
additional transmitter and/or receiver transducers are also included at
other angles on the rotatable hub 40. Alternatively to using a motor to
rotate a single transducer, several transducers may be mounted on the hub
40. This eliminates the motor mechanism at the expense of reducing the
resolution of the measurements.
The equation given below correlates the velocity measurements to the
frequency measurements. That is, velocity can be determined by the use of
this equation resulting from the frequency measurements made on the
transmitted signal and received signal. The equation shows how the
transmitted and received signals differ in frequency as a result of the
frequency shift derived from the Doppler effect. This data of course is
collected as a function of depth and is subsequently recorded at the
surface on the recorder 26 and is available for later careful diagnostics
and analysis.
Typically, the present apparatus operates at a frequency of from about 500
kilohertz to about 2 megahertz, and is a pulsed system. Typically, the
transmitted pulse is from perhaps 2 to about 20 microseconds in length.
After the transmitted pulse is formed, an interval is permitted for the
received pulse to return. The typical duty cycle involves formation of a
pulse of the duration just mentioned and a pause for an interval of up to
perhaps 40 microseconds. The duty cycle can be relatively small that is,
the transmitters on only a small percentage of the time. The pulse
repetition rate, i.e. the spacing between adjacent transmitted pulses, can
be varied so that the pulses are perhaps every 50 microseconds to every
500 microseconds. By choosing the pulse rate or time interval at which the
frequency comparison is made, the velocity of scatters at different
distances from the transducer can be determined. Thus, a profile of
scattering velocity across the well borehole can be determined. This is
one reason for the use of a pulsed Doppler system. This is exemplified in
FIG. 4 and the Doppler frequency shift relates to velocity in the
following equation:
##EQU1##
In the foregoing, .DELTA.f is the shift in frequency between the
transmitted and received signals, c is the speed of sound in the fluid,
V.sub.T is tool velocity and V.sub.S is the desired velocity of the
scattering particles.
Attenuation of the ultrasonic energy can be used in fluid identification.
For example, backscatter reflections from gas bubbles are generally larger
than reflections from sediment or oil-water emulsions. Also, gas bubbles
attenuate ultrasonic energy more than sediment or emulsion. Reflections
from the wall of the casing or pipe can be used for casing diameter and
corrosion measurements.
Several types of ultrasonic pulsed Doppler approaches can be utilized for
signal generation and processing depending on the requirements of the
desired velocity measurements. One such technique uses random signals to
enhance both range and velocity resolution. (C. P. Jethwa M. Kaveh, G. R.
Cooper and F. Saggio, Blood Flow Measurements Using Ultrasonic Pulsed
Random Signal Doppler System, IEEE Trans. on Sonics and Ultrasonics. V.
SV-22, No. 1, 1-11, 1975). IEEE Trans. on Sonics and Ultrasonics. V.
SV-22, No. 1, 1-11, 1975). Transducer frequency can be adjsuted so that
variations in fluid attenuation can be accommodated.
While the foregoing is diected to the preferred embodiment, the scope
thereof is determined by the claims which follow.
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