CA1140663A - Automatic liquid level monitor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Apparatus for determining the location of the liquid surface in a well or bore hole has a source of acoustic pressure pulses coupled to the surface casing so that they are transmitted down the well and reflected by the gas-liquid interface. A transducer is also coupled to the well to produce an electrical signal in response to direct and reflected acoustic pulses occurring in the well. A clock pulse source is provided together with a pulse counter for producing one pulse output for a certain number of input pulses and having its input connected to the clock pulse source to apply the clock pulses as input pulses. The pulse counter is connected to a calibrating device capable of adjusting the number of input pulses required to produce a single output pulse. Gating circuitry responds to the transducer for starting and stopping the counting of clock pulses by a digital counter and a digital read out device is utilized to indicate the depth to be liquid level in the well.
The source of acoustic pulses is actuated automatically at predetermined times, and the depth to the liquid level and the time are recorded automatically. The monitor can be used to control the operation of a pump in the well.

Description

66~

Back~round of the Invention This invention relates to well sounding and more particularly to apparatus for and methods of automatically continuously deter~ining the depth of the liquid surface of a well and for automatically controlling the depth. This ap-plication is a àivision of our co-pending Canadian Patent Application No. 241,616, filed December 12, 1975, now Canadian Patent No. 1, G91,338, issued December 9, 1980.

Prior Art The state-of-the-art techniques are represented by the Sonolog'`(trademark) instrument made by the Keystone Development Corporation of Houston, Texas, and the Echometer instrument made by the Echometer Company of Wichita Falls, Texas. The Echometer instrument is of the type shown in U.S. Patent No.
3,316,997 - McCoy. The recordings of acoustic amplitude vs.
time are essentially the same for both instruments and are illustrated in Figures 8 and 9 of U.S. Patent No. 2,232,476 - Ritzman. When the electrical output signal of the acoustic transducer is passed through a high-pass filter, the short, high-frequency reflections from tubing collars are emphasized.
~hen a low-pass filter is used, the low-frequency pulse from the gas-liquid interface is more easily distinguished on the record. The depth to the liquid surface is then determined by counting the number of tubing collars above the liquid interface and multiplying by the average distance between the collars.

Commonly, the initiating acoustic pulse is generated in the annulus between the well casing and the string produc-tion tubing in a producing oil well. The source of the initiating acoustic pulse is uslially a blank cartridge fired into this annulus, such as described in U.S. Patent No. 3,100,023 - Clements. Another less-used technique is to discharge com-pressed gas into the annulus as is described in PETROLEUM
PRODUCTION ENGINEERING - OIL FIELD EXPLOITATION, Third Edition, by Lester C. Uren, McGraw sook Company, Inc., New York, Toronto, 5 London (1953), pp. 143-14~. State of the art well sounding techniques such as these require an operator to trigger the acoustic source, manipulate the filter circuit and obtain the resultant record on a time-driven stip chart recorder. Because of this, it is not economical to use this technique to obtain 10 ~requent measurements of the liquid level to trace the movement of the liquid column over long periods of time. The movement of this liquid column in response to the inflow of fluid into a shut-in well or during pump down after the well has been shut in and allowed to build up is especially important to the 15 petroleum engineer in analyzing the well performance character-istics.
This invention relates to a method of repetitively sounding a well over a long period of time which does not re-quire an operator to be present at each sounding. Another 20 advantage for repetitively taking well soundings over long periods of time is that it is often not possible to obtain the uid level on a one-shot basis because of foam or a bubble blanket on the liquid surface. When this condition exists~ the acoustic pulse is absorbed and no reflected pulse is generated.
25 It has been demonstrated that in most cases the formation of foam on the liquid surface is a transient condition. Thus, by ~ .

~ -3-repetitive sounding the true liquid level can be determined during the occasional periods when the foam disappears and a reflection is generated. Only a fortuitous firing using the usual single-shot method would produce a usable well sounding record.
Another source of error in the state-of-the-art techniques is the lack of accuracy in detexmining the liquid depth. Since the tu~ing collars are used to measure the depth, the actual distance between these collars or the length of each joint of tubing must be known. This is seldom the case except 10 where the tubing is strapped as it is being placed in the well.
As an exampler tubing lengths assembled in a string may be nominally 30 = 2 feet in leng~h. In a well having, for example~
150 joints of tubing, the variations from nominal length may be cumulative and the depth error may be appreciable. Also, it is 15 difficult by this method to determine the depth to an accuracy greater than one joint of tubing (`about 30 feet).
Another source of error involves the operator's in-terpretation of the record and his skill in manipulating the filter settings. The problem of interpretation is especially ~0 noticeable in deep wells that exhibit a relatively high bottomhole temperature. The acoustic velocity increases with an increase in temperature. Therefore, the tubing collars near the bottom of the well will appear closer together on the chart record than the upper collars. In most cases the 25 reflection from these lower collars are too small to be recorded and are ignored. In establishing the depth scale in this case, only the upper collars are generally used. This results in an interpretation error that may be large. This also emphasizes the lack of consistency in interpreting many successive soundings on one well.

Summary of the Invention _, This invention overcomes many of the shortcomings of the state-of-the-art techniques described above. It provides a method of obtaining the buildup and drawdown characteristics of 10 a producing well with great accuracy. By calibrating at the maximum depth to the fluid interface, an accurate determination of the average acoustic velocity in the well can be determined.
Variations in the fluid depth of only a few feet can be observed by this method without any interpretive errors. No change in 15 filter or gain settings is required. The low-frequency return pulse fro~ the liquid surface is the only reflected pulse necessary for satisfactory operation. The automatic firing of the initiating pulse and recording of the liquid depth and time do not require an operator to be present and provide a method ~0 o~ measuring fluid levels under transient foaming conditions wh~n taken over long periods of time. This ability to record li~uid depth while the apparatus is unattended is particularly important in logging a marginally producing well in which pressure buildup must ~e measured over a long period of time.
In accordance ~ith this invention~ the location of the liquid surface of a well is determined by repetitively 6~

