Measuring and testing – Liquid level or depth gauge
Reexamination Certificate
1998-06-08
2001-05-29
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
C073S291000, C367S118000, C367S120000, C367S099000
Reexamination Certificate
active
06237410
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to a method for controlling the speed of a fluid pump in accordance with a measured fluid level and a method to determine the fluid level by transmitting a sonic pulse into the well casing, detecting reflections of the sonic pulse from collars on the tubing inside the well casing, detecting the pulse reflected from the fluid surface and accumulating distance for the collars from the top of the well to the point of reflection from the surface.
According to the invention, fluid production from a well can be increased without pumping the well dry, by controlling pumping capacity of the pump. Pumping capacity can be controlled by, for example, adjusting the speed of a reciprocating, rotary or submersible pump. To prevent the pump from running dry, the level of the fluid is measured using a fluid level measuring instrument and a minimum fluid head is maintained above the inlet to the pump. The fluid level instrument operates by sending an acoustic pulse into an annulus between the well casing and the production tubing and sensing reflections of the acoustic pulse from collars on the tubing and a reflection from the fluid surface. Since the collar spacing on the production tubing is known, the distance from the top of the well to the surface of the fluid can be obtained by accumulating the distance associated with the collars detected in the reflection signal.
DESCRIPTION OF RELATED ART
When a well is pumped more quickly than the formation is able to supply fluid to the well casing, the pump will not completely fill and the pump will try to pump gases as well as liquid. This pump-off condition will cause damage to the pumping equipment and reduces the efficiency of the pump. To optimize the production from the well, it is desirable to pump as quickly as possible without creating a pump-off condition.
Prior art pump controllers such as described in U.S. Pat. No. 4,286,925 by Standish use a measurement of the load on the polished rod of the pump to determine when pump-off has occurred. The pump is then stopped for a period of time to allow the formation to replenish the fluid in the well casing before the pump is restarted.
Prior art pump controllers such as described in U.S. Pat. No. 3,953,777 by McKee measure the electric current consumption of the pump motor and when the load on the motor decreases because of the pump-off condition, the motor is turned off for a period of time.
Prior art pump controllers such as described in U.S. Pat. No. 4,318,674 by Godbey et al use a fluid level measuring device with upper and lower limits that determine when the pump should be started or stopped.
Prior art pump controllers such as described in U.S. Pat. No. 4,973,226 by McKee vary the pumping speed to maintain a condition of partial pump-off. The input signal that is used by the controller to determine if the speed of the pump should be increased or decreased is based on the load on the polished rod of the pump. It is not desirable to operate the pump continuously in a partial pump-off state.
There are other advantages to maintaining a continuous output from the well rather than having the well turned off for periods of time. For example, in locations where the above-ground pump works could freeze in cold weather if the flow of fluid is stopped, it is advantageous to maintain a continuous output.
In the present invention, the controller periodically measures the fluid level in the well and adjusts the speed of the pump to maintain a minimum desired head of fluid above the inlet to the pump. This will optimize the production of the well, maintain a continuous output from the pump and prevent even partial pump-off.
It is critical to the operation of a pump controller that the fluid level measurement be reliable. There have been prior attempts to produce a fluid level measuring instrument that will return a suitable signal for use by a controller. Most fluid level measurement methods utilize an estimated acoustic velocity and the return time of the echo of an acoustic pulse that is generated at the top of the well and allowed to travel down the well casing and reflect from the fluid surface. There are problems with these prior methods because of changes in the acoustic velocity in the gases above the fluid surface due to changes in gas composition and changes in pressure in the well casing.
Prior attempts have been made to calibrate for these changes. For example the acoustic velocity device described in U.S. Pat. No. 5,200,894 by McCoy uses reflections off the collars on the production tubing string to calculate an estimate of the acoustic velocity.
Prior methods for determining depth in a well as described in U.S. Pat. No. 5,200,894 by McCoy use a method of counting tubing joints and applying the average distance between collars to the number of joints counted. A disadvantage of these methods lies in the method of detecting the collars. The present invention provides an improvement in the method of detecting collars because it uses an amplitude demodulation of the amplitude envelope of a broad-band signal received from a microphone rather than using the fundamental frequency component of the collar reflection signal.
The present invention also uses an acoustic pulse transmitted into the well casing, but it uses an improved method to detect the collar reflections from the top of the well to the point of reflection from the fluid surface and then accumulates the known distance between the collars which hold together the tubing sections to determine the distance from the top of the well to the fluid surface.
Prior art methods of analyzing the signal received from the microphone caused by the reflection from collars assumes that the signal is the sum of several sinusoidal terms as shown in the following equation, which is reproduced in
FIG. 6
a:
A
1
sin(&ohgr;
1
t+&dgr;
1
)+A
2
sin(2&ohgr;
1
t+&dgr;
2
)+A
3
sin(3&ohgr;
1
t+&dgr;
3
)+ . . . (1)
The sinusoidal term (the A
1
sin(&ohgr;
1
t+&dgr;
1
) term) representing the fundamental frequency caused by an acoustic wave front reflecting from the collars is filtered from the signal.
The collar reflection frequency depends on the collar spacing and the speed of sound in the gases in the annulus. Considerable effort is used in prior art methods to determine this fundamental frequency and filter the signal from the microphone with a very selective bandpass filter to exclude all of the signal except the collar reflection frequency. Prior art methods of determining the distance from the top of the well to the fluid surface measure the return time of the reflection signal from the fluid surface and by applying the average speed of sound in the annulus they can calculate the distance. In prior art arrangements the purpose for detecting collar reflections in the reflection signal is to allow for calculation of the speed of sound at various depths in the annulus.
In developing the present invention, it was determined that if the acoustic pulse transmitted down the annulus has a fast rise time then the signal received by the microphone appears to be an amplitude modulated signal that can be represented by the product of sinusoidal terms of the following equation:
[B
1
sin(&sgr;
1
t+&tgr;
1
)+B
2
sin(&sgr;
2
t+&tgr;
2
)+ . . . ]*
[C
1
sin(&lgr;
1
t+&kgr;
1
)+C
2
sin(&lgr;
2
t+&kgr;
2
)+ . . . ] (2)
The acoustic system in the well appears to be underdamped and a reflection of the incident acoustic pulse from a collar will generate an amplitude modulated acoustic reflection signal. By receiving a wide band signal from the microphone and performing an amplitude demodulation, the coefficients of the sinusoidal term representing the collar reflection frequency in the demodulated signal provides a significantly better signal than the signal obtained by filtering the sinusoidal term (the A
1
sin(&ohgr;
1
t+&dgr;
1
) term) representing the fundamental frequency caused by an acous
Baker James A.
Dyck John G.
Halisky Ronald W.
Circa Enterprises Inc.
Loo Dennis
Sughrue Mion Zinn Macpeak & Seas, PLLC
Williams Hezron
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