actuating a source of acoustic pulses and continuously indicating a digital reading of elapsed time between each initiating acoustic pulse and the reflected pulse.
In accordance with another aspect of this invention, the well sounding apparatus is calibrated for the differing 5 acoustic velocity characteristics of each well by pumping the liquid in the well down to the known level of the pump inlet in the well end changing the number of pulses digitally counted until the indicated digital count corresponds with the known level of the pump in the well.
In accordance with a specific embodiment, a manually adjustable pulse counter produces one pulse output ~or an ad-justable number of input clock pulses. By manually adjusting this pulse counter, calibration for acoustic velocity is ; obtained.
In accordance with another aspect of the invention, the circuits which produce the starting and stopping signals for the digital counter have adjustable trigger levels to make them immune from noise which might otherwise cause erroneous `~ indication of elapsed time or depth.
In accordance with another aspect of the invention, a calibrated mute time circuit is actuated coincidentall~ with the initiating acoustic pulse and renders the circuit which stops the digital counter ;noperative for a known adjustable period of time after the occurrence of each initiating 25 acoustic pulse. This calibrated mute time circuit allows the operator to gate out any undesired noise or reflected signal, especially those generated near the surface.
In accordance with another aspect of the invention, an adjustable firing time rate circuit provides means for adjusting the repetitive firing time of the source of acoustic 5 pulses.
In accordance with another aspect of the invention, a digital measure of the depth of the liquid level in the well is recorded together with a recording of the time of each initiating acoustic pulse with respect to the time that automatic operation 10 o~ the equipment was started.
In accordance with another aspect of this invention, the initiating acoustic pulses are generated by a solenoid valve releasing compressed gas in response to a timing circuit which determines how long the valve is open. In this way, shape and 15 duration of the initiating acoustic pulse can be changed in accordance with the mode of operation of the equipment.
In accordance with another aspect of the invention, the digital output of the well sounding apparatus of this in-vention is used to control the pump which maintains the liquid 20 level in the well at a desired depth or between two selected depths in the well. This arrangement has many advantages: (1) it prevents the well from "pumping off" so that "pounding"
does not occur, thus reducing maintenance costs~ (2) by using .
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~4~16~3 a running time meter on the motor and a total time clock, changes in the productivity of the well can be recorded and used to evaluate well performance, and (3) the entire operation call be monitored by a remote computer system so that the system can be monitored, optimized, and controlled.
Other objects/ features, and advantages of the in-vention will be better understood from the following description and appended claims.

Brief Descriptioh-of the Drawings FIG. 1 is a block diagram of the automatic liquid level ~onitor;
FIG. lA shows a typical well makeup;
FIG. lB shows an alternate connection for the sounder using a two-port valve on a pressured casing;
FIG. 2 is a block diagram of the automatic liquid level monitor used for intermittent pump control;
FIG. 3 shows the manner in which FIGS. 3A-3E together show a circuit diagram of the automatic liquid level monitor;
FIG. 4 shows a digital tape readout of time and depth 20 o~tained with the monitor;
FIG. 5 shows a buildup curve on a well using data obtained ~ith the monitor; and FIG. 6 shows a typical drawdown curve on a well using data obtained with the monitor.
~5 Description of th-e Preferred Embodiment FIG. lA shows a typical ~ell in which the automatic monitor may be used. ~ string of tubing 11 is suspended by a tu~ing hanger 12 ~ithin a casing 13. The tubing includes a num~er of sections joined ~y tubing collars 14. The tubing has 5 a pump inlet 15 and usually terminates in a gas anchor 16. A
sucker rod 17 extends through a packing gland 18. ~ pump 19 is connected to the suckex rod. The casing nipple 2a is used to couple acoustic pulses to the annulus 21 ~et~een the string of tubing 11 and t~e' casing 13.
lQ Referring to FIG. 1, the source of acoustic pulses in-cludes a compressed gas cylinder 22 which commonly contains ni-trogen. Gas from cylinder 22 flows through pressure regulator 23 and through the three-way solenoid valve 24 to the expansion chamber 25. Solenoid valve 24 repetitively couples gas from the 15 chamber 25 to the casing nipple 20 to generate initiating acoustic pulses in the well annulus. If the gas pressure in the well annulus is a~ove about 30 psig, then a two-port valve 24a (FIG. lB~ can ~e used to produce atmosphere. This method can be used for generating the initial pulse in all the embodiments ~0 disclosed here, if the annulus pressure is sufficient to meet the requïrements.
The opening of soleno~d valve 24 is cantxolled ~
valve driver 26 whi`ch has a manual adjustment 27 for controlling the duration of each acoustic pulse. Digital electronics in-25 cluding a firing time d;vider circu;t 23 and a firing time .

control circuit 29 are used to obtain an initiating acoustic pulse according to any desired time schedule. ~lternatively, the source may be manually initiated by depressing the firing push button 30.
The firing time circuits are controlled by the clock pulse source 31. Clock pulses from this source are also used in the digital readout circuitry.
An acoustic transducer 32 is coupled to the well and produces an electric signal in response to the occurrence of 10 acoustic pulses in the well annulus. Electric signals from thetransducer 32 are applied to gating means, including a first voltage com-parator circuit 33 and a second voltage comparator circuit 34. Voltage comparator circuit 33 produces a start pulse. Upon the occurrence of the initiating acoustic pulse in the well, this starts counting of clock 15 pulses by digital means, includin~ preset counter 35 and depth counter 36.
The second voltage comparator 34 produces a stop pulse upon detection of the acoustic pulse reflected from the liquid surface of the well. The stop pulse is connected to the preset counter 35 to stop the counting of clock pulses.
The first voltage comparator circuit 33 has an ad-justable trigger level set by the potentionmeter 37. This allot~ls the operator to set the level of the signal which pro-duces a start pulse so that the circuit detects the initiating acoustic pulse. Similarly, the second voltage comparator 25 circuit 34 has an adjustable trigger level set by the -lQ-;3 potentiometer 38 so that the circuit detects the desired re-flected pulse.
The mute time circuit 39 is actuated by the solenoid valve driver 26 coincidentally with the initiating acoustic pulses. The mute time circuit 3g is connected to the second 5 voltage comparator circuit 34 to render it inoperative for a known adjustable period of time after the occurrence of each initiating acoustic pulse. Mute time circuit 39 includes a calibrated time adjustment 40.
Manually adjustable pulse counter 35 produces one 10 output pulse for a given number of input pulses and the given number is adjustable. Clock pulses from source 31 are applied as input pulses. The manually settable switch 41 is used to change the number of input pulses producing an output pulse.
This switch 41 is used to calibrate the liquid level monitor.
15 The pulse output from counter 35 is applied to the depth counter 36 which accumulates one pulse for each increment of depth in the well, for example, one pulse for each foot of depth Digital readout devices are responsive to the count in counter 36 for indicating the depth of the liquid surface.
20 These include a digital readout 42 and a digital printer 43.
These are controlled by the controller 44 which determines whether each of the devices is to display depth or elapsed time. The elapsed time accumulator 45 records time with re-spect to the start of automatic operation. That is, when a 25 well is being automatically logged by repetitive firing of 6;~

the acoustic pulse source, the accumulator 45 counts every one-minute pulse which occurc after the start of this automatic op-eration. This accumulated time can be displayed on the digital readout 42. The time is also recorded by the printer43 Printer 43 produces a printout 43a ~FIG. 4) of the depth of the liquid 5 level together w;th the time of occurrence of the initiating pulse producing that depth reading.
The operation of the apparatus in performing a well logging operation is as follows. The firing time rate circuitry including a divider circuit 23 and firing time control circuit 10 29 controls the firing time of initiating acoustic pulses which are coupled to the casing nipple 20. These acoustic pulses are detected by the transducer 32~ Clock pulses from source 31 are counted by counter 36 through the preset counter 35 in the time interval between the generation of an initiating acoustic pulse 15 and the detection of a reflected acoustic pulse. Digital read-out device 42 and printer 43 continuously produce an indication of the digital count as a measure of the depth of the liquid level in the well. At the beginning of an operation, the ap-paratus is calibrated for differing acoustic velocity character-20 istics as follows.
First, the pump 19 is started and the well is pumpeddown to the known level of this pump~ Assume that this is 2,000 feet. Assume that the acoustic velocity characteristic is 1. oaa millisecond per ~oot which is not known by the operator. The time 25 interval between the initiating pulse and the detection of the reflected acoustic pulse will be 4 seconds. The operator ~12-6~

adjusts the switch 41 until the digital readout 42 displays2,000 feet in depth upon manual actuation of the initiating pulse. At this point the manually adjustable counter 35 is producing one output pulse for each 1,000 input pulses. Therefore, the counter 36 is counting one thousand pulses per second. There is a divide-5 by~two set into the counters to convert two-way pulse travel into depthr so the depth counter actually receives 2,QOO pulses.
This procedure determines the acoustic velocity char-acteristic which can be read from the manually settable switch in milliseconds per foot. This acoustic velocity characteristic 10 can be used in logging other wells in the same field. These other wells will normally have the same acoustic velocity characteristic. When it is desired to log them, the acoustic velocity characteristic is set by the switch 41 without the need for going through the calibrating procedure.
lS Another calibration procedure can be performed with the apparatus described in this invention that can be used when the well cannot be pumped down and the actual liquid levelcannot be determined from other wells producing from the same formation.
The manually adjustable preset counter 35 is set to any conven-~0 ient velocity setting, such as one millisecond per foot, and the liquid level depth determined, for exampler 1818 feet. The trigger level adjustment of the voltage comparator circuit 34 is then set at a low level so that the acoustic pulses reflected from tubing collars are detected. The mute time adjustment 40 .

ii3 is then increased so that only the collars near the center of the gas column can be detected, for example, 909 feet. Under these conditions, the depth of a collar near the midpoint of the gas column will be indicated on the depth display. Assume a depth of 924 feet is recorded. The mute time is then increased 5 so that the next collar is detected at a recorded value of, for example, 951 feet. The difference indicates a collar separation of 27 feet when it should be about 30 feet. To correct the erro~, the velocity set on the preset counter 35 should be in-creased by the ratio of 30 to 27, thus yielding an average velo-10 city setting of 1.111 milliseconds per foot. This processshouldbe repeated on successive pairs of collars to obtain a more ac-curate average velocity figure. The new velocity of 1.111 milli-seconds per foot is then set on the manually adjustable preset counter 35, the mute time is reduced to eliminate only the near 15 surface reflections, and the trigger level adjustment isreturned to the normal setting for detecting the liquid level. The new liquid level is found to be more accurately located at 2020 feet and the calibration procedure is complete.
This calibration procedure assumes a linear tempera-~0 t~lre gradient in a well so that the average velocity in the an-nulus gas is near the middle of the gas column.
The voltage comparator 34, used to detect the return si~nal, can be preceded by an adjustable filter. This filter 25 can be adjusted to accept only low-frequency pulses for the 6~i~

detection of the liquid level or adjusted to only accept higher frequencies for the detection of the tubing collars. The ad-dition of the filter can greatly enhance the discrimination be-tween signals reflected from the liquid surface and the collars.
FIG. 2 shows the liquid level monitor of this invention 5 used to control the pump. The same circuits used in the FIG. 1 embodiment are used except that digital readout 42, printer 43, and associated circuits axe omitted.
In their place, two manually preset dividers 46 and 47 are used. Divider 46 produces a pu]se when the liquid level rises 10 above the high level set point manually set into the divider. Di-vider 47 produces a pulse when the liquid level drops below the low level set point. The output of divider 46 is used to start the motor, and the output of divider 47 is used to stop the motor. By this means, the well can be pumped in its most ef-15 ficient mode. The high level set point would be adjusted so thatthe liquid level would not rise high enough to limit the inflow to the well and the low level set point would be adjusted to a level just above the pump inlet.
As an optional feature, a running time meter 52 can be ~0 installed in the motor line to indicate the total ON time. A
total time indicator 54 can also be used to measure the total time the well is under local control. The total ON and OFF times can then be determined and a record of these times would in-dicate a change in productivity of the well.

This entire system can be monitored and recorded by a remote computer system so that a complete record of well per-formance can be maintained. This system would also indicate a broken rod condition, loss in pump efficiency, and loss of con-trol of the system as well as the well performance. The actual 5 sounding of the well could also be initiated from a remote point at any time by overriding the solenoid timing circuit. Theactual depth to the liquid in this case could be observed by placing an appropriate modem on the output of the adjustable preset counter 35.
The velocity of the acoustic wave in the automatically controlled well is preset manually upon installation, as in the case of the portable unit. This system works well with the pump running. The mute time control is used as in the case of the portable unit. The firing time circuit can be adjusted to any 15 convenient cycle time and can even be modified to sound the well at more frequent intervals during pump down than during buildup.
This change in sounding time may be necessary since most wells pump down much faster than they build up. This modification con-serves the gas supply. Of course, if the gas pressure in the ~0 ~nnulus is high enough, the two-port valve system can be used and no external gas supply is necessary.
If a variable speed motor control system is usedr this circuit can be used to maintain a constant liquid level under continuous pumping conditions. In this case, the depth ~5 to the liquid would be measured in the same way. However, only one set-point detector would be used. If the liquid rises in the well, the set-point detector furnishes an analog voltage pro-portional to the deviation from the set point. This voltage is used to increase the motor speed. If the liquid falls below the set point, the motor speed is decreased until equilibrium is at-5 tained A digital-to-analog linear converter is included in the set-point circuit of this embodiment.
The automatic control of the pumping rate, either by ON-OFF or proportional control operation, is a feature of the invention. Not only will direct pumping control maximize the 10 pumping efficiency and decrease maintenance, it also provides very important information on the rate of buildup, or the inflow rate and its change over extended time periods.
The circuitry of this invention may take many forms within the skill of a person working in this art. The circuitry 15 shown in FIGS. 3A-3E is an example only. The function of the in-dividual integrated circuits in this diagram is indicated either by the shape of each unit or letters within a rectangle. These symbols conform to the American Standard Graphic Symbols AIEE 91-ASA Y 32.14-1962. These integrated circuits can be ~0 obtained from a number of manufacturers.
FIG. 3A shows the transducer 32 which produces an electric signal in response to the occurrence of acoustic pulses in the annulus. This electric signal is applied to the first voltage comparator circuit 33 and to the second voltage compar-25 ator circuit 34. soth circuits have a potentiometer, 37 and 38, 6~

respectively, for adjusting the trigger level. The outputs of circuits 33 and 34 are applied to circuitry including NAND gates 61 and 62 and flip-flops 63 and 64. AND gate 65 produces an output which controls the starting and stopping of the preset counter 35, shown in FIG. 3D.
FIG. 3A also shows a switch 66 which controls whether the readout 42 and the printer 43 display time of depth. The out-put from the delay flip-flop 67 starts the printer 43 recording time with respect to the start of automatic operation.
FIG. 3B shows the mute time circuit 39 having a po-10 tentiometer 40 which is used to adjust the mute time. The solen-oid valve driver 26 is a flip-flop having a potentiometer 27 for controlling the time length of the initiating acoustic pulses.
The flip-flop receives a fire signal from the firing time con-trol circuit 29, shown in FIG. 3C, or it receives a fire signal 15 from the manual fire push button 30.
FIG. 3C shows the firing time divider circuit 28 and the time control circuit 29. These circuits receive clock pulses at one-minute intervals and produce fire pulses at intervals of five minutes, ten minutes, or 30 minutes.
FIG. 3D shows the manually adjustable preset counter 35. The digital switch is indicated diagrammatically. The manually settable digital switch 41 is of the type manufactured by Electronic Engineering Company of California, four-digit unit, Model 4B1776206. The connections are as indicated.

'' ~4(~66~

FIG. 3E shows the depth counter 36 and the elapsed time accumulator 45. In one actual embodiment of the invention, the L-100 incandescent four-digit readout manufactured by Luminetics Corporation of Fort Lauderdale, Florida, was employed as the display 42. The digital printer 43 was the model PN107 5 Sodeco* Impulse Counter manufactured by a division of Landis &
GYR, Elmsford, New York. An interface unit P-105 is used to convert from the sCD code to the printer code. The multiplexing of the input to the display 42 and printer 43 is controlled by the Time/Depth Print Controller of the circuit shown in Fig. 3A
10 so that the time and depth of any one reading are printed sequentially on the printer tape.
FIG. 5 shows a buildup curve 55 that can be derived by plotting the data shown in FIG. 4. These data would be obtained with an automatic firing time rate of every five minutes during 15 the first hour, every ten minutes during the second and third hours, and every half-hour thereafter.
FIG. 6 shows a drawdown curve 56 that can be derived by plotting data obtained with the monitor of the present in-vention. The timing of the initiating pulses is the same as in ~0 the buildup curve.
Smooth curves such as shown in FIGS. 5 and 6 can be expected to result from data obtained with the monitar if the produced fluid is absent gas. With the presence of gas slugs which brea~ out, there will appear sharp changes in the curves, 25 particularly during a buildup test. However, because of the large number of data points r corrections may be made to smooth the shape of the curve.
*Trade Mark 6~

Curves 55 and 56, shown in FIGS. 5 and 6, are useful in the interpretation of well performance as well as equipment per-formance. From these curves may be determined the efficiency of the pump and to establïsh pump cycle time. For example, in esta-blishing efficiency of a pump, a straight line 57 is drawn as an 5 extension of the straight line segment 58 of the curve 56. The intersection of the line 57 with the abscissa establishes the data for computing pump efficiency. In the example shown, the pump is withdrawing liquids at the rate of 85 ~eek per hour.
Knowing the volume per foot of the annulus in the well, one may 10 readily convert feet per hour to barrels per hour and thus com-pare this figure with the rate at which the pump should be pro-ducing under the field-operating conditions for that pump. If the value established from the curve of FIG. 6 is well below that of the field rating of the pump, then the operator may de-15 sire to pull the pump and make repairs to seals or to movingparts thereof to restore efficiency.
Pump cycle time can likewise be established from the data of the curves of FIGS. 5 and 6. It is desirable that the amount of buildup in the well be held to a level such that the 20 back pressure does not materially impede the inflow rate of liquids. Likewise, it is undesirable to continue the operation of the pump after the liquid level has fallen below the level of the pump inlet. To do so would cause physical damage by reason of pounding.

Having information as provided by the data in FIGS. 5 and 6, one has knowledge as to the rate of inflow as well as the pump rate and therefore may establish pump cycle time for most efficient production of the well.
The data represented ln the buildup curve 55 of FIG. 5 5 can be utili2ed to determine an estimate of formation pressure.
There are a number of techniques to determine formation pressure, one of them being described in an article entitled "Use of Data on the Build-up of Bottom-hole Pressures" by Morris Muskat, ap-pearing in SPE Reprint Series No. 9, Pressure Analysis Methods, 10 AIME, 1967.
While a particular embodîment of the invention has been shown and describedr various modifications are within the true spirit and scope of the inventïon. One modification is to utilize a digital-to-analog converter driven by the depth 15 counter to provide an analog strip chart recording of the depth of the liquid surface vs. time. The following claims are, thereforet intended to cover all such modifications.

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SUPPLEMENTARY DISCLOSURE

The liquid level monitor shown in Fig. 2 can be used as described to operate a pump for the well. Pumping wells generally have a pumping capacity greater than the inflow into the well. This is done to compensate for the loss of efflciency due to wear on the valves and plunger with time and to eliminate the need to change the pump if repressuring of the formation is contemplated or the gas cut is expected to increase. As a result of this over-sizing, most of the rod pumped wells will "pump-off," or draw the produced liquids down to the pump inlets. When this condition occurs, solution or free gas will be drawn into the pump chamber forming a gas cap on the upstroke o~ the pump. On the downstroke, the plston will fall freely through the gas cap and strike the liquid surface. This condition is called "pounding" and pro-duces high stress reversals in the sucker rod strong. This pump~n~ condition should be avoided to prevent rod failure.

Most state-of-the-art control systems are pump-off controllers which reduce the number of times a pump "pounds"
but do not eliminate the problem . Ideally, the liquid in the well should be allowed to build up to approxi-mately 10 to 15 percent of its shut-in height and then pumped down to within a few feet of the pump inlet.

. . . . :

~:~4~
This method of pumping not only eliminates "pounding"
but will produce the well more efficiently. The con-troller of this invention accomplishes this method of pump control.

Pump-off controllers are popular because of their low cost and ease of installation. One type, which is shown in rJ. S. Patents 3,817,094 and 3,838,597 Montgomery et al, measures the load on the walking beam of the pump by means of a strain gage welded to the up-per web of the beam, and is manufactured by End Device, Inc. Of Midland, Texas for the Halliburton Co. When the !
well is pumped off, the ~eam load changes rapidly because of "pounding." This rapid change in load is sensed by the strain gage transducer and`the well is shut off after three such changes in load are sensed. The off time of the pump is controlled by a manually adjustable timing clock.

Another type of pump-off controller responds to a change in the pumping motor current when pump-off occurs. Again at least three pounding cycles are de-tected to p~event~false triggering and the,,,off time is set by the timing clock.

Another controller measures the actual flow rate of the well by means of a flow line differential pressure ga~ge. The down time and pump up time are set by the operator. If the well pumps-off, the flow trans-ducer will shut off the pump. The operator must then change the various times until the well is operating pro-perly. This system is not self-adaptive to changes in the productivity of the well and therefore is not a truly automatic pump controller.

~.~41~

Another controller measures the bottom-hole pressure during pumping. This system requires a cable clamped to the production tubing to provide the control signal at the surface.
A hi~h fluid level is set on the monitor and the pump is shut o~f on pump down. Because of the labor costs of installing the wire line and the problems involved in servicing the well, this system is relatively expensive and has not proved to be popular.

The pump controller of the present invention employs the circuit shown in Fig. 2 of the drawings to operate the pump. The acoustic puIses are produced and are detected after reflection from the liquid surface. The time difference be-tween each acoustic pulse and the detected reflected pulse is determined. This time difference is used to control the operation of the pump.

In accordance with a preferred aspect of the invention, man~ally settable digital registers calibrated in units of depth contain the high level and low level set points for the pump and the mute depth. A multiplexer sequentially compares these registers to the digital liquid depth as de-termined by counting clock pulses between the initiating acoustic pulse and the detection of the reflected acoustic pulse.

In accordance with another aspect of this invention, measurements of liquid depth which turn the pump on are confirmed by generating several initiating acoustic pulses and counting clock pulses at frequent intervals to prévent turn off of the pump by an erroneous reading.

In accordance with another aspect of the inven-tion, an alarm is actuated when the measured liquid depth - ~4~366;3 continues to exceed the high level set point after the counted clock pulses exceed the high set point level.

In cccordance with another aspect of the invention, the well cont.oller is used to pump a set allo~Jable volume of liquid fro~l a well during a given time period.

BRIEF DESCTI?TION OF THE DRAWINGS ACCOMPANYING
THE SUPPLF~.~IENTARY DISCLOSURE

Figs, 2A and 2B show the gating circuit, counter and firing control circuit for the well controller;

Fig, 2C shows the digital registers for the controller;

Fig. 2D shows the digital circuitry for controlling the pump~ and Fig. 7 depicts a portion of a well.

Referring to Figs. 2A-2D, further details of the pump controller of Fig. 2 are shown. Figs. 2A and 2B show the circuitr~- of the counter 35 and the firing time circuit 29.
Acoustic transducer 32 is coupled to the well and produces an electric signal in response to the occurrence of acoustic pulses in the well annulus. This electric signal is applied to first voltage comparator 33 which produces a start pulse.
Upon the occurrence of the initiating acoustic pulse in the well, this starts counting the clock pulses by the preset counter 35.

The output of the transducer 32 is also connected to the filter 72 which normally passes 5 Hz pulses reflected from the liquid level in the well. Filter 72 can also be set to pass 20 Hz pulses reflected from tubing sollars. The ~4~

center frequency can easily be changed to satisfy a particular well.

The second voltage comparator circuit 34 produces a stop pulse upon detection of the reflected acoustic pulse. The stop pulse is connected to the preset counter 35 to stop count-ing. The stop pulse is applies to the counter through the mute gate 39A. The mute gate 39A is disabled and enabled by a MUT~
signal which is produced in the circuit of Fig. 2D. The mute signal disables the gate 39A until the count in depth counter 36 (~ig. 2C) reaches a selected count corresponding with a selected depth. At this time the MUTE signal enables the mute gate 39A.

Figs. 2A and 2B also show the firing time circuit 29, scillator clock 31 and peak meter 73, all of which are more fully described in the original application.

Fig. 2C shows the manually settable registers 46B and 47B included in the digital high level and low level set point circuits 46 and 47, the manually settable register 39B for setting mute depth. Pulses representing one ~oot increments are produced by the counter 35 of Fig. 2. These pulses, referred to as "foot clock," are applied to the depth counter 36 which includes counting stages 74-77.

The manually settable digital high depth register 46B includes stages 78-81. The manually settable digital low depth register 47B includes stages 82-85. A manually settable mute depth register 39B includes stages 86-89. A multiplexer 90 sequentially applies the contents o~ these three registers to a comparator 91 which includes stages 92-95. The contents o~ the register are sequentially compared ~ith the depth as represented by the count in the depth counter stages 74-77.

The operation can best b`e understood with reference Ei6:~

to Fig. 7 which depicts a portion o~ the well between about ],900 feet and 2,200 ~eet. Assume that the mute depth register 39B
has been set to a mute depth of 1,900 feet; the high depth re-gister 46B has been set to a high depth of 2,000 feet; and the low depth register 47B has been set to a low depth of 2,200 feet.
5 After the occurrence of an initiating acoustic pulse, the depth counter 36 starts counting pulses. When the number counted ex-ceeds 1,900, comparator 91 produces a "compare" output which is applied to the timing circuit 96 of Fig. 2D. The timing circuit 96 responds to a compare signal to produce either a "mute", "low"
10 or "high" signal depending upon the condition of multiplexselect timing circuit 97. If the multiplexer is comparing the contents of the mute register with the depth register, a MUTE signal is produced. Thisenables the mute gate 39A (Fig. 2B) so that the next occurring detected reflection will stop the counting of 15 preset counter 35 (Fig. 2) and of the depth counter 36 (Fig. 2C).
When the depth counter 36 is stopped, its contents are compared with the contents of hi~h depth register 46B and low depth re-~ister 47B. MUX select timing circuit 97 (Fig. 2D) controls this sequential comparison. If the multiplexer 90 is comparing the ~0 contents of the low depth register 47B with the depth counter 36, a "low" signal ma~ be produced; and if the multiplexer is comparing the contents of the high depth register 46B with the depth counter 36, a "high" signal may be produced.

Assume the liquid level is between the high and low ~5 depth set points as shown in Fig. 7. In this case, no "compare"
output is produced and the pump is neither started nor stopped, but continues in its present state of energization. I~ the depth counter 36 iS greater than 2,000 ~eet, a compare output is produced which starts the pump. If the contents of depth counter 36 are greater than 2,200 feet, a compare is produced which stops the pump.
Fig. 2D also shows the circuitry which turns the pump on and off.
Before the pump is turned on, reverification readings are required. Counter 98 counts the number of high signals produced. After three have been counted, pump control 99 is turned on. If any one of the three shots should show the level to be below the set point, the timer will revert to its normal 10 selected timing c~cle and the next indication of a high level will again cause the one minute confirming cycle to take effect.
After the pump is started, the firing time will still be at its normal selected timing interval. After this time~ if the liquid level is still high and is confirmed by three outputs from 15 counter 98, the alarm counter 100 produces an output. This actuates an alarm indicating pump failure.
When the well is pumped down to the lower set point, timing circuit 96 produces a "low" signal. Verification counter 101 requires one confirming shot at a one minute interval to 20 shut off the pump.
A permanent record of the liquid level and on and off times is produced on the strip chart recorder 102. The recorder will indicate the depth to the liquid relative to the high set point on the main portion of the chart and an event marker on ~5 the right hand edge of the chart will indicate whether the pump motor is on or off. Since the chart is driven by a timing motor, the resultant plot is liquid level vs. time and the length of the on and '663 off t~mes of the pump. A selector switch is used to change the span of the recorded liquid depth scale from 50 to 250 feet in 50-foot intervals. This selection of ranges will cover a majority of the pumping wells, but can be changed to other selected intervals, i~ required.

This information should prove valuable to the opera-tor in determining changes which can a~fect production.
lf the pump on time begins to increase, it may indicate pump wear and incipient failure. Likewise, an increase in the off time indicates a reduction in the inflow rate due to decrease in formation pressure or an increase in apparent skin effect, or both. Of course, if the inflow rate increases, the pump time should also increase, but the off time would decrease proportionately. This could provide valuable information, especially in a secondary recovery project.

The measured depth of the on and off cycle are stored in memory 103. The buildup counter 104 registers the liquid level as referred to the high depth setting and is stored ~n latch 105 and transferred to the recorder 102. This pro-duces a plot of the liquid buildup and drawdown throughout the controlling cycle. The liquid depth may also be displayed in feet on a digital liquid depth display.

The circuitry of this invention may take many forms within the skill of a person working in this art. The cir-cuitry shown in Figs. 3A-3~ of the original disclosure directed to the well monitor is one example only of the circuitry used to implement Fig. 1.

The c~libration of the controller can be accomplished by setting the mute depth to approximately halfway down the ~ - 29 -gas column. The first collar reflection below this depth will stop the counter and display its depth when the filter is set to p25S high frequencies. The muke depth is then increased so that the next collar is detected. The difference in these depth readings is the tubing joint length and provides 2 means o~ calibration based on the known distance be~ween collars.
Of course, several tubing joints should be me2sured in this .anner to improve the accuracy of the calibration.

Normally the controller can most easily be calibrated to the pump-down depth as more fully described herein.

After the acoustic velocity calibration has been accomplished, the high and low set points must be selected.
This selection is most easily done if a buildup and draw-down cha~acteristic of the well is obtained. This can be obtained b~ use of the monitor as described hereinabove The high set-point is usually selected to be at a liquid level which is 10 to 15 percent of the final buildup level to be expected if the well is shut in ~or a very long time. This rule-of-thumb was arrived at by observing on the buildup curve that the inflow of the well is not restricted appreciably du~ing this period of the buildup.
Referring for example to the buildup curve shown in Fi~. S of Canadian Application No. 241,616, now Canadian Patent No. 1,091,338, the high and low depths are selected so that the well is always operating on the linear portion of the buildup curve. For example, the well whose buildup characterlstic is depicted in Fig. 5 might have a low depth of 3,400 feet and a high depth of 3,300 feet. The lower set point is selected to be a few feet above the pump inlet.

i6~

The repetitive ~iring time is selected next by usin~
the buildup and draw-down curves. The liquId level can be de-termined at intervals of 1, 5, 10 and 15 minutes. In hi~h volume wells, it may be necessary to set the firing time at one minute intervals. In low volume wells, fifteen minute intervals may be 5 adequate.
After these adjustments are made, the controller can be placed in automatic operation and will perform the control functions without any further attention.
The well controller can also be used to pump a set 10 allowable from a well during each 24-hour period. This can be accomplished by first obtaining an accurate well test which will define the ratio of water-oil produced by the well. Since the amount of oil in the liquid phase is known and the volume in terms of feet of liquid per barrel in the annulus is also known, 15 the controller will produce a known amount of oil between the high and low control points set on the instrument during one pump cycle. The requisite number of pump cycles can then be set to make the oil allowable. After the allowable is produced, the well is shut-in until the next 24-hour period arrives and the 20 process is repeated. Since the liquid level will rise above the upper set point during this off period, the instrument can be made to measure this liquid level even though it is above the high set pointand simply accumulate the actual number of feet the liquid level has dropped.
All of the information accumulated by the well controller can be transmitted to a remote readout or central computer for observation of the operation. The alarm signal from the controller and the on-off pump cycle-times can be transmitted to a remote location to allow early detection of an abnormal situation. It is also possible to control the high and low set points, ana to control on the actual volume of liquid produced from the annulus by use of a central control 5 computer.
While a particular embodiment of the invention has been shown and described, various modifications are within the true spirit and scope of the invention. One modification is to utilize a digital-to-analog converter driven by the depth 10 counter to provide an analog strip chart recording of the depth of the liquid surface vs. time. The following claims are, therefore intended to cover all such modifications.

' .

Claims (31)

WE CLAIM:
A. CLAIMS BASED ON THE ORIGINAL DISCLOSURE

1. Apparatus for controlling a pump suspended in a well comprising:
a source of acoustic pulses coupled to said well;
a transducer coupled to said well producing an electric signal in response to the occurrence of acoustic pulses in said well;
a clock pulse source;
digital counting means;
gating means responsive to the output of said transducer for starting the counting of clock pulses by said digital counting means upon the occurrence of an acoustic pulse in said well and for stopping said counting upon detection of the acoustic pulse reflected from the liquid surface in said well; and a digital controller responsive to said digital counting means for controlling the on and off times of the pump, a digital indication of liquid depth from said digital counting means being applied to said controller and the controller operating to maintain the location of the liquid surface in the well at a desired depth or between two selected depths.

2. Apparatus as claimed in claim 1 wherein said digital controller includes a high level set-point circuit which turns the pump on when the liquid level in the well exceeds the high set-point level; and a low level set-point circuit which turns the pump off when the liquid level falls below the low set-point level.

3. Apparatus as claimed in claim 1 further comprising:
means responsive to said clock pulse source for repetitively actuating said source of acoustic pulses so that said digital controller controls said pump at predetermined times over a period of time.

4. Apparatus as claimed in claim 1 or claim 2 wherein said digital counting means includes:
a manually adjustable pulse counter producing one pulse output for an adjustable number of input pulses, said clock pulses being applied as input pulses, said adjustable pulse counter having means for manually changing the number of input pulses producing an output pulse in order to calibrate said depth counter for variations in acoustic velocity; and a depth counter for accumulating a pulse for each increment of depth in said well.

5. Apparatus as claimed in claim 1 wherein said gating means includes:
a first circuit having an adjustable trigger level, said transducer being connected to said first circuit to produce a start pulse upon the occurrence of the initiating acoustic pulse in said well, said start pulse being applied to start the counting of said clock pulses; and a second circuit having an adjustable trigger level, said transducer being connected to said second circuit to produce a stop pulse upon indication of an acoustic pulse reflected from the liquid surface in said well, said stop pulse being connected to stop said counting.

6. The apparatus recited in claim 5 further comprising:
a calibrated mute time circuit actuated coincident-ally with the initiating acoustic pulses and connected to said second circuit to render it inoperative for a known adjustable period of time during and after the occurrence of each initiating acoustic pulse.

7. Apparatus for controlling a pump suspended in a well comprising:
a source of acoustic pulses coupled to said well;
a transducer coupled to said well producing an electric signal in response to the occurrence of acoustic pulses in said well;
a clock pulse source;
digital counting means;
gating means responsive to the output of said transducer for starting the counting of clock pulses by said digital counting means upon the occurrence of an acoustic pulse in said well and for stopping said counting upon detection of the acoustic pulse reflected from the liquid surface in said well; said gating means including:
a first circuit having an adjustable trigger level, said transducer being connected to said first circuit to pro-duce a start pulse upon the occurrence of the initiating acoustic pulse in said well, said start pulse being applied to start the counting of said clock pulses;
a second circuit having an adjustable trigger level, said transducer being connected to said second circuit to produce a stop pulse upon indication of an acoustic pulse reflected from the liquid surface in said well, said stop pulse being connected to stop said counting; and a calibrated mute time circuit actuated coincident-ally with the initiating acoustic pulses and connected to said second circuit to render it inoperative for a known adjustable period of time during and after the occurrence of each initiating acoustic pulse.

8. Apparatus as claimed in claim 5 further including a digital mute depth circuit comprising:
a manually settable digital register calibrated in units of depth;
a comparator, said digital mute depth register and said counting means being connected to said comparator; and a mute depth circuit actuated coincidentally with the initiating acoustic pulses and connected to said second circuit to render it inoperative, said comparator being connected to render said second circuit operative when the count in said digital depth counting means equals the count in said digital mute depth register.

9. Apparatus as claimed in claim 3 wherein said means for repetitively actuating said source of acoustic pulses includes:
a firing time rate circuit, said clock pulses being connected to said firing time rate circuit, said firing time rate circuit having means for adjusting the repetitive firing time of said source of acoustic pulses.

10. Apparatus as claimed in claim 1 or claim 2 wherein said well has a string of tubing disposed within a casing and wherein said source of acoustic pulses and said transducer are coupled to the annulus between said tubing string and said casing.

11. Apparatus as claimed in claim 1 or claim 2 wherein said well has a casing with said pump suspended from a tubing string disposed within said casing, said apparatus further comprising:
a selectable band pass filter connected between said transducer and said gating means, said filter being settable for first band pass which passes transducer outputs representing acoustic pulses reflected from the liquid surface of said well and a second pass which passes transducer outputs representing acoustic pulses reflected from collars in said tubing string.

12. Apparatus as claimed in claim 1 or claim 2 and including a digital printing recorder responsive to said digital counting means for selectively printing the stored digital depths.

13. A method of controlling the pumping of liquid from a well which has a pump suspended therein comprising:
repetitively generating initialing acoustic pulses in the well;
detecting acoustic pulses reflected from said liquid surface;
measuring the time difference between the initiating acoustic pulse and the detected reflected acoustic pulse by digitally counting clock pulses in the intervals between the generation of each initiating acoustic pulse and the detection of the reflected acoustic pulse;
repetitively producing a digital count as a measure of the depth of the liquid level in said well; and turning the pump on and off in response to said digital count to maintain the depth of liquid level in said well at a desired level or two selected levels, and recording said time difference as a measure of the depth of the liquid level in said well while said depth is changing to produce a record of the rate of change in depth in said well.

14. The method recited in claim 13 wherein said well has a casing with said pump suspended from a tubing string disposed within said casing, further including calibrating for variations in the acoustic velocity characteristics of the well by the steps of:
pumping the liquid in said well down to the known level of the pump inlet in said well and while maintaining the liquid level approximately at the pump inlet repetitively generating initiating pulses; and between initialing pulses changing the number of clock pulses counted until the digital count corresponds with the known level of said pump inlet in said well.

15. The method recited in claim 14 further comprising:
recording the digital count which measures the depth of the liquid level and recording the time of each initiating acoustic pulse.

16. The method recited in claim 13 wherein the generation of initiating acoustic pulses is by the steps of:
opening a solenoid-operated valve which communicates with said well to release gas under pressure; and changing the time that said solenoid valve remains open to change the acoustic properties of said initiating pulses.

17. The method claimed in claim 13 wherein said well has a casing with said pump suspended from a tubing string disposed within said casing, further including calibrating for variations in the acoustic velocity characteristics of the well by the steps of:
filtering the detected acoustic pulses to produce an output representing only acoustic pulses reflected from collars in said tubing string;
muting the circuit, which stops said counting of clock pulses for a time after the generation of each initiating acoustic pulse so that only reflections from collars near the middle of said string are detected;
changing the muting depth to detect reflections from the next adjacent collar; and calibrating the acoustic velocity from known distance between said tubing collars.

18. The method claimed in claim 13 further comprising simultaneously recording the on and off intervals of said pump.

19. The method recited in claim 13 further comprising:
storing a digital representation of said measured time difference; and thereafter printing out a record of liquid level depth at each of said preset times.

20. The method recited in claim 13 wherein the step of recording includes:
recording said digital count as a measure of the depth of the liquid level in said well while said depth is changing to produce a record of the rate of change in depth in said well.

B. CLAIMS BASED ON THE SUPPLEMENTARY DISCLOSURE

21. Apparatus for controlling a pump suspended in a well comprising:
a source of acoustic pulses coupled to said well;
a transducer coupled to said well producing an electric signal in response to the occurrence of acoustic pulses in said well;
a clock pulse source;
digital counting means;
gating means responsive to the output of said transducer for starting the counting of clock pulses by said digital counting means upon the occurrence of an acoustic pulse in said well and for stopping said counting upon detection of the acoustic pulse reflected from the liquid surface in said well; and a digital controller responsive to said digital counting means for controlling said pump; said digital controller including:

a high level set-point circuit including a manually settable digital register calibrated in units of depth;
a low level set-point circuit including a manually settable digital register calibrated in units of depth;
a comparator, said digital registers and said counting means being connected to said comparator, said comparator being connected to turn said pump on when the liquid in said well exceeds the high set-point level and to turn said pump off when the liquid in said well falls below the low set-point level.

22. Apparatus as claimed in claim 21 wherein the gating means includes a first circuit having an adjustable trigger level, said transducer being connected to said first circuit to produce a start pulse upon the occurrence of the initiating acoustic pulse in said well, said start pulse being applied to start the counting of said clock pulses; and a second circuit having an adjustable trigger level, said transducer being connected to said second circuit to produce a stop pulse upon indication of an acoustic pulse reflected from the liquid surface in said well, said stop pulse being connected to stop said counting; and the apparatus further including a digital mute depth circuit comprising a manually settable digital register calibrated in units of depth;
multiplexing means sequentially connecting said digital registers to said comparator; and a mute depth circuit actuated coincidentally with the initiating acoustic pulses and connected to said second circuit to render it inoperative, said comparator being connected to render said second circuit operative when the count in said digital depth counting means equals the count in said digital mute depth register.

23. Apparatus for controlling a pump suspended in a well comprising;
a source of acoustic pulses coupled to said well;
a transducer coupled to said well producing an electric signal in response to the occurrence of acoustic pulses in said well and representative of the level of liquid in said well;
means responsive to an electric signal representative of a predetermined low liquid level in said well for turning the pump off; and means responsive to an electric signal representative of a predetermined high liquid level in said well for turning said pump on; and means for producing said electric signal representative of the level of liquid in said well in at least two intervals whereby a measure of depth is confirmed before said means responsive to said electric signal turns the pump on or off.

24. The apparatus recited in claim 23 further comprising:
a digital counter for counting clock pulses in the intervals between the generation of each initiating acoustic pulse and the detection of the reflected acoustic pulse; and means responsive to the digital count in said counters for generating digital high set point and digital low set point electric signals representative of a predetermined high liquid level and a predetermined low liquid level.

25. The apparatus recited in claim 24 wherein said means for producing said electric signals in at least two inter-vals includes:
means for counting said clock pulses in at least two intervals in which clock pulses exceed said digital high set point signal or fall below said digital low set point signal.

26. The apparatus recited in claim 24 further comprising:
a counter for counting the number of intervals in which clock pulses exceed said digital high set point signal;
and means for actuating an alarm when the counted number is more than predetermined amount.

27. Apparatus for controlling a pump suspended in a well comprising:
a source of periodic acoustic pulses continuously coupled to said well;
a transducer coupled to said well producing an output in response to the occurrence of acoustic pulses in said well and in response to the detection of the acoustic pulse reflected from the liquid surface in said well;
a clock pulse source;
digital counting means;
gating means responsive to the output of said transducer for starting the counting of clock pulses by said digital counting means upon the occurrence of an acoustic pulse in said well and for stopping said counting upon detection of the acoustic pulse reflected from the liquid surface in said well;
and a digital controller responsive to said digital counting means for controlling said pump, said digital controller including:
a high level set-point circuit including a manually settable digital register calibrated in units of depth;
a low level set-point circuit including a manually settable digital register calibrated in units of depth; and a comparator, said digital registers and said counting means being connected to said comparator, said comparator being connected to turn said pump on in response to counting clock pulses between said periodic acoustic pulses and said detection of the acoustic pulse reflected from the liquid surface when the liquid in said well exceeds the high set-point level and to turn said pump off in response to counting clock pulses between said periodic acoustic pulses and said detection of the acoustic pulse reflected from the liquid surface when the liquid in said well falls below the low set-point level.

28. A method of controlling the pumping of liquid from a well which has a pump suspended therein comprising:
repetitively generating initiating acoustic pulses in the well;
detecting acoustic pulses reflected from said liquid surface;
measuring the time difference between the initiating acoustic pulse and the detected reflected acoustic pulse by digitally counting clock pulses in the intervals between the generation of each initiating acoustic pulse and the detection of the reflected acoustic pulse;
repetitively producing a digital count as a measure of the depth of the liquid level in said well;
controlling the operation of asid pump from said digital count to maintain the depth of liquid level in said well between desired levels; and recording said time difference as a measure of the depth of the liquid level in said well while said depth is changing to produce a record of the rate of change in depth in said well; and turning said pump on when the counted clock pulses exceed a digital high set point level;

turning said pump off when the counted clock pulses fall below a digital low set point level;

digitally changing said high and low set point levels so that said record of the change in depth in said well is linear.

29. The method recited in claim 28 further comprising:
recording the time at which said pump is turned on and off.

30. The method recited in claim 28 further comprising:
confirming a measure of depth which turns said pump on or off by generating initiating acoustic pulses and counting clock pulses in at least two intervals in which clock pulses exceed said digital high set point level or fall below said digital low set point level.

31. The method recited in claim 28 further comprising controlling the volume per unit time of liquid pumped from said well by the steps of:
turning said pump on when the counted clock pulses exceed a digital high set point level;
turning said pump off when the counted clock pulses fall below a digital low set point level;
digitally changing said high level; and digitally changing the intervals between the generation of initiating acoustic pulses to pump the desired volume per unit time of liquid.